EMBRYOPSIDA Pirani & Prado

Gametophyte dominant, independent, multicellular, initially ±globular, not motile, branched; showing gravitropism; acquisition of phenylalanine lysase* [PAL], flavonoid synthesis*, microbial terpene synthase-like genes +, triterpenoids produced by CYP716 enzymes, phenylpropanoid metabolism [development of phenolic network], xyloglucans in primary cell wall, side chains charged; plant poikilohydrous [protoplasm dessication tolerant], ectohydrous [free water outside plant physiologically important]; thalloid, leafy, with single-celled apical meristem, tissues little differentiated, rhizoids +, unicellular; chloroplasts several per cell, pyrenoids 0; glycolate metabolism in leaf peroxisomes [glyoxysomes]; centrioles/centrosomes in vegetative cells 0, microtubules with γ-tubulin along their lengths [?here], interphase microtubules form hoop-like system; metaphase spindle anastral, predictive preprophase band + [with microtubules and F-actin; where new cell wall will form], phragmoplast + [cell wall deposition centrifugal, from around the anaphase spindle], plasmodesmata +; antheridia and archegonia +, jacketed*, surficial; blepharoplast +, centrioles develop de novo, bicentriole pair coaxial, separate at midpoint, centrioles rotate, associated with basal bodies of cilia, multilayered structure + [4 layers: L1, L4, tubules; L2, L3, short vertical lamellae] (0), spline + [tubules from L1 encircling spermatid], basal body 200-250 nm long, associated with amorphous electron-dense material, microtubules in basal end lacking symmetry, stellate array of filaments in transition zone extended, axonemal cap 0 [microtubules disorganized at apex of cilium]; male gametes [spermatozoids] with a left-handed coil, cilia 2, lateral; oogamy; sporophyte +*, multicellular, growth 3-dimensional*, cuticle +*, plane of first cell division transverse [with respect to long axis of archegonium/embryo sac], sporangium and upper part of seta developing from epibasal cell [towards the archegonial neck, exoscopic], with at least transient apical cell [?level], initially surrounded by and dependent on gametophyte, placental transfer cells +, in both sporophyte and gametophyte, wall ingrowths develop early; suspensor/foot +, cells at foot tip somewhat haustorial; sporangium +, single, terminal, dehiscence longitudinal; meiosis sporic, monoplastidic, MTOC [MTOC = microtubule organizing centre] associated with plastid, sporocytes 4-lobed, cytokinesis simultaneous, preceding nuclear division, quadripolar microtubule system +; wall development both centripetal and centrifugal, 1000 spores/sporangium, sporopollenin in the spore wall* laid down in association with trilamellar layers [white-line centred lamellae; tripartite lamellae]; nuclear genome [1C] <1.4 pg, main telomere sequence motif TTTAGGG, KNOX1 and KNOX2 [duplication] and LEAFY genes present, ethylene involved in cell elongation; chloroplast genome with close association between trnL_UAA and trnF_GAA genes [precursors for starch synthesis], tufA gene moved to nucleus; mitochondrial trnS(gcu) and trnN(guu) genes +.

Many of the bolded characters in the characterization above are apomorphies of subsets of streptophytes along the lineage leading to the embryophytes, not apomorphies of crown-group embryophytes per se.

All groups below are crown groups, nearly all are extant. Characters mentioned are those of the immediate common ancestor of the group, [] contains explanatory material, () features common in clade, exact status unclear.

STOMATOPHYTES

Abscisic acid, L- and D-methionine distinguished metabolically; pro- and metaphase spindles acentric; class 1 KNOX genes expressed in sporangium alone; sporangium wall 4≤ cells across [≡ eusporangium], tapetum +, secreting sporopollenin, which obscures outer white-line centred lamellae, columella +, developing from endothecial cells; stomata +, on sporangium, anomocytic, cell lineage that produces them with symmetric divisions [perigenous]; underlying similarities in the development of conducting tissue and of rhizoids/root hairs; spores trilete; shoot meristem patterning gene families expressed; MIKC, MI^*K^*C^* genes, post-transcriptional editing of chloroplast genes; gain of three group II mitochondrial introns, mitochondrial trnS(gcu) and trnN(guu) genes 0.

[Anthocerophyta + Polysporangiophyta]: gametophyte leafless; archegonia embedded/sunken [only neck protruding]; sporophyte long-lived, chlorophyllous; cell walls with xylans.

POLYSPORANGIOPHYTA†

Sporophyte well developed, branched, branching apical, dichotomous, potentially indeterminate; hydroids +; stomata on stem; sporangia several, terminal; spore walls not multilamellate [?here].

TRACHEOPHYTA†

Vascular tissue + [tracheids, walls with bars of secondary thickening]; stomata numerous, involved in gas exchange.

EXTANT TRACHEOPHYTA / VASCULAR PLANTS

Sporophyte with photosynthetic red light response, stomata open in response to blue light; plant homoiohydrous [water content of protoplasm relatively stable]; control of leaf hydration passive; plant endohydrous [physiologically important free water inside plant]; sporophyte with polar auxin transport, PIN [auxin efflux facilitator] involved; (condensed or nonhydrolyzable tannins/proanthocyanidins +); xyloglucans with side chains uncharged [?level], in secondary walls of vascular and mechanical tissue; lignins +; roots +, root hairs and root cap +; stem apex multicellular [several apical initials, no tunica], with cytohistochemical zonation, plasmodesmata formation based on cell lineage; vascular development acropetal, tracheids +, in both protoxylem and metaxylem, G- and S-types; sieve cells + [nucleus degenerating]; endodermis +; leaves/sporophylls spirally arranged, blades with mean venation density ca 1.8 mm/mm² [to 5 mm/mm²], all epidermal cells with chloroplasts; sporangia adaxial, columella 0; tapetum glandular; ?position of transfer cells; MTOCs not associated with plastids, basal body 350-550 nm long, stellate array in transition region initially joining microtubule triplets; suspensor +, shoot apex developing away from micropyle/archegonial neck [from hypobasal cell, endoscopic], root lateral with respect to the longitudinal axis of the embryo [plant homorhizic].

Sporophyte growth ± monopodial, branching spiral; roots endomycorrhizal [with Glomeromycota],lateral roots +, endogenous; G-type tracheids +, with scalariform-bordered pits; leaves with apical/marginal growth, venation development basipetal, growth determinate; sporangium dehiscence by a single longitudinal slit; cells polyplastidic, MTOCs diffuse, perinuclear, migratory; blepharoplasts +, paired, with electron-dense material, centrioles on periphery, male gametes multiciliate; nuclear genome size [1C] = 7.6-10 pg [mode]; chloroplast long single copy ca 30kb inversion [from psbM to ycf2]; mitochondrion with loss of 4 genes, absence of numerous group II introns; LITTLE ZIPPER proteins.

LIGNOPHYTA†

Sporophyte woody; stem branching lateral, meristems axillary; lateral root origin from the pericycle; cork cambium + [producing cork abaxially], vascular cambium bifacial [producing phloem abaxially and xylem adaxially].

SEED PLANTS†

Growth of plant bipolar [roots with positive geotropic response]; plants heterosporous; megasporangium surrounded by cupule [i.e. = unitegmic ovule, cupule = integument]; pollen lands on ovule; megaspore germination endosporic [female gametophyte initially retained on the plant].

EXTANT SEED PLANTS / SPERMATOPHYTA

Plant evergreen; nicotinic acid metabolised to trigonelline, (cyanogenesis via tyrosine pathway); microbial terpene synthase-like genes 0; primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignin chains started by monolignol dimerization [resinols common], particularly with guaiacyl and p-hydroxyphenyl [G + H] units [sinapyl units uncommon, no Maüle reaction]; root stele diarch to pentarch, xylem and phloem originating on alternating radii, cork cambium deep seated; stem apical meristem complex [with quiescent centre, etc.], plasmodesma density in SAM 1.6-6.2[mean]/μm² [interface-specific plasmodesmatal network]; eustele +, protoxylem endarch, endodermis 0; wood homoxylous, tracheids and rays alone, tracheid/tracheid pits circular, bordered; mature sieve tube/cell lacking functioning nucleus, sieve tube plastids with starch grains; phloem fibres +; cork cambium superficial; leaf nodes 1:1, a single trace leaving the vascular sympodium; leaf vascular bundles amphicribral; guard cells the only epidermal cells with chloroplasts, stomatal pore with active opening in response to leaf hydration, control by abscisic acid, metabolic regulation of water use efficiency, etc.; axillary buds +, exogenous; prophylls two, lateral; leaves with petiole and lamina, development basipetal, lamina simple; sporangia borne on sporophylls; spores not dormant; microsporophylls aggregated in indeterminate cones/strobili; grains monosulcate, aperture in ana- position [distal], primexine + [involved in exine pattern formation with deposition of sporopollenin from tapetum there], exine and intine homogeneous, exine alveolar/honeycomb; ovules with parietal tissue [= crassinucellate], megaspore tetrad linear, functional megaspore single, chalazal, sporopollenin 0; gametophyte ± wholly dependent on sporophyte, development initially endosporic [apical cell 0, rhizoids 0, etc.]; male gametophyte with tube developing from distal end of grain, male gametes two, developing after pollination, with cell walls; female gametophyte initially syncytial, walls then surrounding individual nuclei; embryo cellular ab initio, suspensor short-minute, embryonic axis straight [shoot and root at opposite ends; plant allorhizic], cotyledons 2; embryo ± dormant; chloroplast ycf2 gene in inverted repeat, trans splicing of five mitochondrial group II introns, rpl6 gene absent; whole nuclear genome duplication [ζ - zeta - duplication], two copies of LEAFY gene, PHY gene duplications [three - [BP [A/N + C/O]] - copies], 5.8S and 5S rDNA in separate clusters.

ANGIOSPERMAE / MAGNOLIOPHYTA

Lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, apigenin and/or luteolin scattered, [cyanogenesis in ANA grade?], lignin also with syringyl units common [G + S lignin, positive Maüle reaction - syringyl:guaiacyl ratio more than 2-2.5:1], hemicelluloses as xyloglucans; root cap meristem closed (open); pith relatively inconspicuous, lateral roots initiated immediately to the side of [when diarch] or opposite xylem poles; origin of epidermis with no clear pattern [probably from inner layer of root cap], trichoblasts [differentiated root hair-forming cells] 0, hypodermis suberised and with Casparian strip [= exodermis]; shoot apex with tunica-corpus construction, tunica 2-layered; starch grains simple; primary cell wall mostly with pectic polysaccharides, poor in mannans; tracheid:tracheid [end wall] plates with scalariform pitting, wood parenchyma +; sieve tubes enucleate, sieve plate with pores (0.1-)0.5-10< µm across, cytoplasm with P-proteins, not occluding pores of plate, companion cell and sieve tube from same mother cell; ?phloem loading/sugar transport; nodes 1:?; dark reversal P_fr → P_r; protoplasm dessication tolerant [plant poikilohydric]; stomata randomly oriented, brachyparacytic [ends of subsidiary cells level with ends of pore], outer stomatal ledges producing vestibule, reduction in stomatal conductance with increasing CO₂ concentration; lamina formed from the primordial leaf apex, margins toothed, development of venation acropetal, overall growth ± diffuse, secondary veins pinnate, fine venation hierarchical-reticulate, (1.7-)4.1(-5.7) mm/mm², vein endings free; flowers perfect, pedicellate, ± haplomorphic, protogynous; parts free, numbers variable, development centripetal; T +, petal-like, each with a single trace, outer members not sharply differentiated from the others, not enclosing the floral bud; A many, filament not sharply distinguished from anther, stout, broad, with a single trace, anther introrse, tetrasporangiate, sporangia in two groups of two [dithecal], each theca dehiscing longitudinally by a common slit, ± embedded in the filament, walls with at least outer secondary parietal cells dividing, endothecium +, cells elongated at right angles to long axis of anther; tapetal cells binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, tectum continuous or microperforate, ektexine columellate, endexine lamellate only in the apertural regions, thin, compact, intine in apertural areas thick, pollenkitt +; nectary 0; carpels present, superior, free, several, spiral, ascidiate [postgenital occlusion by secretion], stylulus at most short [shorter than ovary], hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, dry; suprastylar extragynoecial compitum +; ovules few [?1]/carpel, marginal, anatropous, bitegmic, micropyle endostomal, outer integument 2-3 cells across, often largely subdermal in origin, inner integument 2-3 cells across, often dermal in origin, parietal tissue 1-3 cells across, nucellar cap?; megasporocyte single, hypodermal, functional megaspore lacking cuticle; female gametophyte lacking chlorophyll, four-celled [one module, nucleus of egg cell sister to one of the polar nuclei]; ovule not increasing in size between pollination and fertilization; pollen grains bicellular at dispersal, germinating in less than 3 hours, siphonogamy, pollen tube unbranched, growing towards the ovule, between cells, growth rate (20-)80-20,000 µm/hour, apex of pectins, wall with callose, lumen with callose plugs, penetration of ovules via micropyle [porogamous], whole process takes ca 18 hours, distance to first ovule 1.1-2.1 mm; male gametophytes tricellular, gametes 2, lacking cell walls, ciliae 0, double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; mature seed much larger than fertilized ovule, small [<5 mm long], dry [no sarcotesta], exotestal; endosperm +, ?diploid, cellular, development heteropolar [first division oblique, micropylar end initially with a single large cell, divisions uniseriate, chalazal cell smaller, divisions in several planes], copious, oily and/or proteinaceous, embryo short [<¼ length of seed]; plastid and mitochondrial transmission maternal; Arabidopsis-type telomeres [(TTTAGGG)_n]; nuclear genome [1C] <1.4 pg [mean 1C = 18.1 pg, 1 pg = 10⁹ base pairs], whole nuclear genome duplication [ε/epsilon event]; ndhB gene 21 codons enlarged at the 5' end, single copy of LEAFY and RPB2 gene, knox genes extensively duplicated [A1-A4], AP1/FUL gene, palaeo AP3 and PI genes [paralogous B-class genes] +, with "DEAER" motif, SEP3/LOFSEP and three copies of the PHY gene, [PHYB [PHYA + PHYC]]; chloroplast chlB, -L, -N, trnP-GGG genes 0.

[NYMPHAEALES [AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]]: wood fibres +; axial parenchyma diffuse or diffuse-in-aggregates; pollen monosulcate [anasulcate], tectum reticulate-perforate [here?]; ?genome duplication; "DEAER" motif in AP3 and PI genes lost, gaps in these genes.

[AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]: phloem loading passive, via symplast, plasmodesmata numerous; vessel elements with scalariform perforation plates in primary xylem; essential oils in specialized cells [lamina and P ± pellucid-punctate]; tension wood + [reaction wood: with gelatinous fibres, G-fibres, on adaxial side of branch/stem junction]; anther wall with outer secondary parietal cell layer dividing; tectum reticulate; nucellar cap + [character lost where in eudicots?]; 12BP [4 amino acids] deletion in P1 gene.

[[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]] / MESANGIOSPERMAE: benzylisoquinoline alkaloids +; sesquiterpene synthase subfamily a [TPS-a] [?level], polyacetate derived anthraquinones + [?level]; outer epidermal walls of root elongation zone with cellulose fibrils oriented transverse to root axis; P more or less whorled, 3-merous [?here]; pollen tube growth intra-gynoecial; extragynoecial compitum 0; carpels plicate [?here]; embryo sac bipolar, 8 nucleate, antipodal cells persisting; endosperm triploid.

[MONOCOTS [CERATOPHYLLALES + EUDICOTS]]: (extra-floral nectaries +); (veins in lamina often 7-17 mm/mm² or more [mean for eudicots 8.0]); (stamens opposite [two whorls of] P); (pollen tube growth fast).

MONOCOTYLEDONS / MONOCOTYLEDONEAE / LILIANAE Takhtajan

Plant herbaceous, perennial, rhizomatous, growth sympodial; non-hydrolyzable tannins [(ent-)epicatechin-4] +, neolignans 0, CYP716 trterpenoid enzymes 0, benzylisoquinoline alkaloids 0, hemicelluloses as xylan; root epidermis developed from outer layer of cortex; endodermal cells with U-shaped thickenings; cork cambium [uncommon] superficial; stele oligo- to polyarch, medullated [with prominent pith], lateral roots arise opposite phloem poles; stem primary thickening meristem +; vascular development bidirectional, bundles scattered, (amphivasal), vascular cambium 0 [bundles closed]; tension wood 0; vessel elements in roots with scalariform and/or simple perforations; tracheids only in stems and leaves; sieve tube plastids with cuneate protein crystals alone; stomata oriented parallel to the long axis of the leaf, in lines; prophyll single, adaxial; leaf blade linear, main venation parallel, the veins joining successively from the outside at the apex and forming a fimbrial vein, transverse veinlets +, unbranched [leaf blade characters: ?level], vein/veinlet endings not free, margins entire, Vorläuferspitze +, base broad, ensheathing the stem, sheath open, petiole 0; inflorescence terminal, racemose; flowers 3-merous [6-radiate to the pollinator], polysymmetric, pentacyclic; P = 3 + 3 T, each with three traces, median T of outer whorl abaxial, aestivation open, members of whorls alternating, [pseudomonocyclic, each T member forming a sector of any tube]; stamens = and opposite each T member [A/T primordia often associated, and/or A vascularized from T trace], anther and filament more or less sharply distinguished, anthers subbasifixed, wall with two secondary parietal cell layers, inner producing the middle layer [monocot type]; pollen reticulations coarse in the middle, finer at ends of grain, infratectal layer granular; G [3], with congenital intercarpellary fusion, opposite outer tepals [thus median member abaxial], placentation axile; compitum +; ovule with outer integument often largely dermal in origin, parietal tissue 1 cell across; antipodal cells persistent, proliferating; fruit a loculicidal capsule; seed small to medium sized [mean = 1.5 mg], testal; embryo long, cylindrical, cotyledon 1, apparently terminal [i.e. bend in embryo axis], with a closed sheath, unifacial [hyperphyllar], both assimilating and haustorial, plumule apparently lateral; primary root unbranched, not very well developed, stem-borne roots numerous, hypocotyl short, (collar rhizoids +); no dark reversion P_fr → P_r; duplication producing monocot LOFSEP and FUL3 genes [latter duplication of AP1/FUL gene], PHYE gene lost.

[ALISMATALES [PETROSAVIALES [[DIOSCOREALES + PANDANALES] [LILIALES [ASPARAGALES + COMMELINIDS]]]]]: ethereal oils 0; (trichoblasts in vertical files, proximal cell smaller); raphides + (druses 0); leaf blade vernation supervolute-curved or variants, (margins with teeth, teeth spiny); endothecium develops directly from undivided outer secondary parietal cells; tectum reticulate with finer sculpture at the ends of the grain, endexine 0.  Back to Main Tree

Age. Magallón and Castillo (2009) suggest ca 162 m.y. for relaxed and 126.5 m.y. for constrained penalized likelihood datings of the divergence of Alismatales from other monocots. 140-130 m.y.a. covers many estimates: ca 130.8 m.y.a. in Magallón et al. (2015), ca 131 m.y. (Janssen & Bremer 2004), (156-)138(-130) m.y. in Nauheimer et al. (2012: sampling), and (139-)132(-125) m.y. in Givnish et al. (2016b). Close are (123-)128(-133) m.y. in Merckx et al. (2008a), ca 142 m.y. in Tank et al. (2015: Table S1, [stem] Petrosaviidae, younger than Alismatales) and (139-)132, 128(-123) m.y. in Hertweck et al. (2015). A mere 108.1 or 101.2 m.y. is the estimate in in Xue et al. (2012), but 137-116 m.y. in Mennes et al. (2013, see also 2015), (147-)136, 118(-107) m.y. in Bell et al. (2010), (163.1-)147.1(-130.6) m.y. in Eguchi and Tamura (2016), ca 148 m.y. in Foster et al. (2016a: q.v. for details) and ca 170 m.y.a. in Z. Wu et al. (2014).

Evolution. Ecology & Physiology. For a possible major reduction of plant height around here, see the monocots node.

Plant-Animal Interactions. Overall, herbivory in this clade is relatively low (Turcotte et al. 2014: see caveats).

Chemistry, Morphology, etc. Although raphides occurring in bundles and largely filling the cells containing them are common in this clade, druses may at least sometimes be found along with them (e.g. Prychid et al. 2008). Root hair development is poorly known, thus although Clowes (2000) records no trichoblasts from Pandanales, Dioscoreales, Asparagales, Liliales or Petrosaviales, sampling was hardly extensive (one orchid examined...).

Phylogeny. For an [Acorus + Alismatales] relationship, see Acorales; this is unlikely.

ALISMATALES Dumortier  Main Tree.

(Cyanogenesis + [triglochinin]); starch grains pteridophyte-type, amylophilic; inflorescence ± scapose; anthers extrorse; tapetal cells uninucleate; carpels free, plicate or mostly so, with completely unfused canals, styles +, stigma dry [common]; fruit a follicle; endosperm helobial; embryo (chlorophyllous), large, cotyledon large; seedling with hypocotyl and root well developed, collar rhizoids +. - 14 families, 166 genera, 4560 species.

Age. Crown-group Alismatales are dated to 124-111 m.y. by Wikström et al. (2001), ca 128 m.y. by Janssen and Bremer (2004) and around 103 m.y. by Bremer (2000b); Magallón and Castillo (2009) suggested ca 147 m.y. and 126 m.y., Bell et al. (2010) ages of (138-)122, 102(-93) m.y., and Magallón et al. (2013, 2015) suggested ages of around 122.6 m.y.a. and 128.9 m.y.a. respectively (see also Hertweck et al. 2015, dates at ca 125-120 m.y.a. are in the same bailiwick); estimates were (133-)123(-97) m.y. in Merckx et al. (2008a), (146-)138(-130) m.y. in Nauheimer et al. (2012b), 134-90 m.y. in Mennes et al. (2013, Mennes et al. 2015 is similar), around 145.8 m.y. in Tank et al. (2015: Table S2), ca 146 m.y.a. in Z. Wu et al. (2014), (144.8-)132.1(-120) m.y. in Eguchi and Tamura (2016) and as little as ca 91.2 m.y. in Tang et al. (2016). These dates all need checking in the context of the topology suggested by Luo et al. (2016).

See Stockey 2006 for a review of fossils that have been placed in Alismatales; see also Araceae below.

Note: Boldface denotes possible apomorphies, (....) denotes a feature common in the clade, exact status uncertain, [....] includes explanatory material. Note that the particular node to which many characters, particularly the more cryptic ones, should be assigned is unclear. This is partly because homoplasy is very common, in addition, basic information for all too many characters is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there are the not-so-trivial issues of how character states are delimited and ancestral states are reconstructed (see above).

Evolution: Divergence & Distribution. Les et al. (2003) discuss the distributions of a number of hydrophytic clades here; most of them are likely to be rather young.

Furness (2013a) optimised a number of palynological characters in Alismatales (surface sculpture, microsporogenesis, tapetum type); those connected with hypohydrophily in marine members are homoplasious; for optimisation of other characters associated with hydrophily, see Du and Wang (2014), and for granulate exine infratectum, see Doyle (2009: Aponogetonaceae, Aroideae). Sokoloff et al. (2015c) discuss and optimize the various manifestations of syncarpy in Alismatales (under both Tofieldiaceae- and Araceae-basal scenarios). Relationships at the base of the tree affect how variation in features like tapetum, endosperm and carpel connation is optimized; here, as elsewhere, treat suggestions of possible apomorphies sceptically.

Ecology & Physiology. Alismatales include the only marine angiosperms that grow fully submerged, the so-called sea-grasses (see below). They also include many other aquatics and hydrophytes and plants of marshy habitats, and this may well be the ancestral condition for the group as a whole (for Araceae, see Nauheimer et al. 2012). It is not surprising that a number of taxa lack vessels.

Bacterial/Fungal Associations. The absence of mycorrhizae in many Alismatales (see also Brundrett 2017b) may be because many of its members grow in water, either rooted in the mud or free-floating; mycorrhizae are usually absent in such situations (e.g. Safir 1987). However, Araceae occupy a variety of habitats.

Plant-Animal Interactions. Caterpillars of Pyralidae-Schoenobiinae are found on aquatic monocots, as are larvae of Chrysomelidae-Donaciinae (Powell et al. 1999; Jolivet 1988; esp. Kölsch & Pedersen 2008); the latter, at least, are found on aquatic plants in general and so are also to be found on Nymphaeaceae, Haloragaceae, etc..

Genes & Genomes. There has been an increase in the GC content of the genome in this clade (Smarda et al. 2014). For chromosome number evolution, see Sousa and Renner (2015).

For comments on chloroplast genome arrangement and evolution, see Luo et al. (2016). Chloroplast ndh genes have been mostly or all lost (or exist only as pseudogenes) four times (Ross et al. 2015; Ruhlman et al. 2015).

Chemistry, Morphology, etc. The root stele is often tri- to pentarch. A number of wholly aquatic Alismatales lack vessels in any part of the plant (Cheadle 1944), and some, but not all, lack a root hypodermis (e.g. Kroemer 1903). For what is known of leaf development in Alismatales, see the Acorales page. When the leaves are petiolate, the vascular bundles are in an arc; inverted bundles are also common. Riley and Stockey (2004) describe the venation of a number of net-veined members of this order in considerable detail; such leaves usually have tertiary veins.

There are suggestions that the bract subtending the flower and the abaxial tepal may be fused or otherwise developmentally linked, so producing a hybrid organ, or the bract may be lost, and both phenomena can occur in the one genus (Buzgo 2001; Remizowa et al. 2013a, b), and it has been suggested that in some Alismatales the flowers are pseudanthial (Soltis et al. 2005; Remizowa et al. 2011). Is the pollen endexine ever lamellate (it is in Acorus)? It has been said that Alismatales, inc. Araceae, are the only monocots with green (chlorophyllous) embryos (Seubert 1993), although they are also known from some Amaryllidaceae-Amaryllidoideae-Amaryllideae, for example. Although endosperm is usually not well developed in seeds here, when it does occur it at least sometimes contains starch as a reserve (Dahlgren et al. 1985).

For leaf anatomy of aquatic members, see Sauvageau (1891), for pollen, see Grayum (1992), Furness and Banks (2010), for ovules, see Igersheim et al. (2001), and for seedling morphology, see Tillich (1985).

Phylogeny. Basal relationships in Alismatales were summarized by Les and Tippery (2013). Tofieldiaceae were placed here with only moderate support (Källersjö et al. 1998; Chase et al. 2000a. They were sometimes sister to the rest of the order (Graham et al. 2006, but sampling; Iles et al. 2013: support weak; Thadeo et al. 2015: including morphology; Eguchi & Tamura 2016) or sister to Araceae (Tamura et al. 2004b; Ross et al. 2015: most analyses; Petersen et al. 2015). In a study involving 22 taxa and 79 protein-coding plastid genes, Luo et al. (2016) found strong (100% bootstrap, 1 p.p., bootstrap support lower only in anayses of combined first and second codon positions) for this position, and this topology is followed here. However, Tamura et al. (2004a), Janssen and Bremer (2004), Givnish et al. (2006b), Chase et al. (2006: strong support), Soltis et al. (2011: little support, but sampling), Nauheimer et al. (2012b), Hertwick et al. (2015) and Z.-D. Chen et al. (2016) all placed Araceae as sister to the rest of the order, and Tofieldiaceae sister to the remaining taxa. X.-X. Li and Zhou (2007) even recovered a [Tofieldiaceae + Araceae] clade. For the inclusion of Acoraceae in Alismatales in some analyses, see the monocot node.

For relationships between other Alismatales, see Les et al. (1997b: the rest of the tree here is largely based on this), also Y. Kato et al. (2003), J.-M. Chen et al. (2004a, b), G. Petersen et al. (2006c: 2 mitchondrial and 1 chloroplast genes), Liu and Li (2010), Nauheimer et al. (2012b), L.-Y. Chen et al. (2013), Du and Wang (2014), Du et al. (2016: focus on aquatic taxa) and Z.-D. Chen et al. (2016: Chinese taxa). In a chloroplast analysis with good generic-level sampling these relationships were again recovered, support for the relationships between the sea-grass groups was strengthened, and support in general was strong (Ross et al. 2015).

Aponogetonaceae and Scheuchzeriaceae are sister taxa and in turn are sister to the other members of the group in a rbcL analysis of Y. Kato et al. (2003) and also in Liu and Li (2010). Nauheimer et al. (2012b), Iles et al. (2013), Les and Tippery (2013), L.-Y. Chen et al. (2013), Petersen et al. (2015), Du and Wang (2014), Ross et al. (2015) and Du et al. (2016), recovered the relationships followed here, [Aponogetonaceae [Scheuchzeriaceae ...]], although support was not always strong. The positions of Aponogetonaceae and Scheuchzeriaceae are sometimes reversed, as in Janssen and Bremer (2004), and relationships between these two families were unclear in von Mering and Kadereit (2010).

Some uncertainties remain. Thus G. Petersen et al. (2006c) did not even recover a monophyletic [Hydrocharitaceae + Alismataceae + Butomaceae], and Petersen et al. (2015) recovered Alismataceae as sister to other core Alismatales, Acorus being sister to the combined group. The tree in Janssen and Bremer (2004) is largely similar to that below, while L.-Y. Chen et al. (2013), Du and Wang (2014) and Du et al. (2016) recovered a clade [Butomaceae + Alismataceae]. Butomaceae were embedded in Hydrocharitaceae in a rbcL analysis of Y. Kato et al. (2003), but this position has not been confirmed. Von Mering and Kadereit (2010) were not sure of the exact position of Maundia, ex-Juncaginaceae, and they also found weak support for a clade [Araceae + Tofieldiaceae]. However, Maundia is usually sister to the main sea-grass clade, as in Petersen et al. (2015), Ross et al. (2015), Du et al. (2016), etc.. For details of relationships within the main sea-grass clade, see also below.

Classification. Many small families have been described in Alismatales, especially in the sea grasses, because the adaptations associated with the aquatic habitat are so striking. A more restricted circumscription of Alismatales has also been suggested given the past uncertainty over the relationships of Acoraceae and Tofieldiaceae, although the two have very rarely been placed ouside the clade recently; this also entails the recognition of Potamogetonales, etc. (Les & Tippery 2013). Eguchi and Tamura (2016) suggest a largely age-based dismemberment of Alismatales.

Previous Relationships. Most of the taxa here included in Alismatales have long been recognised as being related, thus most are in Engler's (1892) Helobieae + Spathiflora and Cronquist's (1981) and Takhtajan's (1997) Alismatidae. However, in some classifications a Spadiciflorae were recognized, and they included all taxa with a spadix, i.e. Cyclanthaceae, Pandanaceae, Araceae and Arecaceae, but these families are now placed in three immediately unrelated orders, Pandanales, Alismatales and Arecales. The relationships of Tofieldiaceae have been unclear, although they have usually been placed in a quite different part of the monocots, having a flower that fitted into the concept of the old Liliaceae sensu latissimo (see below, also Dahlgren & Clifford 1982; Tamura 1998).

Includes Alismataceae, Aponogetonaceae, Araceae, Butomaceae, Cymodoceaceae, Hydrocharitaceae, Juncaginaceae, Maundiaceae, Posidoniaceae, Ruppiaceae, Potamogetonaceae, Scheuchzeriaceae, Tofieldiaceae, Zosteraceae.

Synonymy: Alismatales Dumortier, Aponogetonales Hutchinson, Arales Dumortier, Butomales Hutchinson, Cymodoceales Nakai, Elodeales Nakai, Hydrocharitales Dumortier, Juncaginales Hutchinson, Lemnales Link, Najadales Dumortier, Orontiales J. Presl, Pistiales Richard, Posidoniales Nakai, Potamogetonales Dumortier, Ruppiales Nakai, Scheuchzeriales B. Boivin, Tofieldiales Reveal & Zomlefer, Vallisneriales Nakai, Zosterales Nakai -- Alismatidae Takhtajan, Aridae Takhtajan - Aropsida Bartling, Hydrocharitopsida Bartling, Najadiopsida Hoffmannsegg & Link

TOFIELDIACEAE Takhtajan   Back to Alismatales

?Mycorrhizae; steroidal saponins, chelidonic acid +; vessels?; fibres mixed with phloem; sieve tube plastids also with polygonal crystals; endodermal cells with U-shaped thickenings [distribution around here?]; stomata anomocytic; also prismatic crystals +; leaves two-ranked, ventralized isobifacial [oriented edge on to the stem], (unifacial); inflorescence bracts +, (flower single - Isidrogalvia); floral bracts +, ± foliaceous (0), calyculus below individual flowers (not some Tofieldia); T free (basally connate), with one trace [Tofieldia], median member of outer whorl adaxial [Tofieldia] or not; A (9-12 - Pleea; adnate to base of P; basally connate), introrse to latrorse, (filaments with three traces - some Isidrogalvia); microsporogenesis simultaneous; pollen di(trichotomo)sulcate (monosulcate - Isidrogalvia); septal or tepal nectaries +; G stipitate, carpel periphery completely postgenitally fused, (placentation parietal), (style + - Isidrogalvia); ovules 5-many/carpel, ana-campylotropous, (unitegmic), (nucellar cap +), hypostase +, integumentary obturator +; (embryo sac with long chalazal extension), antipodal cells not multinucleate; fruit also a septicidal capsule ["ventricidal" capsule], (P persistent); seeds with terminal appendages, obliquely stacked; phlobaphene +, tegmen thin; ?embryo; n = (14) 15 (16), chromosomes 0.9-2.5 µm long; radicle 0?.

3-5[list]/31: Isidrogalvia (14). S.E. U.S.A., N. South America, N. temperate (map: see Hultèn 1961; Meusel et al. 1965; Hultén & Fries 1986; Fl. N. Am. 26: 2002; Campbell 2010; Campbell & Dorr 2013). [Photo - Flowers] [Photo - Flowers.]

Age. Crown-group Tofieldiaceae are dated to ca 100 m.y. (Janssen & Bremer 2004); other age estimates are 80-75 m.y. (Wikström et al. 2001) and (95-)64, 61(-35) m.y. (Bell et al. 2010).

• Tofieldiaceae can be recognised by their equitant, isobifacial leaves; racemose inflorescences in which there is usually a 1-3-membered "calyculus" below the flower; and undistinguished monocot flowers with the tepals usually free and the tips of the carpels each with a clearly separate stylulus. The fruit is usually a septicidal capsule or follicle.

Chemistry, Morphology, etc. The leaves of Tofieldia may lack palisade mesophyll (Kao 1989). Branching in Tofieldiaceae needs study. Remizowa et al. (2005a) suggest that the first two leaves of axillary shoots in Tofieldia (the prophyll and the next leaf) are both adaxial; this would be very unusual, if true. Generalised comparisons between the calyculus of Tofieldiaceae, made up of two or three connate scales, with the spathe of Hydrocharitaceae and pseudowhorls of bracts in Alismataceae have been made (Remizowa and Sokoloff 2003; Remizowa et al. 2006b). Remizowa et al. (2010a) found that the calyculus is usually supplied by three vascular traces, but in Tofieldia pusilla it is basal on the pedicel and supplied by a single trace - part bract and part calyculus?

The nectaries of Tofieldiaceae may be unique. They are triradiate, being borne on the inner bases of the connate carpellary stipes (Remizowa et al. 2006a and references), and so appear to be the perfect "intermediate" between the septal nectaries found in many other monocots and the nectary-less condition.

For additional general information, see Ambrose (1975, 1980), Zomlefer (1997b), Tamura (1998 - as Nartheciaceae), and Campbell (2010), for the single-flowered Harperocallis, described from Florida in 1968 but recently synonymized in Isidrogalvia, c.f. McDaniel (1968) and Remizowa et al. (2011b), for sieve tube plastid type, see Behnke (2000, 2003), for inflorescence and floral morphology, see Gatin (1920), Remizowa et al. (2006a, 2010b, 2013b), for ovary morphology, see Sterling (1979), for general embryology, see Cave (1968 and references), and for ovule development, see Holloway and Friedman (2008).

Phylogeny. Pleea is sister to the rest of the family (Tamura et al. 2004b; Azuma & Tobe 2011; Iles et al. 2103). Pleea is the only genus that has two stamens opposite members of the outer tepal whorl and single stamens opposite members of the inner whorl - as in a few Alismataceae, Butomaceae and Hydrocharitaceae (Azuma & Tobe 2011). There is a fair amount of phylogenetic structure in the rest of the family, but little in the way of morphology to support the clades. L.-Y. Chen et al. (2013) found that Isidrogalvia was embedded in Petrosaviaceae, which is rather remarkable, although they did not comment on its position (but see Luo et al. 2016: likely contamination or misidentification).

Classification. For a checklist of the family, see World Checklist of Monocots.

Previous Relationships. Tofieldiaceae have often been included in other families. Dahgren et al. (1985) placed them, along with representatives of Nartheciaceae and Petrosaviaceae, in Melianthaceae, Tamura (1998) placed them in Petrosaviaceae (along with Nartheciaceae), and Cronquist (1981) considered Tofieldia to be an archaic genus of his broadly-drawn Liliaceae. Tofieldiaceae were included in Melanthiales by Takhtajan (1997).

[Araceae [[Alismataceae [Butomaceae + Hydrocharitaceae]] [Aponogetonaceae [Scheuchzeriaceae [Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]]]]]]: septal nectaries 0; tapetum amoeboid.

Age. This node is (132-)118.5(-100.5) m.y.o. (Givnish et al. 2016b).

ARACEAE Jussieu, nom. cons.   Back to Alismatales

Cyanogenic glucoside triglochinin, flavone C-glycosides +; root hairs from unmodified rhizodermal cells, root hypodermis with dimorphic cells; sieve tube plastids also with starch; pseudopetiole bundles scattered; stomata unorientated, also anomo- ["basal" genera] and tetracytic; leaves spiral; inflorescence scapose, spicate, with large associated inflorescence bract [spathe], unbranched; flowers dense [spadix], sessile, small; flowering period up to weeks, floral bracts, bracteoles 0; flowers 2-3-merous, in latter case median member of outer whorl of T adaxial, T ± hooded, with single trace, free (connate); (microsporogenesis simultaneous); pollen (often starchy), ektexine +; carpels connate, (basally) ascidiate], fusion usu. congenital, loculus with mucilage [usu. secretory trichomes], style at most short, stigma also wet; ovules ± unvascularized, nucellar cap + (0); fruit a berry; testa multiplicative, >5 cells across, often parenchymatous, or with exotesta and/or endotesta and mesotesta lignified, tegmen collapsed; x = 16; endosperm +/0, ?cellular, cotyledon not photosynthetic, radicle +, (collar rhizoids or collar roots +).

123[list]/Boyce & Croat 2016 - The Überlist of Araceae//4,365 (3,525, 3,750, 5,422 other estimates...) - 8 subfamilies below. Mostly tropical, few in Africa-Madagascar and Australia.

Age. Crown-group Araceae have been dated to (132-)122(-112) m.y. by Nauheimer et al. (2012b); other dates include 98-89 m.y. (Wikström et al. 2001), ca 128 m.y. (Janssen & Bremer 2004) or to (114-)89, 79(-55) m.y.a. (Bell et al. 2010).

Distinctive pollen assigned to Pothoideae-Monstereae has been found in Early Cretaceous deposits of the late Barremian-early Aptian of some 120-110 m.y. old in Portugal (Friis et al. 2004); other pollen types that may also be Araceae were found at the same place (see also Hesse & Zetter 2007). However, Mayoa portugallica, one fossil involved, may be an individual of Laganella, an euglenid alga... (Hoffmann & Zetter 2010). The site is now, alas, developed.

[Gymnostachydoideae + Orontioideae]: stomata parallel; parallel veins in 2(+) orders; tectum ± continuous, ovules straight, 1-2/carpel; seedling with cataphylls.

Age. The age for this node was estimated as (115-)96(-73) m.y. by Nauheimer et al. (2012b).

1. Gymnostachydoideae Bogner & Nicolson

2. Orontioideae Mayo, Bogner & Boyce

Flavonols +, glycoflavones 0; vessels 0; (collenchyma in cortical bands), bundle-associated fibre strands +/0; (laticifers + - Orontium); (biforine raphides +); (stomata anomocytic); leaves with weakly differentiated petiole and blade; inflorescence bract large, coloured [spathe] (0); flowers 2-3-merous; (A also ventrifixed); ovary inferior (not - Orontium), secretory trichomes 0 - Symplocarpus; ovules ± basal, (hemianatropous - Orontium), outer integument 22+ cells across, (lobed - Symplocarpus), inner integument 5-10 cells across, funicular obturator; tegmen also multiplicative, endosperm nuclear, ± 0; n = 13-15.

3/6. Temperate East Asia, W. and E. North America (map: from Mayo et al. 1997).

Age. The age for crown-group Orontioideae is estimated at (77-)53(-28) m.y. by Nauheimer et al. (2012b).

The fossil Spixiarum kipea, from the Crato formation in Brazil, is dated to 115-112 m.y., so if it were included in the crown group (a possibility: Coiffard et al. 2013b) both its age and locality would be rather discordant.

Synonymy: Orontiaceae Bartling

[Lemnoideae [[Pothoideae + Monsteroideae] [Lasioideae [Zamioculcadoideae + Aroideae]]]]: raphides twinned [?level]; nucellar tissue disappears during ovule maturation, endothelium +; endosperm ± cellular.

Age. The age for this node was estimated as (113-)104(-93) m.y. by Nauheimer et al. (2012b).

3. Lemnoideae Engler

Floating aquatic herbs; roots monarch (0); collenchyma and bundle fibres 0; vessels 0; (prophyll 0); biforine raphides +; stomata anomocytic [on adaxial surface only]; plant made up of thalloid stem-leaf units, (prophyll 0), primary vein alone, two (one) series of axillary buds; (spathe 0), spadix not discernable; P 0; A 1-2, (monothecal), wall with secondary parietal layers not dividing; pollen grains tricellular, mixed with raphides, globose to ellipsoid, ulcerate, spiny; G 1, stigma funneliform; ovules 1-7/carpel, (straight), (outer integument to 4 cells across), parietal tissue to 4 cells across; embryo sac bisporic [chalazal dyad], 8-nucleate [Allium-type]; fruit an achene [sort of]; seed operculate [operculum tegmic], testa ca 4 cells across; endosperm starchy, ?all, copious, chalazal haustorium +, embryo undifferentiated; n = 10, extensive polyploidy and dysploidy; hypocotyl and primary root 0, cataphylls 0; n (10), 20, etc; chromosomes 0.1-1.7 um long, 1C value = 0.15-1.63 pg; two genome duplications; plastid infA gene lost.

5/37. World-wide (map: Hultén 1962; Meusel et al. 1965; Landolt 1986: "absence" in tropical America and Africa may be due to undercollecting; Fl. Austral. 39: 2011). [Photo - Wolffia.]

[[Pothoideae + Monsteroideae] [Lasioideae [Zamioculcadoideae + Aroideae]]]: (velamen +) [?here]; shoots consisting of reiterated sympodial units, branching from the axil of the penultimate foliar organ outside the inflorescence spathe; leaves with petiole and blade, blade with midrib, secondary veins ± palmate, fine venation reticulate, base with lateral (auriculate) flanges, (ligule +); inflorescence bract large, coloured [spathe], peduncle +; (calcium oxalate crystals, inc. raphides, mixed with pollen); (endosperm ± 0, cotyledons green); seedling cataphylls +/0.

Age. The age for this node was estimated to be (107-)97(-87) m.y. by Nauheimer et al. (2012b).

[Pothoideae + Monsteroideae]: stem usu. aerial, plants (hemi)epiphytes, climbers; (separate stem cortical vascular system +); (vessels in stem); H- or T-shaped trichosclereids + (0); styloids +; fibres ensheathing bundles; stomata oriented randomly [?level]; leaf blade elliptical to complex, petiole apically geniculate; crystals often surrounding the embryo.

Age. The age for this node was estimated at (94-)81(-68) m.y. by Nauheimer et al. (2012b).

4. Pothoideae Engler

Climbers and epiphytes common; (main axis monopodial); (biforine raphides +); (leaves 2-ranked), (blade with two or more submarginal veins at some distance from the margin), (petiole flattened); (inflorescences terminal on branches), (several in panicles - Pothoidium); spathe not enclosing spadix, ± reflexed; flowers 2-3-merous; (T connate), anther thecae often forming tip above slit; (pollen 3-4 porate - Anthurium); placentation basal/parietal, (with apical septum), (stylar canal occupied by intertwined trichomes); ovules 1-2/carpel, apotropous, outer integument 6-8 cells across, inner integument 6-8 cells across, parietal tissue? ca 1 cell across; (micropylar megaspore germinates); spathe persistent in fruit; (testa not multiplicative), (inner integument to 8 cells across); endosperm with starch, (chlorophyllous), (embryo chlorophyllous); n = (10) 12 (14-15), (interstitial telomere repeats +); (seedling internodes long; very short unifacial part of cotyledon).

4/900: Anthurium (825), Pothos (70). Tropical America, Madagascar to South and Southeast Asia, Malesia and N.E. Australia (map: from Mayo et al. 1997). [Photo - Flowers, Fruits.]

Age. Crown-group Pothoideae are some (77-)65(-56) m.y.o. (Nauheimer et al. 2012b).

Synonymy: Pothaceae Rafinesque

5. Monsteroideae Schott

(Climbers +); (fibres only capping bundles); trichosclereids + (0); spathe often deciduous; flowers 2(-3 - Spathiphylleae)-merous; P often 0; pollen inaperturate, or sulcus extended or encircling, (ektexine dissected - 0); style with abundant trichosclereids; ovules 1-4(-many)/carpel, often basal, (hemianatropous); spathe soon deciduous in fruit; seed often embedded in mucilage; endosperm + [Spathiphylleae] or 0; n = 12, 14, 15, 21 [much polyploidy, n = 30 common].

12/360: Rhaphidophora (100: paraphyletic), Rhodospatha (75). Tropical South and Southeast Asia to the Pacific, South America (Africa) (map: from Mayo et al. 1997).

Age. The age of crown-group Monsteroideae is (64-)55(-47) m.y. (Nauheimer et al. 2012b).

Synonymy: Monsteraceae Vines

[Lasioideae [Zamioculcadoideae + Aroideae]]: plant tuberous or rhizomatous.

Age. The age of this node is estimated at some (101-)90(-80) m.y. (Nauheimer et al. 2012b), however, stem node Lasioideae are dated to around 48.7 m.y.a. (Iles et al. 2015).

6. Lasioideae Engler

(Rooted aquatics), often prickly; sieve tube plastids with a little starch; fibres ensheathing or capping bundles; petiole long, warty, aculeate, or strikingly coloured, ± geniculate apically; spathe often spirally twisted, inflorescence flowering basipetally; P (0 [Pycnospatha], 4-9); A (-12), anthers with oblique pore-like slits; pollen grains lacking starch, sulcus with lamellate ectexine and thick bilayered endexine [outer: flakes or lamellae; inner: spongy]; G 1 [2-3(-16)], 1 (-6) locular, placentation various, inc. basal-diffuse; ovules 1-2(-many - esp. Cyrtosperma)/loculus, apotropous, ana-campylotropous, ?parietal tissue, ?endothelium, hypostase +, podium +; seed surface lamellate or warty (not Cyrtosperma), exotesta papillate, inner part of mesotesta and endotesta both lignified; endosperm thin (0), embryo chlorophyllous, curved; n = 13.

10/58. Pantropical (Africa - Lasimorpha) (map: from Mayo et al. 1997).

Age. Crown-group Lasioideae can be dated to some (38-)26(-15) m.y.a. (Nauheimer et al. 2012b).

Bogner et al. (2005) described a ca 72 m.y.o. Late Campanian spadix with perfect, tepalline flowers and a stout, conical, stigma/style from Alberta, Canada, and pollen fossils of a similar age from Eastern Siberia have been identified as Lasioideae (Hoffmann & Zetter 2010). Middle Eocene seeds ca 48.7 m.y.o. from the Princeton Chert been assigned to this subfamily (Smith & Stockey 2003).

Synonymy: Dracontiaceae Salisbury, Lasiaceae Vines

[Zamioculcadoideae + Aroideae]: plants monoecious (dioecious); spathe differentiated into tube plus blade, spadix differentiated into zones with staminate and carpellate flowers, (zone of sterile female flowers, sometimes much enlarged, in between), flowers opening simultaneously in each zone, flowering period usu. 2-4 days; flowers imperfect; pollen inaperturate; (cotyledonary hypophyll expanded, flattened, photosynthetic), (radicle 0, hypocotyl 0, roots break through body of cotyledon).

Age. This node can be dated to (97-)87(-77) m.y.a. (Nauheimer et al. 2012b).

7. Zamioculcadoideae Bogner & Hesse

(Leaves compound, petiole geniculate, fine venation reticulate - Zamioculcas); staminate flowers: A (connate), (introrse); pollen also encircling sulcate, columellae winding and forming a sort of internal tectum as well as the external tectum, endexine lamellate, intine thin, (calcium oxalate crystals mixed with pollen); pistillode +; carpellate flowers: G [2], placentation axile; ovule 1/carpel, ascending, ?parietal tissue; n = (14), 17.

3/21. Africa, Liberia to Kenya and Natal (map: from Mayo et al. 1997).

Age. The age of crown-group Zamioculcadoideae was estimated at (42-)23(-6) m.y. by Nauheimer et al. (2012b).

8. Aroideae Arnott

Habit various, (epiphytes); (plant monopodial); (glucomannans +); laticifers +, articulated, (anastomosing), (0); collenchyma in cortical bands or bundle-associated strands (0); fibres variously associated with bundles; biforine raphides + [H-shaped in T.S., wall thick, except for papillae at the two ends, lignified, cell contents mucilaginous] (0); leaves very variable, (blade developing basipetally from hyperphyll/hypophyll junction), (parallel veins of different orders), (vein endings free), (foliar extrafloral nectaries - Philodendron, etc.); (inflorescences several together), (spadix with sterile zones); P 0; staminate flowers: A connate (not - Philodendron), connectives thick, (anthers introrse), (dehiscing by pores); pollen (bicellular), (mixed with raphides), atectate, (spiny, smooth, striate, etc.), (in tetrads), ectexine thin [pollen not resistant to acetolysis], sporopollenin 0, or +, wall with polysaccharides, endexine bilayed, (thick, spongy), intine massive; pistillode +; carpellate flowers: staminodes +, (G 1?, [2-4(-47)], placentation parietal, apical, basal, (styles connate), (stylar canals as many as carpels - Philodendron), stigma various; ovules 1-many/carpel, (straight), parietal tissue 0/1-3 cells across, ovule base broad, massive [Theriophonium]; (testa not multiplicative - Arisaema); (endosperm chlorophyllous), (with starch), chalazal haustorium +, unicellular, (storage cotyledons), (embryo chlorophyllous); n = 7+, but 13, 14, 17 common; (cotyledon sheath photosynthetic, bifacial [e.g. Colocasia, Philodendron, Xanthosoma], even leafy; collar rhizoids +).

75/2520: Philodendron (500), Homalomena (500), Amorphophallus (220), Xanthosoma (204), Arisaema (170), Alocasia (140), Schismatoglottis (120), Chlorospatha (68), Bucephalandra (70), Cryptocoryne (60). Tropical and warm temperate (the latter - Arum and relatives) (map: from Mayo et al. 1997, distribution attributable to Calla alone in green). [Photo - Flowers.]

Age. Crown-group Aroideae are estimated to be some (92-)82(-73) m.y.o. (Nauheimer et al. 2012b).

Macrofossils apparently of Araceae-Aroideae, although with a loosely reticulate tectum, have been discovered in deposits 120-110 m.y.o. in Portugal (Friis et al. 2010, 2011). Afrocasia (Araceae) from deposits in Egypt ca 73 m.y.a. has been placed in crown-group Aroideae (Coiffard & Mohr (2016).

Synonymy: Arisaraceae Rafinesque, Caladiaceae Salisbury, Callaceae Bartling, Colocasiaceae Vines, Cryptocorynaceae J. Agardh, Philodendraceae Vines, Pistiaceae C. Agardh

• Araceae are herbs with leaves that are usually divided into a petiole and expanded blade. They have a distinctive inflorescence of a more or less petal-like spathe plus a spadix made up of densely-packed, sessile, ebracteate flowers - in many taxa the spadix has a large, terminal, sterile part. The fruit is a berry.

Evolution: Divergence & Distribution. Distinctive pollen assigned to Pothoideae-Monstereae has been found in Early Cretaceous deposits of the late Barremian-early Aptian of some 120-110 m.y.a. in Portugal (Friis et al. 2004; see also Hesse & Zetter 2007). Although the identity of some of these grains has been questioned (Hoffmann & Zetter 2010), macrofossils apparently of Araceae-Aroideae (a decidedly non-basal clade) have been discovered in other deposits of a similar age in Portugal (Friis et al. 2010). From the fossil record, all eight subfamilies of Araceae would have seemed to have diverged in the early Cretaceous, and there are early Albian mesofossils ca 102 m.y.o. from Portugal (Friis et al. 2010, 2011; see also Isles et al. 2015). For fossils, see also Wilde at al. (2005) and Bogner et al. (2007), and also Herrera et al. (2008) for leaf fossils.

Molecular estimates of diversification are rather younger than those based on these Portugese fossils, but even they suggest separation of the subfamilies before the K/P boundary, early evolution in the family possibly occuring in Laurasia (Nauheimer et al. 2012b: table S4, esp. S5 with 140+ dated nodes and further discussion of diversification in the family). If Limnobiophyllum scutatum is stem group Lemnoidae (Iles et al. 2015 think that it definitely goes there), much diversification in Araceae could be Palaeogene since the fossil is only some 66 m.y.o. - but of course the ages of stem groups have little to do with those of crown groups.

For the ages of some intercontinental disjunctions within Lemnoideae, see Les et al. (2003). It has been estimated that Alocasia, centred in Borneo, diversified ca 13.5 or 19.3 m.y.a. (stem group ages are ca 10 m.y. more), and there were many subsequent dispersal events through the whole Southeast Asian/Malesian region (Nauheimer et al. 2012a: m.l. trees with little support).

Araceae are the fifth most speciose family in Amazonian forests (Cardoso et al. 2017).

For the possible base chromosome number of the family, see Cusimano et al. (2012 and references).

Ecology & Physiology. See Croat (1990, 1991) for life forms and ecology in Araceae, while Schuyler (1984) discusses classifications of growth forms of aquatic species - largely unecessary (Cook 1996). The original habitat for the family is likely to have been more or less marshy (Nauheimer et al. 2012b).

Climbers/lianes and epiphytes - not exclusive categories here - are notably common in Pothoidaeae and Monsteroideae, particularly in the former; all told, about 570 species of Araceae are climbers (Gentry 1991, ca 400 species in the New World; see also Benzing 1990), and they are root climbers. Skototropism, the movement of the seedling to dark areas, i.e. where there might be a tree trunk, rather than the reverse, movement from dark to light, is known from Araceae (Strong & Ray 1975). The attachment of Syngonium podophyllum (Aroideae-Caladieae) to its support is largely mediated by root hairs which initially produce a mucilaginous substance which hardens and attaches the root to the surface, later the walls of the root hairs break down into spirals (= helical crack root hairs) that are probably effective energy-dissipating units like tendrils - and as with tendrils, the direction of the spiral may even reverse (Yang & Deng 2016). Climbing Araceae are often strongly heteroblastic, the leaves of a plant in the climbing phase being notably smaller and sometimes simpler than when it is reproductive. Hemiepiphytic Araceae quite commonly have positive root pressures, and this is also a feature of climbers, perhaps reducing the dangers of irreversible cavitation (Fisher et al. 1997), however, embolism formation and refilling may controlled by the activity of living cells around the vessels (Knipfer et al. 2016) and/or lipid surfactants in the xylem that i.a. coat nanobubbles, so preventing the formation of full-fledged embolisms (Schenk et al. 2017). For epiphytes in Araceae, most of which are in Anthurium (ca 1/4 of its species), see Zotz (2013).

In Malesia in particular Aroideae-Schismatoglottideae are common rheophytes, the rheophytic habit having evolved several times; some rheophytic species are restricted to a single stream (Wong 2013). Wong (2013) discussed features that might be adaptations to the rheophytic habit, including the ability of the shoot to break off from the root system when the water flow becomes too strong; roots develop both from the rootstock and from the shoot, wherever it ends up downstream. Rheophytic taxa also have a distinctive ligule and seeds (see below).

Many Araceae are plants of shaded conditions, and net-veined leaves and fleshy fruits are associated with this habitat (Givnish et al. 2005).

Lemnoideae are widespread in aquatic ecosystems, and Spirodela in particular efficently takes up nitrogen, including in the form of ammonia, from polluted systems - aside from having 15-45% protein dry weight, comparable with soybeans and alfafa, it can produce about 50% dry weight of starch, depending on the conditions (Cheng & Stomp 2005). Pistia (Aroideae) can also be dominant in lakes, etc., where it is an important aquatic weed.

Pollination Biology & Seed Dispersal.

Pollination Biology.

Gibernau (2003, 2011, 2016) and Gottsberger (2016a) summarize information on pollinators. More or less unpleasant (to us) smells are common in araceous inflorescences, as is evident from common names like skunk cabbage (Symplocarpus foetidus) and dead horse arum (Helicodiceros [Dracunculus] muscivorus). Pollination is often by a variety of flies and beetles - Gottsberger (2016a) estimated that together they might pollinate ca 1,500 species - and bees may also be pollinators. Beetle pollination may be the plesiomorphic condition in the family (Sannier et al. 2009; c.f. Bröderbauer et al. 2012; Chartier et al. 2014a) - and there are oviposition/pollination mutualisms. Schiestl and Dötterl (2012) argue that the ability to detect particular volatile organic compounds in aroid-pollinating scarab beetles developed in the Jurassic, while the plants they pollinate evolved in the Cretaceous/Palaeocene; an example of the quite common disconnect between the evolution of the pollinator and that of the plant. Interestingly, 4-methy-5-vinylthiazole plays an important role in attracting cyclocephaline scarabs both in Caladium and in Annona (Maia et al. 2012; also M. R. Moore & Jameson 2013 for cyclocephaline pollinators). The beetles seem to have no particular preferences for compounds unique to the plant (Schiestl & Dötterl 2012). For a general treatment of chemical mimicry of oviposition sites of different kinds, including carrion, see also Jürgens et al. (2013), and for osmophores, see Gonçalves-Sousa et al. (2017).

Thermogenesis has been detected in the inflorescences of some Araceae (e.g. Meeuse & Raskin 1988; Gottsberger 2016a; Milet-Pinheiro et al. 2017). It is caused both by proteins that uncouple the components of the glycolytic pathway and by the mediation of an alternative oxidase, the net result being that glycolysis results in heat, not energy in the form of adenosine triphosphate, and rates of respiration can be very high - the spadix may become warm at particular times of the day, photoperiodicity being involved, too, and the trigger may be in the staminate flowers (Meeuse & Raskin 1988Gibernau et al. 2005; Watling et al. 2006; Onda et al. 2008; Barthlott et al. 2008 and references; Chouteau et al. 2009; Seymour 2010). The heat may volatilize compounds that attact pollinators, and/or provide a warm roost for them inside the spathe, and temperatures reach over 30⁰ above ambient in Xanthosoma robustum when the plant is kept at 10^o (Meeuse & Raskin 1988).

Some species of neotropical Araceae, including members of both Anthurium and Spathiphyllum, are pollinated by euglossine bees (orchid bees) which show fair visitor specificity despite the apparently unspecialised flowers - the scent bouquets of the attractants are different (N. Williams & Dressler 1976; Schwerdtfeger et al. 2002; Roubik & Hanson 2004; Hentrich et al. 2010b; Schiestl 2012). A nectar-like but sometimes foul-tasting exudate may be produced by stigmatic hairs, etc., as in Anthurium (Daumann 1931; see also Fahn 1979), and this attracts pollinators. Schwerdtfeger et al. (2002; see also Croat 1980 for pollination here) found that rather limited data showed that euglossine bees were common pollinators of Anthurium, although both cecidomyid midges and drosophilid flies were also visitors. Duckweeds produce sucrose-containing drops of liquid at the stigmatic apex, and pollination is probably by small flies (Landolt 1986).

The spathe of Aroideae is usually differentiated into tubular and blade-like portions. Sterile female (?sometimes male - see Low et al. 2016) flowers may be at the very bottom, then female flowers, then sterile male flowers, then male flowers, and then there is a sometimes much elongated sterile portion. The fertile flowers may be more or less enclosed by the sometimes inflated tubular portion of the spathe, mostly obviously in Aroideae - indeed, there are a variety of traps, and they have evolved at least ten times (Bröderbauer et al. 2012; see also Delpino 1873). Pollinators of Aroideae, mostly flies and beetles, attracted by the color of the blade of the spathe, or the smell, or even the dangling apical portion of the spadix (flies: e.g. some Arisaema) may be temporarily trapped inside the tubular basal portion of the spathe by hairs, radiating sterile flowers, etc., but they are usually released when the staminate flowers open and they get covered with pollen as they leave (see Bröderbauer et al. 2012). In most situations the pollinators are not rewarded and while they are trapped they also deposit pollen (Chartier et al. 2014a). There are over 250 species of Schismatoglottidae (Aroideae), most Bornean, and Low et al. (2016 and references) describe the diversity of their pollination mechanisms, where complex movements of the spathe, or its splitting in various ways, sometimes very irregularly, even its abscission (e.g. Boyce & Wong 2007) is part of the whole process; there the drosophilid Colocasiomyia is one of the pollinators. The volatiles produced by Arum attract insects in search of brood sites, thus the smell of A. palaestinum is that of rotting fruit, and it attracts Drosophila flies (Linz et al. 2010; Urru et al. 2011; Nauheimer et al. 2012a: fruit flies and Alocasia; Bröderbauer et al. 2014: fruit flies and Colocasia). Punekar and Kumaran (2010) described the pollination of Indian species of Amorphophallus (see also Claudel et al. 2017 for pollination: inc. mimicry of mammalian dung); the pollen wall is sometimes shed before the pollen tube develops (Ulrich et al. 2017). There is a variety of odours in the genus, including dimethyl sulphides that are "gaseous" or smell like rotting meat, phenylethanol derivatives that smell of fruit or anise, trimethylamine that smells like fish, and isocaproic acid that smells (I think) like cheese or goats; there was some correlation both with phylogeny and with ecology/geography - thus some dours seemed to have evolved only once, others several times, Amorphophallus species that smelled like fish were not all immediately related but were found only in northern Borneo, and sister species that can be sympatric smell of dung and corpses respectively... (Kite & Hetterscheid 2017).

Some Aroideae are pollinated by cyclocephaline dynastine scarab beetles that use the inflorescences as mating sites (Chartier et al. 2014a; Gibernau 2015; Milet-Pinheiro et al. 2017). Caladium is a member of a clade characterised by being pollinated by these beetles (Mayo & Bogner 1988; Maia & Schlindwein 2006). Each species of Philodendron attracts usually but a single species of scarab (see also Maia et al. 2010), and Gottsberger (2016a: Fig. 2) shows a marvellous scrum of Erioscelis emarginata beetles at the bottom of an inflorescence of P. selloum, and inflorescences of Philodendron are highly thermogenic. Thus temperatures reach 20⁰ above ambient in some species of Xanthosoma, another large genus that is thought to be predominantly pollinated by these beetles (Milet-Pinheiro et al. 2017 and references). The scent of some species has been analysed, and although it was suggested that the distinctiveness of a scent was the result of the predominance of "unique dominant compounds", whether or nor visitors were unique to a species and/or were elements in barriers separating sympatric species is currently unknown (Milet-Pinheiro et al. 2017). The arborescent South American Montrichardia has "explosive" pollen; the massively thick intine swells to an elongated structure ca 400 µm long within a few seconds, perhaps aiding its attachment to the visiting hairless beetle (Weber & Halbritter 2007).

Arisaema is another member of Aroideae in which pollination has been studied in some detail. This genus is serially monoecious, i.e. the one plant can change "sex" from year to year, although some tetraploids seem to have reverted to the normal monoecious condition for Aroideae (Renner et al. 2004). Dipteran fungus gnats are the pollinators, and they can escape from any male inflorescence they visit via a hole at the base of the spathe. Visiting female inflorescences is another story. Covered in pollen, they slide down the slippery slope inside the spathe and pollinate the female flowers - but there is no basal exit hole, they cannot climb out of the inflorescence, and so they die (Vogel & Martens 2000). Gnats are attracted by the smell produced by osmophores which are usually at the tip of the apadix, the spadix itself - up to 80 cm, some reports suggest 1.5 m long - dangling from the mouth of the spathe in some species, the insects climbing up the spadix from the ground (Vogel & Martens 2000).

For a discussion on the evolution of the distinctive pollen that characterises most Aroideae, see Hesse (2006b). In a number of members of this subfamily the pollen is extruded from the anthers in toothpaste-like threads, and the anthers of several genera open by pores. A discriminant analysis of thirteen putatively pollinator-related characters, but not pollen surface, identified bee- and also less sharply differentiated beetle- and fly-pollinated morphologies (Gibernau et al. 2010; Gibernau 2003 for references), although how such analyses would fare if pollinator groups were not assumed is unclear. Grayum (1986) had earlier looked at features of the pollen surface and how they might correlate with pollinators; smooth and sometimes rather larger pollen and beetle pollination were linked, as were spiny pollen and fly pollination, and so on. Sannier et al. (2009: but see their caveats, Lemnoideae, Gymnostachys not included, only 2 Orontioideae, etc.) looked at pollinator and pollen across the whole family, and also suggested there was some correlation between pollen morphology and pollinator.

Raphides, prismatic calcium oxalate crystals, and other crystal types from the walls of the anther may become mixed with the pollen (Barabé et al. 2004b; Barabé & Lacroix 2008b, Coté 2010; Coté & Gibernau 2012; Low et al. 2016); their exact function is unclear. In Monsteroideae there are numerous trichosclereids in the stylar tissue and the spathe is deciduous; the trichosclereids may protect the exposed ovary against insects (there can be a variety of crystalline forms in different cells and tissues of the one plant - see Coté 2009). A variety of crystals is also found in the ovary (Coté & Gibernau 2012).

Seed Dispersal.

Understory Araceae in the Neotropics are a particularly important source of food for bats (Lobova et al. 2009). Monstereae are frequently epiphytic and often have seeds embedded in pulp, whether from the testa, trichomes, or the inner pericarp, perhaps to help them stick to branches (Mayo et al. 1998); in South America birds like the mistletoe specialist friar birds (Euphoniinae, near Fringillidae) may disperse the seeds of epiphytic taxa like Anthurium (Snow 1981; Restrepo 1987; Reid 1991). In some Araceae the basal part of the spathe remains fleshy and encloses the fruits; when the latter are mature it splits irregularly, exposing them to the dispersing animals (see also Boyce & Wong 2007 for the mechanics of spathe senescence in Schismatoglottideae; Uhl et al. 2013). A number of small Malesian Aroideae-Schismatoglottideae growing on the forest floor or on rocks have splash-cup dispersal mechanisms, the shape of the persistent spathe directing the dispersal of the seeds/fruits, or the fruits may be corky and water-dispersed, or the seeds of rheophytic taxa may have distinctive micropylar appendages that act as grapnels for anchoring the seeds as they germinate (Wong 2013; Boyce & Yeng 2015 for references). The aquatic Cryptocoryne is the only Araceae to have dehiscent fruits; the seeds float and are dispersed by water (Bown 2000).

Plant-Animal Interactions. Araceae are not much liked as food by butterfly caterpillars (Ehrlich & Raven 1964). A number of species of galerucine beetles (Aplosonyx) have been found feeding on laticiferous Aroideae from South East Asia where they make circular trenches in the leaves to interrupt the latex flow and then eat out the portion of the leaf so isolated - the resultant holes in the blade look as if they were made by a paper punch (Darling 2007); galerucines are known from other monocots and beetle herbivory in Araceae may be geographically more widespread.

In the New World in particular species of Anthurium and Philodendron may be inhabitants of ant gardens (Orivel & Leroy 2011).

Vegetative Variation. Although many Araceae appear to be monopodial, the stem is usually a complex sympodium built up of repeating units each made up of expanded and reduced leaves and a terminal inflorescence (Engler 1877, translated by Ray & Renner 1990; Ray 1987, 1988). These stems often have conspicuous circular scars enclosing another scar; this represents a leaf scar surrounding the stem apex/inflorescence, the termination of a unit of the sympodium. Growth continues by the development of an axillary bud; axillary buds may be superposed, and the upper (developing) bud may be displaced some way from the axil of the leaf that subtends it.

Leaves and their development vary considerably in Araceae. A number of taxa, including Monstera, the swiss cheese plant, and relatives are fenestrate (see Melville and Wrigley 1969) because of localised cell death (Kaplan 1984; Gunawardena & Dengler 2006), and in Anchomanes localised cell death results in what is an initially simple leaf blade with entire margins separating into a complex structure with numerous "leaflets". The leaves of Zamioculcas appear to be truly compound, with the blastozone, the marginal leaf meristem, becoming discontinuous and producing the individual leaflets (Kaplan 1984; Gunawardena & Dengler 2006). Aroid leaves can be huge, in Dracontium gigas and Amorphophallus titanum the dissected foliar part, which can reach up to 4 m in diameter, is born on a massive erect petiole up to ca 5 m tall (Bown 2000). The leaves of Anthurium are notably variable, being entire to deeply lobed or compound. Pulvini occur along the petiole of taxa like Dracontium.

The leaves of Scindapsus develop in a "typical" monocot fashion, i.e. from the leaf base (Troll & Meyer 1955; Bharathan 1996; Doyle 1998b), while in taxa like Arisaema, Orontium, and Zamioculcas the blade develops from the upper part of the leaf primordium, i.e., they are similar in this to broad-leaved angiosperms (Troll 1955; Periasamy & Muruganathan 1986: Arisaema; Bharathan 1996). Kaplan (1973) thought that the blade in Zantedeschia aethiopica developed from the lower part of the leaf, although he also noted that it developed acropetally, and in this was like other Araceae (unspecified) and broad-leaved angiosperms. Ertl (1932) found that leaves with more normal monocot venation may be common in taxa now placed in the basal pectinations of Araceae, but blades with highly reticulate venation are common in the family (Mayo et al. 1997; Coiffard & Mohr 2016).

The highly reduced vegetative body of Lemnoideae is variously interpreted as being some combination of leaf and shoot. Wolffia and Wolffiella lack both roots and veins in the thallus, although W. microscopica has a root-like structure of uncertain function. The thallus of Wolffia may be less than 1 mm across; it is the smallest flowering plant known (see Lemon & Posluszny 2000b; Sree et al. 2015). The reproductive parts have been interpreted as either a reduced but perfect flower or a very highly reduced inflorescence; given the phylogenetic position of Lemnoideae, the former is perhaps more likely. W. Wang et al. (2014), in a genome analysis of Spirodela, found that genes promoting the juvenile phase of growth and inhibiting adult features like flowering were preferentially enhanced, suggesting that the adult body is paedomorphic, retaining juvenile traits, whether by progenesis, speeding up sexual development, or neoteny, slowing down vegetative development, is unclear, although perhaps the former is the better fit. The aquatic Pistia has a much less unconventional plant body, and its inflorescence, although reduced, is basically similar to that of other Aroideae; however, its vegetative shoots are monopodial (Lemon & Posluszny 2000a). Both Lemnoideae and Pistia have supernumerary axillary buds which increases the complexity of their branching patterns, however, the two are not immediately related (c.f. Lemon & Posluszny 2000b). Both Lemnoideae and the Pistia clade are quite old, even the latter being perhaps 90-76 m.y.o. (Renner & Zhang 2004), although that is older than some estimated for Lemnoideae (see above).

Although the early Caenozoic fossil Limnobiophyllum seems "intermediate" between Lemnoideae and Pistia (Stockey et al. 1997), those two groups are not at all close in molecular phylogenies - indeed, Limnobiophyllum is to be assigned to stem Lemnoideae (Iles et al. 2015) while Pistia itself is Aroideae. The palynomorph Pandaniidites is associated with flowers of Limnobiophyllum (Stockey et al. 1997; Stockey 2006). There are yet other unrelated fossil floating aquatics in the family (Stockey et al. 2007). The recently-described Aquaephyllum auriculatum from rocks ca 67 m.y.o. of the Crato formation in Argentina is sister to Lemnoideae in morphological phylogenetic analyses, Pistia and some other fossil aquatic Araceae are in turn sister to that clade, and the whole lot are embedded in Aroideae (Gallego et al. 2014; some analyses in Stockey et al. 2016b). Aquaephyllum has a peltate petiolate lamina with a crenate margin and veins proceeding to the margin, and it is difficult to know what to make of it, and it is placed apart from all other aquatic Araceae in some analyses in Stockey et al. (2016b); interestingly, Pandaniidites is known from these Crato sediments. In general, all the aquatic taxa tend to group together in morphological analyses, parallel evolution of adaptations ro the aquatic habitat overwhelming signals of other relationships (Stockey et al. 2016b).

Genes & Genomes. For chromosome number change, descending dysploidy being common, see Sousa and Renner (2015). Even species with chromosome numbers as high as 2n = 60 showed no evidence of polyploidy (Sousa & Renner 2015), which is remarkable. For chromosome numbers, see also Bogner and Petersen (2007).

DNA substitution rates are particularly high in the free-floating Araceae, Lemnoideae and Pistia (Nauheimer et al. 2012b). For the highly autapomorphic chloroplast genome of Zantedeschia and to a lesser extent that of Anchomanes (both Aroideae, close to each other?), see Henriquez et al. (2014); the organization of the chloroplast genome in Lemna is also distinctive (Mardanov et al. 2008). The mitochondrial genome of Spirodela has been substantially rearranged, and it shows no synteny with other mitochondrial genomes (W. Wang et al. 2012); analyses of mitochondrial sequences give rather different relationships than do those of chloroplast sequences (Henriquez et al. 2014).

For the nuclear genome of Spirodela, quite small, see W. Wang et al. (2014); despite its size, it has a very large number of microsatellite tandem repeats, ca 1 Mb in total. There is evidence of two whole genome duplications in S. polyrhiza, and these have been dated to around 95 m.y.a. (Wang et al. 2014), suggesting that these duplications might be unique to Lemnoideae (see ages above). Bliss and Suzuki (2012) found substantial variation in genome size in Anthurium, but there was little correlation with anything.

Chemistry, Morphology, etc. Raphides in those taxa that have been studied are twinned calcium oxalate crystals, H-shaped in transverse section, and often with lateral barbs (Sakai & Hanson 1974; Cody & Horner 1983); Lemna also has such raphides, but I do not know what their general distribution is. Raphides develop earlier than druses, at least in Amorphophallus, and may help protect young tissue, as well as helping to regulate calcium (Prychid et al. 2008).

Xylem and phloem are mixed in the medulla of roots of Monstera, Heteropsis and Philodendron (Huggett & Tomlinson 2010) at least: I do not know the broader distribution of this distinctive feature. Gonçalves et al. (2004) noted that some taxa with perfect flowers may have collenchyma at the apex or base of the petiole; their comparative data is of collenchyma presence at the middle of the petiole. This may perhaps explain the apparent conflict in the literature. Thus although Keating (2000b) recorded collenchyma for a few members of Lasioideae and Pothoideae, Gonçalves et al. (2004) failed to find it for some of the same taxa.

Ray (1987b) questioned whether all taxa have axillary buds; these are sometimes much displaced from the leaves with which they are normally associated. In addition to Gymnostachys, I have seen one taxon (unnamed, from Thailand) with a leaf blade that had softly dentate/spinulate margins. The leaves of Scindapsus, but not Arisaema, Orontium, Typhonodorum and Zamioculcas, and even Acorus (Acoraceae) itself, may develop in a "typical" monocot fashion (Troll 1955; Troll & Meyer 1955; Bharathan 1996; Doyle 1998b). Kaplan (1973) thought that the blade of the leaf of Zantedeschia developed from the lower part of the leaf, but noted that its leaf, and that of other (unnamed) Araceae he had examined, developed acropetally like the lamina of most broad-leaved angiosperms...

Buzgo (2001) suggested that Orontium was more like core Araceae in floral development than was Lysichiton or Symplocarpus. Orontieae have a long internode (not always obvious) between the base of the spike and the subtending leaf or spathe and there may be common A-C primordia (Buzgo 2001). The sterile flowers that are often found between the staminate and carpellate zones of the inflorescences of many Aroideae develop in a variety of ways, but whether this implies that there are correspondingly different evolutionary pathways is unclear (c.f. Barabé et al. 2004a). When flowers are 2-merous, the outer pair of tepals are lateral. Some taxa have binucleate tapetal cells (Wunderlich 1954), but they are usually uninucleate. For a discussion on the evolution of the distinctive pollen that characterises most Aroideae, see Hesse (2006b). Pseudomonomery has been documented for the family (Eckardt 1937; see Buzgo 2001). See Buzgo (2001) for discussion on the gyneocial construction of [Gymnostachydoideae + Orontioideae]; the gynoecium often has a single loculus.

Variation in ovule morphology is considerable but confusing. Gymnostachys has no micropyle since the integuments do not cover the nucellus, Pistia is exostomal, and other taxa are bistomal. The ovules of Pothos macrophyllus are shown by Buzgo (2001) to be anatropous and apotropous, although Pothos is described in the same paper as having straight ovules (the former is correct). In a number of taxa the ovules are reported to be tenuinucellate while the nucellus of Pistia is "very well developed" and appears to be in radial files (Mercado-Noriel & Mercado 1978), and both a nucellar cap and integumentary endothelium may be present, and so on. Parameswaran described Theriophonum minutum as being tenuinucellate, but drew a complete layer of cells below the nucellar epidermis, while Jüssen (1929) noted that Spathiphyllum had a doubled epidermal layer, but that is not evident from the illustrations; in general it is difficult to match statements of nucellus type with the illustrations there. Taxa in which the megaspore that germinates is micropylar are scatteed in the family, especially in the "basal" clades (Grayum 1991).

The uninucleate chalazal endosperm haustorium of Arum maculatum is reported to be 24,576 n (Werker 1997). Maheshwari and Khanna (1957) and Tobe and Kadokawa (2010) describe the endosperm as being cellular, but it could be interpreted as being an extreme form of helobial (see also Acorales). The ovules of Pothoidaeae and Monsteroideae are described as frequently being ana-campylotropous (Seubert 1997). According to Mercado-Noriel and Mercado (1978) the seeds of Pistia have large amounts of perisperm as well as some endosperm. Variation in seedling morphology is great; in some taxa the roots are green, and in others they are always white (Tillich 1985, 2003b). The cataphylls of the seedlings of Orontium are relatively long, linear structures (Tillich 2003b). Chromosome number is especially variable in Cryptocoryne (Aroideae).

Much information is taken from Grayum (1990), Mayo et al. (1997, 1998), Bown (2000), Boyce and Yeng (2015: Malesian genera), Buzgo and Endress (1999: Gymnostachys), Hetterscheid and Ittenbach (1996: Amorphophallus) and Croat et al. (2017 and references: Xanthosoma. See also Dring et al. (1995: chemistry), Behnke (1995a: sieve tube plastids), French (1998: stem anatomy, extremely variable), Keating (2000b: collenchyma, 2003a: general anatomy, b: leaf anatomy, 2004a: classification, b: raphides), Gonçalves et al. (2004: collenchyma), Carlquist and Schneider (2013: vessels), Gow (1913), Maheshwari and Khanna (1956), Swamy and Krishnamurthy (1971), and Tobe and Kadokawa (2008: good summary, 2010: endosperm development), all embryology, Gatin (1921: seedlings, unfortunately Gatin died before he could make more than this "première contribution"), Tillich (1985, 2003b, 2014: seedlings), Seubert (1993: starch grains, seeds and seedlings, most of family; 1997: Lasioideae). For floral morphology, see Mayo (1989: Philodendron), Barabé and Lacroix (2008a) and Poli et al. (2012, 2015), all Anthurium, Barabé and Lacroix (2008b: Anaphyllopsis), Barabé et al. (2012: Syngonium), Fukai (2004: Arisaema) and Barabé (2011: floral meristicity of Lasioideae, 2013: general, esp. Lasioideae), for ovary loculus hairs, see French (1987), for floral anatomy, see Eyde et al. (1967). For pollen, see Grayum (1991, 1992: much variation), Weber et al. (1999), Hesse (2002: Lasioideae), Jayalakshmi (2004: phylogenetic framework inadequate), van der Ham et al. (2005: Amorphophallus and relatives), Hesse (2006a, b: summary, phylogenetic framework reasonable), Ulrich et al. (2012: Calla, 2017: Amorphophallus), and French (1986 and references: endothecial thickenings). See Smith and Stockey (2013) for seeds of Lasioideae.

For general information on Lemnoideae, see Plant Biol. 17. (2015), a special issue, for a monograph, see Landolt (1980, 1986) and Landolt and Kandeler (1987) and for general morphology, see Landolt (1998), for embryology, Maheshwari (1954), cytology, Urbanska-Worytkiewicz (1980), for chemistry, etc., see Landolt and Kandeler (1987).

Phylogeny. Mayo et al. (2013) summarize phylogenetic work on the family. Early hypotheses of phylogeny based on restriction site analysis (French et al. 1995) suggested rather pectinate relationships in the family, but a consensus tree of morphological characters (Mayo et al. 1997) showed somewhat less resolution. However, relationships have turned out to be pectinate, although of course that is in part because of what we have decided to call subfamilies. In most analyses the clade [Gymostachydoideae + Orontioideae] is sister to the rest of the family, and Lemnoideae are strongly supported as sister to the remainder, although Isles et al (2013) recovered a topology [Gymostachydoideae [Orontioideae + the rest]], although support was weak and sampling poor. Barabé et al. (2004a: little support) found that Lasioideae were not clearly separated from Aroideae. Although a trnL-trnF phylogeny (Rothwell et al. 2004) placed Callopsis and Asterostigma (both Aroideae) outside a clade with 100% jackknife support that included other Aroideae, Lemnoideae and Pothoideae, Tam et al. (2004: trnL-F sequences, Calla not examined) again suggest the phylogeny is rather pectinate, as do Cabrera et al. (2008: five chloroplast genes). The topology that Cabrera et al. (2008) present, quite well supported, is used here (see also Nauheimer et al. 2012b; Henriquez et al. 2014: chloroplast, but not mitochondrial, sequences).

In Orontioideae, Orontium is sister to the rest of the subfamily (e.g. Nauheimer et al. 2012b). In Lemnoideae, relationships are [Spirodela [[Landoltia + Lemna] [Wolfia + Wolfiella]]] (Ding et al. 2017). Tam et al. (2004) discuss relationships within Pothoidaeae and Monsteroideae; in the latter, Spathiphyllum may be sister to the rest of the subfamily (Henriquez et al. 2014). Carlsen and Croat (2013) have begun to disentangle relationships in Anthurium, the classical sections there are a poor guide to relationships. For the phylogeny of Lemnoideae see Les et al. (2002), Rothwell et al. (2004), Nauheimer et al. (2012b) and Tippery et al. (2015); relationships are [Spirodela [Lemna + The Rest]]. For speciation in Lemnoideae, see Crawford et al. (2006). In Lasioideae, Urospatha is sister to the rest (Nauheimer et al. 2102b).

Cabrera et al. (2008) offer a number of suggestions about tribal relationships in Aroideae; for Chinese taxa, see Z.-D. Chen et al. (2016). Nauheimer et al. (2012b) found little support for relationships between many of the tribes; Callopsis was sister to the rest of the subfamily, but support was weak. For a phylogeny of Philodendron, see Gauthier et al. (2008); Homalomena may be part of the same clade. Yeng et al. (2013) focus on Southeast Asian Homalomena. There H. cochinchinense is sister to all other species; the New World species form a quite separate clade sister to a clade of Philodendron, within which they are embedded. Cusimano et al. (2010) and Ohi-Toma et al. (2010) discuss relationships within Areae, particularly Typhonium and related genera, while Espíndola et al. (2010) focus on Arum. Gonçalves et al. (2007) discuss the phylogeny of the Andean Spathicarpeae, a clade in which the spathe is adnate to the spadix; many of the species grow in very dry and/or high conditions, rather atypical for the family. Renner et al. (2004: support varied from slight to strong depoending on the analysis) found that Arisaema tortuosa was sister to the rest of the genus, a position not recovered by Ohi-Toma et al. (2016) who found relationships along the backbone of Arisaema for the most part poorly supported, as in Renner et al. (2004). For relationships in the speciose Schismatoglottidae, see Wong et al. (2010) and Wong (2013); Alocasia, see Nauheimer et al. (2012a); and for those in Amorphophallus, Sedayu et al. (2010) and in particular Claudel et al. (2017).

There are two areas of particular phylogenetic interest.

1. The exact position of Zamioculcadoideae needs confirmation, but they can reasonably be excluded from Aroideae. Although there is no strong molecular support for a clade [Zamioculcadoideae + Aroideae] in some analyses (e.g. Nauheimer et al. 2012b), it was well supported in the plastid analysis of Henriquez et al. (2014); this clade has several morphological features in common (see above). Bogner and Hesse (2005) raised the group [Zamioculcas + Gonatopus] to subfamilial status as Zamioculcadoideae. Stylochaeton has the same pollen as most Aroideae and simple leaves (Hesse et al. 2001). It is sister to other Zamioculcadoideae (see also Nauheimer et al. 2012b; c.f. some analyses in Chartier et al. 2014a), so its inclusion in that subfamily (see Cabrera et al. 2008) makes it less distinctive morphologically. The phylogeny of Cusimano et al. (2011) is largely similar to that of Cabrera et al. (2008); the former recognise a Zamioculcadoideae s. str.. It may be of interest that the pollen of Lasioideae, at least, has a lamellate endexine rather like that of Zamioculcadoideae.

2. Calla palustris, in early (prior to ed. 7) versions of this site placed in a separate subfamily, now seems best included in Aroideae. Barabé et al. (2004a) found that it was embedded in Aroideae, although without strong support (see also Nauheimer et al. 2012b: well embedded, but again with poor support). Calla came out in a clade of Aroideae along with other rooted aquatics/marsh plants in molecular analyses (see Cabrera et al. 2008), but its position was unclear in both morphological and restriction site analyses. Chartier et al. (2014a) found a clade [Anubias [Calla + Montrichardia]] sister to other Aroideae, but support was low. Calla and Schismatoglottis ("Calla and the rheophytes") form a clade, but with weak support; that clade was sister to one of the two major clades formed by members of Aroideae and there are morphological features found in members of that clade (Henriquez et al. 2014). Just looking at Chinese taxa, Calla was in a separate clade off the backbone of the tree (Z.-D. Chen et al. 2016).

What does morphology have to say about all this? Calla and Aroideae both have laticifers, but the former has bisulcate pollen with sporopollenin and a tectate-columellate ektexine, perfect flowers, etc. (Ulrich et al. 2013) - and it has a much more northerly distribution (see the Map above), overlapping with that of other Aroideae only in Western Europe and N.E. North America. The ovules have parietal tissue, but so do those of some other Aroideae (Ariopsis, Arum). Its perfect flowers and many pollen features would represent reversals, its acropetal flowering is unique (c.f. Cusimano et al. 2011; Ulrich et al. 2013; Barabé 2013: floral morphology), although some other Aroideae do have perfect flowers, albeit atypical. Pollen data suggested to Ulrich et al. (2013) similarities with Stylochaeton (Zamioculcadoideae) and Aroideae on the one hand and Lasioideae on the other. The phylogenetic position of Calla needs confirmation in order to understand the evolution of its morphology, but its inclusion in Aroideae seems reasonable.

Classification. See Mayo and Bogner ( 2013) for Adolf Engler's classification of the family, which lasted for about a hundred years. For a tribal classification, see Cabrera et al. (2008) and especially Cusimano et al. (2011), however, a few genera immediately basal to their Aroideae s. str. and Zamioculcadoideae s. str. are unplaced, and a broad circumscription of both subfamilies is adopted here (see also Mayo et al. 2013 for the classification of the family). For a checklist and bibliography, see Govaerts and Frodin (2002) and the World Checklist of Monocots, and for several keys and much more, see CATE-Araceae. There are lots of monotypic genera!

Ohi-Toma et al. (2016) provide a sectional classification for Arisaema and Claudel et al. (2017) a subgeneric classification for Amorphophallus.

Thanks. I am grateful to Monica Carlsen and Richard Keating for discussions about Araceae and to Simon Mayo for comments.

[[Alismataceae [Butomaceae + Hydrocharitaceae]] [Aponogetonaceae [Scheuchzeriaceae [Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]]]]]: mycorrhizae uncommon; plant rooted aquatic (with floating stems), leaves often emergent; roots often not medullated [pith slight to none], rhizodermal cells dimorphic, root hairs from short cells; (protoxylem lacunae +), stem with lacunae; little oxalate accumulation; raphides and druses 0 (prismatic crystals +); bulliform cells 0 [?this level]; plant glabrous; leaves with intravaginal squamules [squamulae intravaginales, ?= colleters]; inflorescence scapose; pollen grains tricellular; carpel fusion via the central floral axis, partial at the carpel periphery; endosperm 0; seedling collar and collar rhizoids +.

Age. The divergence of the two main clades above is dated at 91-81 m.y.a. by Wikström et al. (2001: note topology), ca 107 m.y.a. (Janssen & Bremer 2004), or a little younger, at (115-)96, 83(-66) m.y. (Bell et al. 2010: note topology) and ca 96.6 m.y.a. (Magallón et al. 2015).

Fossils of Thalassocharis bosquetii ca 72 m.y.o. from the early Maastrichtian of western Europe have been identified as those of a seagrass, although to what clade they should be assigned is unclear - they seem to completely lack intravaginal squamules. Their stem anatomy is rather complex with a well-developed fibrous layer in which the vascular bundles are embedded while the bundles going to the leaves are constricted just before they depart the stem (van der Ham et al. 2017).

Evolution: Divergence & Distribution. L.-Y. Chen et al. (2013: disregard ages in Table 1) thought that the most recent common ancestor of this clade inhabited Eurasia.

For carpel evolution, see J.-M. Chen et al. (2004a); free carpels may have evolved twice, there have been reversions to syncarpy, and the unicarpellate condition also shows a complex pattern of evolution. For the evolution of aquatic growth forms, see J.-M. Chen et al. (2004b).

Ecology & Physiology. All members of this clade grow in more or less marshy or aquatic and sometimes saline conditions, and are common in all aquatic environments in which angiosperms grow except fast-flowing rivers. J.-M. Chen et al. (2004b) discuss the evolution of various life forms in the group - parallelisms are common.

More particularly, the clade is notable for the number of taxa it contains that can tolerate salt concentrations of 200mM (Flowers et al. 2010), and it includes all fully marine angiosperms. These are generally called sea-grasses, although not all are grass-like and none is at all close to grasses. True grasses like Spartina and Puccinellia, which can dominate in estuaries, are not usually included in this ecological group (for the evolution of salt tolerance in Poaceae, see Bennett et al. 2012). The extreme halophytic habit has evolved more than once here, probably two or three times - once in Hydrocharitaceae and again in the [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]] clade. Hartog and Kuo (2006) estimate that there are some 48 species of extreme halophytes in the [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]] clade and another 18 species in Hydrocharitaceae. There are reversals in the former group if extreme halophily there evolved only once (Les et al. 1997b; Les & Tippery 2013), some species of Ruppiaceae, Zosteraceae and Potamogetonaceae in particular tolerating a range of salinities (Barbour 1970; Hartog & Kuo 2006), furthermore, Maundiaceae and many Juncaginaceae happily grow in salt marshes. Like mangroves, sea grasses are most diverse in the area from the western Pacific to east Africa, less so in the Americas and west Africa (e.g. Tomlinson 1986).

Touchette (2007; see also Marbà et al. 2006; Mateo et al 2006, and other papers in Larkum et al. 2006) discuss the physiological problems faced by marine angiosperms, particularly problems with Na, Cl, and P concentrations, Romero et al. (2006) noting that sea-grasses were adapted to nutrient poor conditions. Invers et al. (1999) examined carbon acquisition via dissolved bicarbonates. For the effect of the oxygen concentration of the water on submerged angiosperms in general, see Caraco et al. (2006). Sulphated phenolic compounds and flavones are common in sea-grasses (McMillan et al. 1980), and probably arose in parallel, although their function is unclear, but is perhaps connected with the problems of living in saline environments (McMillan et al. 1980 and references); for sulphates/sulphites, see Marbà et al. (2006), and for sulphated polysaccharides in Zosteraceae, involved in osmotic balance, see Olsen et al. (2016). Wissler et al. (2011) compare gene expression in Posidonia and Zostera with that in grasses and broad-leaved angiosperms; the approach is interesting, but the sampling is too poor to say much about specific adaptations to the marine habitat. In Zostera, at least, adaptation to the marine habitat has involved less the acquisition of novel genes than the loss of a number of genes such as those involved in hormone response and in photosynthesis (H. T. Lee et al. 2016); for further details, see .

The ecological importance of sea-grasses is very great (see e.g. Marbà et al. 2006; Mateo et al. 2006). The sea-grass ecosystem is very productive, supports a considerable amount of diversity, captures much sediment, and stores much carbon (e.g. Marbà et al. 2006; Orth et al. 2006; Kennedy et al. 2010 for summaries; Marbà et al. 2015). Sea-grass stands are now usually little grazed (e.g. Smith 1981; Waycott et al. 2009; Arnaud-Haond et al. 2012), and Valentine and Duffy (2006: p. 465) note that "one of the few existing paradigms of marine ecology is that little of the production of seagrasses is grazed by marine herbivores" - a paradigm that they question. Indeed, grazing of sea-grasses by large vertebrates used to be moderate to intense, but sirenians became much less diverse in the later Pliocene and humans may more recently have had substantial effects on their populations (c.f. Steller's sea cow: Domning 2001; Valentine & Duffy 2006), and their morpho-ecological characterisation of sea grasses parallels that of Poaceae, well known to tolerate grazing by large vertebrates. Now, however, smaller animals like some sea urchins commonly eat the leaves and grazing by small invertebrates of epiphytic algae on sea-grass detritus has become important (Valentine & Duffy 2006). Sea-grasses often grow in monodominant stands made up of clones that may be very extensive indeed (see Cymodoceaceae below). These stands are susceptible to attack by pathogens such as Labyrinthula, a heterokont protist that caused eel-grass wasting disease (Rasmussen 1998; K. A. Moore & Short 2006).

Estimates of the area occupied by sea-grass communities range from 22.8 x 10⁶ (Waycott et al. 2009) to 30 x 10⁶ (Duarte et al. 2005) to ca 40 x 10⁶ ha, but this is less than 0.2% of the area of the oceans (Duarte et al. 2005. The gross primary productivity of sea-grass communities is high, around 1903 gCm²y^-1 (like that of mangroves) and global primary productivity is 628 TgCy^-1, while their net ecosystem production (1211 gCm²y^-1 and globally 400 TgCy^-1) is substantially higher than that of mangroves because of a relatively low respiration rate. Sea-grasses are responsible for 1.13% of all marine primary productivity; they bury 27-44 TgCy^-1, some 12% of the total C storage in the marine ecosystem: macroalgae excluded, area occupied 0.3 x 10¹² m² [30,000,000 ha]; Duarte 2011). Indeed, this burial estimate may be only one half the actual amount (Fourqueran et al. 2012). Although the amount of carbon in sea-grass plants themselves may be small, that stored in the soil/trapped sediment, which can be up to 11 m thick in the Mediterranean, is larger than that in the soils of most forests and comparable with that in terrestrial peats and mangroves. Estimates of global carbon storage by sea-grasses range from 4.2-8.4 or 9.8-19.8 Pg C, depending on the assumptions made, somewhat over 0.5% the global total (Fourqueran et al. 2012). Carbon may be sequestered for maybe 12,000 years or so in the anoxic sea-grass soils (Orem et al. 1999; K. A. Moore & Short 2006; Serrano et al. 2011, 2013); Posidonia oceanica is particularly prone to form massive deposits containing refactory carbon (Mateo et al. 2006; Gobert et al. 2006). Indeed, sea grasses trap not only sediment but allochthonous carbon, too, and when thinking about sea-grass communities as carbon sinks, then an estimate of 169-186 g C m^-2 yr^-1 seems reasonable - net community production of ca 120 g plus 41-66 g of allochthonous C (Kennedy et al. 2010: higher areal estimate above).

Gross primary productivity is 3595 g C m² y^-1 and global primary productivity is 1438 Tg C y^-1, while net ecosystem production is 1585 g C m²y^-1 and globally 634 Tg C y^-1, substantially higher than either mangrove or seagrasses. Estimates of C burial are 60.4-70.0 Tg C y^-1 (Duarte et al. 2005). Furthermore, a substantial amount of sea-grass carbon moves into other marine ecosystems, including the deep sea (Suchanek et al. 1985). Mcleod et al. (2011) suggest a carbon burial rate of (100-)138(-176) g C m^-2 y^-1 (range 45-190), total carbon burial of 48-112 Tg C y^-1), in a sea-grass area of 17.7-60.0 x 10⁶ ha.

Estimates of the ecosystem services provided by sea-grasses are about $20,000/ha y^-1. This value is twice or more those for mangroves, saltmarshes, and coral reefs (Orth et al. 2006). However, sea-grass ecosystems are under considerable pressure from humans and occasional pandemic diseases (Orth et al. 2006).

The age of seagrasses is uncertain. The crown-group age of the sea-grass clade in Hydrocharitaceae-Hydrilloideae is estimated to be only (41.3-)19.4(-15.9) m.y.o. (Iles et al. 2015), although some estimates are far older. Fossils from the late Middle Eocene in Florida have been identified as Hydrocharitaceae (Thalassia) and Cymodoceaceae (Thalassodendron, Cymodocea), indeed, some have been referred to extant species, and these and other records from the Eocene in the Old World suggest a considerable age for the seagrass community (Lumbert et al. 1984; Ivany et al. 1990). Sea grasses may have originated in the eastern Tethys in the Late Cretaceous, with some taxa recorded from the New World by the Eocene that are now known only in the Old World (Ivany et al. 1990). See also Marbà et al. (2015) and Clade Asymmetries for additional information.

Pseudoasterophyllites, ca 97 m.y.o. from the European Cenomanian is possibly the earliest halophyte, and was described as growing in supratidal salt marshes; morphologically, it tends to link Chloranthaceae and Ceratophyllum (Kvacek et al. 2016) so it is not immediately related to Alismatales or any other extant halophytic group.

Pollination Biology & Seed Dispersal. The remarkable pollination devices of water-pollinated Alismatales, both marine and freshwater, have been much discussed (e.g. Cox 1988: review; Cox et al. 1991: computer simulation of underwater pollination; Les et al. 1997b: phylogeny and hydrophily; see also Pettit et al. 1980; McConchie & Knox 1989; Les et al. 2006; Ackerman 2006; Remizowa et al. 2012b; Du & Wang 2014). These include staminate flowers that detach from the parent plant and rise to the surface, the floating flowers themselves transporting pollen to the stigma; pollen variously aggregated and forming masses especially on the water surface; and underwater pollination (hypohydrophily) where the pollen grains are sometimes very much elongated or are aggregated to form elongated strands, so increasing the chances of pollination, monoecy or dioecy is common, the carpellate flowers often have a single ovule per carpel and the fruits have but a single seed, and so on. Interestingly, it has recently been suggested that marine arthropods may carry out night-time pollination of Thalassia testudinum, a species in which mucilage aggregates the pollen into strands (van Tussenbroek et al. 2016), supposedly an adaptation to hypohydrophily. Pollen exine is often absent (Ackerman 2006), and genes involved in exine development may be lost (Olsen et al. 2016). The progamic phase, the time between pollination and fertilization, is notably short in most of this clade, as in at least some other aquatic angiosperms (see Williams et al. 2010: I have not tried to optimize this). These adaptations are so striking that the flowers and inflorescences in particular, but also the vegetative bodies, of the plants appear very different both from one another and from other Alismatales with more conventional life styles.

Sea grasses in particular can be very widely dispersed, whether as fruits or plant fragments, perhaps for hundreds of kilometres (Kendrick et al. 2012). The seed coat is sometimes photoynthetic (Celdran 2016).

Plant/Animal Interactions. As suggested above, the current ecological relationships in sea grass beds, dominated by mostly small herbivores, some grazing algae on sea grasses rather than the plants themselves, and detritivores, may be fairly recent. Domning (2001) notes that sirenians, large marine herbivores from less than 100 to more than 1,000 kg in size, are well known as fossils in the New World from the Eocene onwards, with three or more species being sympatric; sirenians extant today do not reflect past diversity. The different sizes of the tusks of these animals, and the degree of rostral inflection (how much the front of the jaw is bent downwards) allows speculation about their feeding habits, which likely ranged from eating sea grass leaves to grubbing up the sometimes quite large rhizomes (to 10 mm across) of genera like Thalassodendron. However, in the later Pliocene the whole system may have broken down, with some sirenians and sea grasses going extinct, general ecological relationships in the whole Caribbean region showing extensive changes at this time whether because of geological changes or cooling of the climate or both (Ivany et al. 1990; Domning 2001 and references). Extant seagrass communities serve as nurseries for the young of a variety of animals, and this relationship is evident in fossil seagrass communities (Ivany et al. 1990).

Bacterial/Fungal Associations. Seagrasses reduce the relative abundance of bacteria that are potential pathogens of fish and invertebrates living in seagrass communities, although how they do this is unknown (Lamb et al. 2017).

Genes & Genomes. There are a number of reports of sex chromosomes, e.g. in Phyllospadix (Harada 1956). At least some mitochondrial genes show an accelerated rate of change in aquatic Alismatales (G. Petersen et al. 2006).

Chemistry, Morphology, etc. Tomlinson (1974b) described the vegetative morphology of sea grasses. He noted that in some taxa branching patterns were remarkably precise; roots were initiated within apical meristems, and were themselves unbranched, although a few taxa did have branched roots. The growth of sea grasses is highly modular, and members show an allometric scaling of their parts, the sizes of leaves, fruits, shoots and rhizomes all being correlated: species with thin rhizomes grow fast and have short-lived leaves; plants with thicker rhizomes grow more slowly, but have longer-lived leaves and more inter-module integration (Tomlinson 1974b; Duarte 1991). Thickened (nacreous) walls occur in the sieve tubes of a variety of seagrasses (Kuo 1983). Resting buds are produced sporadically throughout this group.

Non-medullated roots are quite common, occurring in e.g. Butomaceae, Alismataceae (inc. Limnocharitaceae) (Stant 1964, 1967), Aponogeton, Triglochin, Potamogeton, and many of the core seagrass clade (Kuo & den Hartog 2006), although roots of e.g. Posidonia are somewhat medullated (von Guttenberg 1968). Maundia (the roots are sometimes branched here) has triarch roots that lack pith (Platonova et al. 2016). Arber (1921) noted that inverted vascular bundles were to be found in the leaf blades of some Hydrocharitaceae, Butomaceae, Alismataceae, Cymodoceaceae and Potamogetonaceae, while other taxa have unifacial leaves; Platonova et al. (2016) discuss foliar vasculature in detail.

Remizowa et al. (2010b) discuss carpel fusion in the clade; when carpels are adnate to the central floral axis, they are often free laterally (see also Nymphaeaceae) and there is no compitum.

For general accounts of sea grasses, see den Hartog (1970), Tomlinson (1982), Kuo and McComb (1989), Hemminga and Duarte (2000), Green and Short (2003: inc. distribution maps, etc.), den Hartog and Kuo (2006, in Larkum et al. 2006: especially useful), Wissler et al. (2011) and Hogarth (2015); see Zidorn (2016) for summary of secondary metabolites, Zindler-Frank (1976) for oxalate accumulation, and Wilder (1975) for vegetative branching, inflorescence morphology, etc.; Sokoloff et al. (2013c and references) describe the rather scattered distribution of taxa with air canals in the testa. For extensive cytological studies, see e.g. Harada (1956), Uchiyama (1989), Sharma and Chatterjee (1967), and Costa and Forni-Martins (2003).

Phylogeny. For a phylogeny of the whole group as well as detailed studies of most of the families within it, see Les and Tippery (2013: main tree 167 taxa, rbcL).

[Alismataceae [Butomaceae + Hydrocharitaceae]] : apical meristems of vegetative axes bifurcating; C-glycosyl flavones +; (adaxial peripheral foliar vascular bundles +, inverted); Vorläuferspitze not on blades of emergent leaves; inflorescence branches determinate; P = K + C, members of both whorls with many traces; (androecium with trunk bundles), (stamen pairs opposite K); placentation laminar; compitum 0; (ovules many/carpel); seeds exotestal; chromosomes (0.8-)2-13.6 µm long.

Age. The age of this clade is mid-Cretaceous, some (127≤-)103.6(-74) m.y. (L.-Y. Chen et al. 2012a), ca 95 m.y.a. (Janssen & Bremer 2004), or around 83.5 m.y. (Tank et al. 2015: Table S2); ca 72.1 m.y.a. is the estimate in Magallón et al. (2015).

Evolution: Divergence & Distribution. L.-Y. Chen et al. (2013) thought that the ancestor of this clade inhabited Eurasia.

I have tentatively put a character of leaf development at this node, but sampling is exiguous in the extreme (Bharathan 1996).

Ecology & Physiology. There are reports of CAM photosynthesis (Keeley 1998) in this clade.

Genes & Genome. Many protein genes involved in the large and small subunits of mitochondrial ribosomal proteins have been lost in this clade (Adams et al. 2002b).

Chemistry, Morphology, etc. The prophylls of Limnocharis (Alismataceae) and Vallisneria (Hydrocharitaceae) may not be in the normal adaxial position (Wilder 1975). Islam (1950) sugested that both Alismataceae and Hydrocharitaceae had tenuinucellate ovules.

For floral development, see Posluszny et al. (2000) and Charlton and Ahmed (1973), for tepal vasculature, see Glück (1919), and for cytogenetics, see Feitoza et al. (2009).

ALISMATACEAE Ventenat, nom. cons.   Back to Alismatales

Plant with latex; (cormose, stoloniferous); (unicellular or stellate hairs); flavone and phenolic sulphates, tannins + (0); rhizome with endodermis; (vessels 0); stomatal subsidiary cells with parallel divisions; leaves two-ranked to spiral, with petiole and blade, (leaf terete), blade elliptic to sagittate, vernation involute, apical subepidermal pore +, primary veins merge or not with each other, (inverted vascular bundles), petiole terete; (plant mon- or dioecious); inflorescence branches whorled; C more or less crumpled in bud, thin, evanescent; nectary at base of C, A, or from staminodes or carpel flanks; A 3-many, centrifugal or centripetal, (outer members staminodial), also latrorse, endothecium with base-plate thickenings; pollen pantoporate (0-3 porate - Caldesia), surface granular [Alisma etc.] or spinose; G 2-many, ± free, with residual floral apex, partly ascidiate, placentation also basal-lateral, style with narrow canal filled with secretion [Sagittaria]; ovules (one/carpel), apotropous [when 1], (parietal tissue none), (nucellar cap ca 2 cells across - e.g. Sagittaria); embryo sac bisporic [chalazal dyad], 6-nucleate [variant of Allium-type], (monosporic, 4-nucleate [Oenothera-type]); fruit also an achene; exotesta with outer wall thickened, (thin-walled, cells with upturned ends [Limnocharis]; with glandular hairs), tegmen ± obliterated or walls ± thickened; endosperm nuclear - Alisma etc., basal cell of suspensor enlarged; embryo strongly curved; n = (5-)7-8(-13), chromosomes 2.4-14.4 µm long.

15[list]/88 (115). Pantropical, also temperate (map: see den Hartog 1957; Hultèn 1961; Meusel et al. 1965; Haynes & Holm-Nielsen 1997; Fl. Austral. 39: 2011; Trop. Afr. Fl. Pl. Ecol. Distr. 7. 2012).

Age. L.-Y. Chen et al. (2012a: 95% HPD) estimated the age of crown group Alismataceae as Upper Cretaceous, some (109.2-)79.4(-68.6) m.y. ago.

[Luronium, Damasonium, Baldellia, Alisma]

(G connate basally - Damasonium).

4/10.

Synonymy: Damasoniaceae Nakai

The Rest.

11/78: Echinodorus (30). [Photo - Limnocharis Flower, Echinodorus Flower, Fruit, Sagittaria Flower, Limnocharis vegetative, Hydrocleys flower.]

Synonymy: Limnocharitaceae Cronquist

• Alismataceae are laticiferous, aquatic herbs. Their leaves, whether floating or aerial, are petiolate and have a prominent midrib, and parallel veins linked by fine transverse veins. Their petals are crumpled in bud and their staminate flowers usually have many extrorse anthers and their carpellate flowers have many free carpels. It is obvious from looking at the seeds that the embryo is strongly bent.

Evolution: Divergence & Distribution. For the fossil record of Alismataceae, see Friis et al. (2011). Ages for clades within the family are given by L.-Y. Chen et al. (2012a), and there are also biogeographical reconstructions.

Pollination Biology & Seed Dispersal. For a summary of pollination in Alismataceae, see also Gottsberger (2016a).

In at least some species of Ranalisma and Sagittaria the pollen tubes grow down the style, out through an opening at the base of the carpel, into the floral axis and thence into adjacent carpels (X.-F. Wang et al. 2002, 2006, 2011; Huang et al. 2014). This could be quite a high-level apomorphy in the family!

The individual fruitlets of Limnocharis separate from the axis and float.

Genes & Genomes. The chloroplast genome of Sagittaria lichuanensis, at a hair over 179,000 bp, is the second largest known (luo et al. 2016), after that of Pelargonium hortorum.

Chemistry, Morphology, etc. The blade of Sagittaria develops from the upper part of the leaf; there is a Vorläuferspitze on cataphylls, but not on the blades of emergent leaves, and the petiole is evident in these leaves early in development (Bloedel & Hirsch 1979). Both leaf form and flower type (staminate, carpellate) are extremely plastic in taxa like Sagittaria latifolia (Dorken & Barrett 2004).

Although there are often many carpels and stamens, organ initiation is basically whorled. Anther initiation may be centrifugal or centripetal; there are common stamen primordia (Sattler & Singh 1977). The pollen often contains starch. The pores of the pollen grains have very irregular margins. The carpels may initiate first in the antesepalous positions, sometimes on gynoecial bulges; or the carpels may be in many whorls (looking spiral!) and completely covering the axis (Singh & Sattler 1972, 1973, 1977a; Charlton 2004 and references; Rudall 2008). Alternatively, there may be a single whorl of carpels with a large, residual floral axis in the center, as in members of the old Limnocharitaceae (e.g. Leins & Stadler 1973); there the carpels are connate laterally, there are many ovules per carpel, and placentation is laminar.

General information is taken from Haynes et al. (1998b: as Alismataceae and Limnocharitaceae) and Hooper and Symoens (1982: as Limnocharitaceae); for general morphology, see Charlton and Ahmed (1973), for vegetative anatomy, see Stant (1964, 1967), for floral development, see Leins and Stadler (1973), Charlton and Ahmed (1973), Charlton (1991 and references), Wang and Chen (1997) and K.-M. Liu et al. (2002), for embryology, see Dahlgren (1928b, 1934b) and Johri (1936 and references), all with discussion on number of cells in the embryo sac, for endothecium, see Manning and Goldblatt (1990), and for cytogenetics, see Feitoza et al. (2010).

Phylogeny. Details of the relationships between and within Alismataceae and the old Limnocharitaceae are still rather unclear (Soros & Les 2002; Y. Kato et al. 2003; J.-M. Chen et al. 2004a, b; L.-Y. Chen et al. 2012a; von Mering & Kadereit 2010; Lehtonen 2009 for a summary). Echinodorus is polyphyletic (Soros & Les 2002; see also Lehtonen & Myllys 2008). Morphological analyses yield poorly supported basal pectinations with Butomopsis, Hydrocleys, and Limnocharis successively sister to the remainder of the clade; Alismataceae in the old sense then form a well supported clade (Lehtonen 2009: several characters show continuous variation). In an analysis with comprehensive sampling and using ITS plus three chloroplast genes a well-supported clade [Luronium, Damasonium, Baldellia, Alisma] was sister to the rest of the family (see also J.-M. Chen et al. 2004a), but there was only moderate to weak support along the basal part of the backbone. Furthermore, this clade was not recovered in a smaller study using additional chloroplast genes, or was in a different position when mitochondrial genes were used (L.-Y. Chen et al. 2012a). It was again well supported in the plastid phylogenomic study of Ross et al. (2015), but Du and Wang (2014) and Du et al. (2016) found that Ranalisma and Burnatia were successively sister to the rest of the family; basal relationships were unclear in Lehtonen (2017).

Classification. Alismataceae include the "old" Limnocharitaceae (first recognized by Takhtajan in 1954) here, and they certainly have much in common.

[Butomaceae + Hydrocharitaceae]: ovary loculi with secretions.

Age. The divergence of these two families is dated to ca 88 m.y. before present (Janssen & Bremer 2004) or around 21.8 m.y.a. (L.-Y. Chen et al. 2012a).

BUTOMACEAE Mirbel, nom. cons.   Back to Alismatales

Plant rhizomatous; monopodial; flavonols?; stomata variable; leaves ± two-ranked, blade triangular; inflorescence umbellate, with subtending bracts, (floral bracts 0), prophylls 2, lateral; flowers protandrous; P = T, petal-like, but whorls not identical; A 9, some latrorse; pollen monosulcate; nectar from carpel flanks; G 6, fusion postgenital, stigma ± decurrent; chalazal cells ± hypertrophied, surounding nucellar cells radially arranged; air canals in testa, outer walls of exotestal cells thickened and with encrustations, tegmen persists; embryo and color?; cell at end of suspensor enlarged; n = 7, 8, 10, 11, 12, etc., chromosomes 3.7-8.3 µm long.

1[list]/1: Butomus umbellatus. Temperate Eurasia, naturalised in N.E. North America (map: from Hultén & Fries 1986). [Photo - Habit © D. Woodland, Inflorescence © E. Parnis.]

• Butomaceae are aquatic monocots that may be recognised by their long, three-angled leaves and axillary, scapose, umbellate, bracteate inflorescence. The flowers are perfect and there are clearly two distinct perianth whorls although both are petal-like; the stigmas are yellow when they are receptive, and then they look very like the pollen-filled anthers immediately after they have dehisced. The fruit is a follicle.

Age. The age of crown-group Butomaceae is estimated at (50-)30(-15) or (10-)9 m.y. (Hertweck et al. 2015).

Evolution: Divergence & Distribution. Diversification rates in this clade are reduced (Hertweck et al. 2015).

Pollination Biology. The staminate and carpellate phases of the flowers in an umbel, possibly in an entire ramet, but not in different ramets, are synchronized (Bhardwaj & Eckert 2001).

Chemistry, Morphology, etc. Cook (1998) hesitantly suggested that the rhizome was monopodial (see also Bhardwaj & Eckert 2001); this should be checked. (Stant (1967) reported crystals "in the form of small rods" (styloids) in the diaphragm cells surrounding the air spaces in the stem; she also suggests that the leaf of Butomus is equivalent to the petiole of Alismataceae (Limnocharitaceae).

For the position of the prophyll, see Eichller (1875). There appear to be C-A primordia, with a pair of stamens differentiating first, and then a single stamen adaxial to that pair (Singh & Sattler 1974). The basal cell of the endosperm may remain undivided but become hypertrophied, or there may be some free nuclear divisions (Fernando & Cass 1996).

Much information is taken from Cook (1998); for additional information, see Roper (1952) and in particular Fernando and Cass (1996) for embryology and Charlton and Ahmed (1973) for morphology.

HYDROCHARITACEAE Jussieu, nom. cons.   Back to Alismatales

Stem ± stoloniferous, leaves in groups; branching?; flavone and phenolic sulphates +; roots unbranched; vessels 0; endodermis obscure or thick-walled; stomata ?type; (prophyll lateral); leaf blade ± linear; plant dioecious (flowers perfect); inflorescence subtended by 2 often connate bracts; androecium with trunk bundles?, (A introrse); pollen inaperturate, exine thin to none [pollen not resistant to acetolysis]; nectaries 3, staminodial (0); G inferior, carpels laterally ± free, closure by secretion only, placentae [= carpel walls] much intruded, style single, short, stigmas usu. branched, papillate adaxially; ovules with outer integument often >3 cells across, micropyle bistomal, parietal tissue 1-2(-more?) layers across, nucellar cap 2-3 cells across (absent); fruit often ± fleshy, dehiscence irregular; seeds with short hairs, all testal cells ± thickened except the outermost wall, or exotestal, stone cells in mesotesta and endotesta, endotegmen with tuberculate inner wall alone persisting; chalazal endosperm haustorium, unicellular suspensor haustorium; n = notably variable; cotyledon bifacial; extensive loss of mitochondrial genes.

18[list]/116 (135) - four groups below. World-wide (map: blue, marine Hydrocharitaceae; red, freshwater members - see Hultèn 1961; Hultén & Fries 1986; Fl. N. Am. 22: 2000; FloraBase 2005; Fl. Austral. 39: 2011; Trop. Afr. Fl. Pl. Ecol. Distr. 7. 2012: van Steenis & van Balgooy 1966; but commonly introduced and original distributions unclear; den Hartog 1970 for marine taxa).

Age. Crown group Hydrocharitaceae are dated to ca 75 m.y. (Janssen & Bremer 2004), while L.-Y. Chen et al. (2012b) suggest a rather younger crown-group age of (72.6-)65.5(-54.7) m.y..

1. Hydrocharitoideae Eaton

Roots branched; foliar vascular bundles inverted; leaves spiral or spirally 2-ranked, with petiole and blade, blade vernation involute or convolute, ligules +, basal, adaxial (paired lateral) [totally enclosing young leaves], ?base; plants monoecious; staminate flowers: A (3-)6-12(-18), (connate in pairs - different whorls), (pistillodes +); pollen tectate-columellate, with (minute) spines; carpellate flowers: (C 0); (staminodes +); G [3-9], (carpels basally ascidiate - Limnobium); ovules straight; testa with 1-3-celled "papillae", exotestal cells much enlarged, variously thickened; n = 7-11, 13-15.

2/5. Temperate and subtropical.

[Stratiotoideae [Anacharidoideae + Hydrilloideae]]: roots unbranched; leaf blade � linear, base not sheathing; plant dioecious (flowers perfect); (ovules straight).

2. Stratiotoideae Luersson

Plant floating, vegetative growth monopodial; stem endodermis 0; leaves spirally 3-ranked, margins strongly spiny; staminate flowers: A many, adaxial 5-17 fertile, staminodes of three kinds, two nectariferous, ± abaxial; pollen baculate, tectum incomplete; pistillode 0; carpellate flowers: staminodes +; G 3, 6, in 2 whorls, outer opposite K, (carpels basally ascidiate); ovules 4-6/carpel; fruit locules filled nwith mucilage; exotestal seed hairs mucilaginous; n = 10+.

1/1: Stratiotes aloides. Eurasian.

Age. The age of the stem node of Stratiotes is estimated to be 55.9 m.y. (Iles et al. 2015).

Synonymy: Stratiotaceae Schultz Sch.

[Anacharidoideae + Hydrilloideae]: plant monopodial; submerged; roots lacking hairs [?all]; (leaves scattered along stem), (margins spiny); inflorescences axillary, emersed or not; placentae usu. not much intruded; ovules usu. few/carpel; fruit fleshy, capsular, or dehiscing irregularly, (fruit indehiscent).

3. Anacharidoideae Thomé

(Plant sub-cormose); root trichoblasts 0 [Blyxa]; leaves whorled, spiral, two ranked, (usually opposite when scales), (petiole + blade - some Ottelia); (flowers perfect [Apalanthe, Blyxa, Ottelia); staminate flowers: released, usually as buds; P 3 + 3 (3); A 3 (+ 3 staminodes)-12, (dorsifixed); pollen (bicellular - Ottelia), (with discontinuous exine, little or no sculpturing), (surface spiny); carpellate flowers: hypanthium +, usu. long; P 3 + 3; staminodes +; G [3(-20+)]; (carpel walls much intruded); (ovules many/carpel), (micropyle bistomal, outer integument to 4 cells across, parietal tissue 1-4 cells across; nucellar cap 2-3 cells across; antipodal cells persist; seeds usually <30; n = ?6, 8, 9, 11, 12, 14, etc.

7/38. Tropical to temperate, esp. America.[Photo - Habit © D. Woodland, Blyxa Habit, Flower © M. Clayton.] and Egeria.

Age. The age of the clade [Apalanthe + Lagarosiphon] is ca 40.5 m.y. (Les et al. 2003).

Synonymy: Blyxaceae Nakai, Elodeaceae Dumortier, Otteliaceae Chatin, nom. illeg.

4. Hydrilloideae Luersson

(Marine); (plant rhizomatous), (annual); (leaves with spines - some Naias); roots unbranched, trichoblasts 0 [Vallisneria]; leaves (spiro)two-ranked or whorled, linear, (base expanded, rounded to 2-lobed - Naias); plant dioecious (monoecious - Najas); perianth biseriate, ± undifferentiated, (3 + 1), uniseriate, (0); staminate flowers: (released as buds); A 1-9, (1 staminode); pistillode 0; carpellate flowers: staminodes (0-)3; hypanthium +; G [2-9], (stigmas commissural), (filiform, smooth); (micropyle bistomal), (obturator +); (seeds smooth), (with starch); (air canals in testa), (exotegmic tuberculae +); n = 6-8, 10, 12, 15. Marine taxa: sulphated flavones and phenolic acids +: Halophila: sulphated phenolic acids 0; Enhalus: roots unbranched; leaves with blade + petiole; pollen exine 0; testa cells with small, peg-like projections, (photosynthetic - Thalassia); fruit a capsule; radicle 0; n = 6, 9, chromosomes 0.6-13.2 µm long; chloroplast ndh genes lost [?level].

8/61: Najas (40), Vallisneria (12). Tropical and subtropical, especially Old World; Najas subcosmopolitan. [Photo - Hydrilla, © H. Wilson, Halophila, Enhalus, flower, © from D. Les website], Thalassia, fruit, © from D. Les website.]

Synonymy: Enhalaceae Nakai, Halophilaceae J. Agardh, Hydrillaceae Prantl, Najadaceae Jussieu, nom. cons., Thalassiaceae Nakai, Vallisneriaceae Dumortier

• Hydrocharitaceae are aquatic plants with petiolate or (usually) undifferentiated leaves. The inflorescence often has two fused (sometimes free) bracts at the base and the ovary is inferior, often with laminar or more or less strongly intruded parietal placentation, and the stigma lobes are bifid.

Evolution: Divergence & Distribution. Y. Kato et al. (2003) proposed ages for Hydrilloideae of (130-)119(-108) m.y., while He et al. (1991) proposed a Cretaceous age and Gondwana origin for Ottelia. Stratiotes has a rich fossil record (as seeds) from the middle of the Eocene onwards (Cook & Urmi-König 1983).

Ecology & Physiology. Hydrocharitaceae such as Hydrilla and Egeria have C₄ photosynthesis with metabolic compartmentalisation occurring within single cells (Bowes et al. 2002 for references). Marine seagrasses in the [Thalassia + Enhalus + Halophila] clade are estimated to be 47.8-38 m.y.o. (Iles et al. 2015: stem node age).

Pollination Biology & Seed Dispersal. Pollination mechanisms in Hydrocharitaceae include entomophily, anemophily, epi- and hypohydrophily, zoobenthophily, and selfing. Parallelism is pervasive, and sex expression of the flowers/plants is very labile, whatever the tree (e.g. see L.-Y. Chen et al. 2012b). For pollination mechanisms, see Cook (1982, 1994-1995, 1996), these correlate with pollen morphology and phylogeny (Tanaka et al. 2004). Hypohydrophily has evolved at least twice (e.g. Najas, marine genera), and staminate flowers that detach from the plant and rise to the surface of the water perhaps five times (Les et al. 2006 and references). In a number of species the hypanthium elongates greatly, and the carpellate flower opens onto the surface of the water. Pollination in those taxa where the staminate flowers are released may be epi- or hypohydrophilous. Small detached staminate flowers floating on the surface of the water on reflexed sepals are caught by the carpellate flowers; these flowers may have two stamens (Nechamandra), three stamens and three erect staminodes that act as little sails (Lagarosiphon), or six stamens (Appertiella). Hydrilla is wind pollinated, the pollen being released explosively by the anthers as they reach the surface in little gas bubbles produced by the submerged staminate flower, while in Elodea the pollen, similarly produced from submerged flowers, floats. In other taxa the hypanthium does not elongate, the carpellate flowers having long pedicels, again, detached staminate flowers are caught by the carpellate flowers. Examples are Maidenia (= Vallisneria) and the marine Enhalus; in the former, there are at most 24 pollen grains per staminate flower. In the marine Halophila the pollen is released embdedded in strands of mucilage, and pollination is underwater. Indeed, marine arthropods may carry out night-time pollination of Thalassia testudinum, the anthers opening at night when the arthropods are active (van Tussenbroek et al. 2016). The insect-pollinated Blyxa has secondary pollen presentation, while Cook and Lüönd (1982) suggested that in Hydrocharis the staminate flowers, which lack nectary, mimic the carpellate flowers, which have nectariferous antepetalous staminodes. For more details, see e.g. Ernst-Schwarzenbach (1945) and Tanaka et al. (2013: details of pollen and stigma morphology).

Vegetative Variation. Marine taxa are rhizomatous, with leaf-bearing short shoots (see Tomlinson 1974b for a summary). Taxa like Elodea have leaves borne all along the stem, while others have whorled leaves, Stratiotes aloides forms floating rosettes in the summer, but the plant sinks to the bottom of the water in winter, rising again in the next year. Leaf shape and margin also vary a great deal.

Genes & Genomes. The chloroplast ndh genes are all lost or are pseudogenes in Naias, but not in Elodea (King et al. 2017); I do not know what happens in other Hydrilloideae.

Chemistry, Morphology, etc. The plants may be tanniniferous. Hydrocharis, apparently alone in the group, has a root epidermis that is of inner epidermal origin.

Growth and branching in Hydrocharitaceae needs more study. In Enhalus and Stratiotes the first leaves produced after the cotyledon are at right angles to it, whereas in most other taxa these leaves are borne in the same plane as the cotyledon (Haccius 1952a). Axillary buds along the stems are commonly precocious (Wilder 1975), and pseudodichotomous branching, often interpreted as being the result of this precocious axillary branching, is also quite common (Tomlinson 1974b, 1982). Posluszny and Charlton (1999) described the extremely complex branching in Hydrocharis morsus-rana, suggesting that it has components of flower/inflorescence morphology. They thought that the sheathing bracts, separated by a short internode, might be comparable to the first two leaves on a branch. Tanaka et al. (1997) noted that flowers and axillary branches frequently arise from the same axil, and there is also variation in bracteole number and position (lateral, paired, etc.: Eichler 1875).

The anthers sometimes lack an endothecium (Ernst-Schwarzenbach 1945). Elodea is shown as having its carpels opposite the inner perianth whorl (Eichler 1875). The staminodes of Vallisneria are opposite the petals/inner P and V. spiralis, at least, has commissural stigmas (Les et al. 2008). The carpels are best interpreted as being more or less free from each other laterally but adnate to the receptacular wall abaxially (Weberling 1989 for references, esp. Kaul 1968). The ovary in at least some taxa is filled with mucilage, but it is unclear if there are intra-ovarian trichomes (Rudall et al. 1998c, c.f. Oriani & Scatena 2012). Blyxa has notably short chromosomes (Uchiyama 1989).

General information is taken from Kuo and McComb (1980), Cook (1998: he and collaborators have revised almost the entire family), Haynes et al. (1998a), Haynes and Holm-Nielsen (2001) and van Tussenbroek et al. (2006: Thalassia); for morphology and anatomy, see Ancibor (1979), for ovules, etc., see Kausik (1940a), Islam (1950) and Govindappa and Naidu (1956), for testa anatomy, see Shaffer-Fehre (1991a, b), for some cytology, see Vanitha et al. (2016).

Phylogeny. Tanaka et al. (1997: two genes) found a series of quite well-supported nodes; the ultimate groupings recognised there are similar to those of Les et al. (1997b). Les et al. (2006) in a four-gene analysis of all genera bar one, and including morphological characters, again found largely similar relationships: Hydrocharitoideae were sister to the rest of the family (support unclear, only a single outgroup used), and then Stratiotoideae, but with only moderate support (72% bootstrap, all characters), and then the clade [Anacharidoideae + Hydrilloideae] (52%); however, the monophyly of all four subfamilies was strongly supported.

Analysis of an eight-gene supermatrix by L.-Y. Chen et al. (2012b) yielded an appreciably different tree. Basic relationships are [Stratiotoideae [Anacharidoideae [Hydrocharitoideae + Hydrilloideae]]]. The position of Stratiotoideae was largely driven by mitochondrial data, otherwise support for the backbone of the tree was strong, but that for the particular position of Hydrilla poor (L.-Y. Chen et al. 2012b). Ross et al. (2015) also recovered this topology, again with strong support for all the subfamilies, but the position of Stratiotoideae was unclear. Les and Tippery (2013) noted the variety of topologies that had been found for the four subfamilies; they themselves found the relationships [Hydrocharitoideae [Anacharidoideae [Stratitoideae + Hydrilloideae]]], althought support was weak, as it was for the monophylyy of Anacharidoideae and Hydrilloideae. Du et al found two main clades in the family, one [Hydrocharitoideae + Anacharidoideae] and the other [Stratiotoideae + Hydrilloideae]. For relationships between Chinese taxa, see Z.-D. Chen et al. (2016).

Within Hydrilloideae, Najas was strongly supported (98%) as sister to Hydrilla in the combined analysis, although not in all individual molecular analyses, yet the two are notably distant in the tree in the morphological analysis (Les et al. 2006). There is a "marine" clade [Enhalus [Halophila + Thalassia]] here; the latter genus was omn a very long branch in the plastid gene analysis of Lam et al. (2016). Despite the obvious morphological differences between Najas and other Hydrocharitaceae, Posluszny and Charlton (1999 and references) noted that both branching and seed anatomy link them, however, Liu and Li (2010) found Najas to be sister to the other Hydrocharitaceae that they examined, while L.-Y. Chen et al. (2012b) found the relationships [sea-grasses [Najas [Hydrilla + the rest]]]. Najas flexilis was on a very long branch in the chloroplast gene analysis of Luo et al. (2016), and had lost eleven ndh genes. For a phylogeny of Vallisneria, see Les et al. (2008), and for that of Naias see Ito et al. (2017a).

• Najas: The leaves have an "auricle" at very base; staminate flowers have two envelopes and a single stamen, the secondary parietal layers of the anther wall do not divide; carpellate flowers have a single floral envelope; single, ascidiate carpel, one- or two-branched style; one, basal ovule and an ?achenial fruit; the seed may have an exotegmen; the endosperm is nuclear, the embryo chlorophyllous, the cotyledon unifacial. The root system is well-developed, the roots are unbranched but they have many root hairs. Variation in chromosome length (1.6-11.7 µm) is considerable. Miki (1937) suggested that the ovary of Najas, apparently superior, might really be inferior; floral development would repay re-examination. Even in a morphologically very variable group, Najas is a distinctive genus.

• Ottelia is distinctive in Anacharidoideae having several apparently plesiomorphic features; it has a petiole, broad leaf blade, and reticulate fine veins; its petals are large and there are up to 15 stamens; the fruits dehisce quite regularly, and the seeds have some endosperm.

Classification. There are eleven family names available for the eighteen genera in Hydrocharitaceae, reflecting the variety of pollination mechanisms and vegetative adaptations to the aquatic life style. I follow the subfamilial classification suggested by Les et al. (2006).

[Aponogetonaceae [Scheuchzeriaceae [Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]]]]: primary root poorly developed; calcium oxalate crystals of any sort absent; bracts 0; P members with a single trace; pollen reticulate; parietal tissue >2 cells across; suspensor unicellular, cell large; chromosomes (0.5-2.3(-4.5) µm long.

Age. The first branch in this clade is dated to ca 98 m.y. (Janssen & Bremer 2004) or to 81.1 m.y. (stem node of Aponogetonaceae: Iles et al. 2015); it is estimated to be around 100 m.y.o. in L.-Y. Chen et al. (2014b).

Evolution: Divergence & Distribution. L.-Y. Chen et al. (2013) thought that this clade began to diverge in Eurasia.

Chemistry, Morphology, etc. A number of taxa in this clade have a radicle that is lateral and exogenous in origin (Yamashita 1970, 1972, 1976).

For a summary of much information about the families below, see Sokoloff et al. (2013c); Markgraf (1936) describes general floral studies in the "simplest Helobiae".

Classification. There are many small families in this clade reflecting the very distinctive floral and vegetative morphologies that have evolved in connection with the aquatic habitat its members favour. Maundiaceae are recognised below, further increasing the number.

APONOGETONACEAE Planchon, nom. cons.   Back to Alismatales

Plant with a short rhizome or corm, apical meristems of vegetative axes bifurcating [?all]; vessels 0; laticifers +, articulated; peripheral vascular bundles in the leaf various; ?stomata; leaves spiral, with petiole and blade, blade vernation involute, primary veins merge with each other, tertiary veins few, apex of old leaves with pore; plants usu. monoecious or dioecious; flowers sessile [inflorescence spicate], (monosymmetric); P (1-4); staminate flowers: (A 16>), (stamen pairs +), (anthers introrse); microsporogenesis also simultaneous; pollen monosulcate, reticulum uniform, muri broad, (micro)echinate; pistillode ?; carpellate flowers: (P 0); staminodes +; G 2-9, free, alt. P, septal nectaries + (0), placentation basal; ovules 1-12/carpel, (unitegmic - integument 5-6 cells across), nucellar cap ca 2 cells across; T etc. persistent in fruit or not; seed coat mucilaginous, air canals in testa, exotesta protective or not, endotegmen tanniniferous, or undifferentiated and translucent; embryo chlorophyllous or not, radicle sublateral, exogenous (0); n = ?12, 16, 19, etc., chromosomes 0.7-2.3 µm long; cotyledon bifacial.

1[list]/50. Old World, esp. South Africa, largely tropical and warm temperate, suspected of being introduced in parts of Southeast Asia-Malesia - localities not on map) (map: from van Bruggen 1985, 1990). [Photo - Aponogeton Flower © H. Wilson, Habit © R. Kowal.]

• Aponogetonaceae are water plants with petiolate leaves; the sometimes fenestrate blades have a midrib, parallel veins, and strong transverse veins. The often densely spicate inflorescence has a long scape and the rather small flowers nevertheless often have more or less conspicuous tepals; the whole inflorescence is coloured.

Age. Crown-group Aponogetonaceae may be ca 23.3 m.y.o. (Les et al. 2003) or (48-)39.8(-32.2) m.y.o. (L.-Y. Chen et al. 2014b).

The distinctive pollen of Aponogeton has recently been reported from western Greenland and North America in deposits around 82-81 m.y.o. (Grímsson et al. 2013); current and past distributions are rather different.

Evolution: Divergence & Distribution. Given the age of the pollen identified as Aponogetonaceae from the northern hemisphere (Grímsson et al. 2013) and the strongly supported sister-group relationship of the Australian Aponogeton hexatepalus to the rest of the genus, the place of origin and diversification of the clade is difficult to ascertain (L.-Y. Chen et al. 2014b: c.f. Fig 3 and abstract). There is a long lag time - perhaps 60 m.y. - between its origin and diversification.

Pollination Biology. Madagascan and Indian species of Aponogeton can be hybridized (Yadav 1995; see also Grímsson et al. 2013).

Chemistry, Morphology, etc. For cell death and the development of the fenestrate leaves of Aponogeton madagascariensis, see Gunawardena and Dengler (2006), Wright et al. (2009) and Dauphinee et al. (2017) and references.

The bract may form an hybrid organ with a tepal, so making the flower slightly monosymmetric; separate bracts were not seen (Buzgo 2001). More pronounced monosymmetry occurs in flowers in which only two perianth members develop; these appear to be the abaxial pair, and in a monocot flower with "normal" orientation these would be members of the inner perianth whorl; the median member of the outer whorl of stamens is abaxial (see Singh & Sattler 1976b); a parallelism with Maundia (Sokoloff et al. 2013c).

Some information is taken from van Bruggen (1990, 1998); for embryology, see Sâné (1939), embryo development, see Yamashita (1976), and for floral morphology, see Remizowa et al. (2010b).

Phylogeny. For the phylogeny of the genus, see Les et al. (2005), Les and Tippery (2013) and especially L.-Y. Chen et al. (2014b). The Australian Aponogeton hexatepalus, with six tepals and quite distinctive pollen, is sister to the rest of the genus.

[Scheuchzeriaceae [Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]]: leucanthocyanins, flavones 0; stem/rhizome endodermis +; fibre bundles in leaf; leaf ± linear, ligulate, base with auricles; filaments shorter than the anthers, anthers ± sessile; pollen inaperturate; carpels with complete postgenital fusion [sampling!], nectary 0.

Age. This node is ca 92 m.y.o. (Janssen & Bremer 2004; see also L.-Y. Chen et al. 2014b).

Evolution: Divergence & Distribution. Volkova et al. (2016) suggest that pollen apertures have been lost and then regained (in Potamogetonaceae, Ruppiaceae) in this clade.

Chemistry, Morphology, etc. For the distribution of sulphated compounds, see especially McMillan et al. (1980).

The nature of the small, tepal-like structures closely associated with the stamens found in many members of this clade has occasioned much discussion. Sattler (1962) and Singh (1965) considered the perianth and androecium of Potamogetonaceae to be distinguishable although there was but a single trace to each P/A pair. This tepal-like structure is called a retinaculum by some, and then considered to be some kind of enation, not tepalline; indeed, von Mering and Kadereit (2010) suggest that the clade [Maundiaceae + the rest] may be characterized by a flower that lacks a perianth (several taxa here do entirely lack perianth-type structures). Rudall (2003b, see also references) suggested that the flowers of all or many of the taxa were some kind of pseudanthia. However, flowers in which the perianth member seems to come from the back of a stamen are here considered to be an extreme form of the perianth-stamen association that is common in monocots.

SCHEUCHZERIACEAE F. Rudolphi, nom. cons.   Back to Alismatales

Plant irregularly sympodial; cyanogenic glucoside triglochinin +, flavonoids 0; peripheral ring of sclerenchyma in peduncle; adaxial peripheral foliar vascular bundles inverted; stomata tetracytic; leaves two-ranked, with apical pore, intravaginal squamules as hairs; inflorescence bracts +, ± foliaceous; bracts ± foliaceous; pollen in dyads, calymmate, with simple cohesion, grains trinucleate; G 3(-6), opposite outer T, basally connate, fusion usually congenital, placentation parietal, stylulus 0 [?level], ?compitum; ovules (1) 2/carpel, subbasal, outer integument ca 4 cells across, inner integument ca 2 cells across, parietal tissue 4-5 cells across, (nucellar cap 2 cells across); T persistent in fruit; testa smooth, mesotesta many layered, cells with thick walls; embryo chlorophyllous; n = 11, chromosomes 0.8-2 µm long; cotyledon not photosynthetic.

1[list]/1: Scheuchzeria palustris. N. Temperate to Arctic (map: see Hultén 1961; Fl. N. Am. 22: 2000). [Photo - Habit.]

• Scheuchzeriaceae are small herbs of marshy places with 2-ranked leaves; the open sheaths have auricles at the top and there is a little pore at the tip of the blade. The racemose inflorescence has large, leafy bracts and all the parts of the flowers (except the bases of the carpels) are free from one another.

Chemistry, Morphology, etc. Although Scheuchzeriaceae are chemically like Juncaginaceae, they are not otherwise particularly similar. The inflorescence is described as being a closed racame; there is a terminal flower (Volkova et al. 2016).

Some information is taken from Haynes et al. (1998b: general); see Stenar (1935) for embryology.

[Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]]: flowers rather small, closely aggregated, inconspicuous; P-A pair with single vascular trace, [P often adnate to A]; G free; carpels the dispersal unit; endosperm nuclear.

Age. The age of this node is estimated at ca 82 m.y. (Janssen & Bremer 2004; see also L.-Y. Chen et al. 2014b), about 77.4 m.y. (Tank et al. 2015: Table S2) or ca 68.8 m.y.a. (Magallón et al. 2015).

Evolution: Divergence & Distribution. The character "ovule one/carpel" is placed at this node by Remizowa et al. (2012b), q.v. for the evolution of a number of characters in this part of the tree. For the evolution of syncarpy around here, see Sokoloff et al. (2013d).

Chemistry, Morphology, etc. See Thadeo et al. (2015) for fruitlet anatomy of some taxa in this clade.

Phylogeny. Iles et al. (2009, 2013) and von Mering and Kadereit (2010) suggested that Juncaginaceae are paraphyletic; the separation of Maundia from the rest of the family in fact clarifies gynoecial variation within Juncaginaceae s.l.. However, von Mering and Kadereit (2010) were not sure of the exact position of Maundiaceae.

JUNCAGINACEAE Richard, nom. cons.   Back to Alismatales

Rosette herb; apical meristems of vegetative axes bifurcating [?all]; O- and C-glycosyl flavones, cyanogenic glucoside triglochinin +; (stem endodermis 0); peripheral ring of sclerenchyma in peduncle; (laticifers - Lilaea); (adaxial peripheral foliar vascular bundles inverted); stomata also tetracytic, subsidiary cells with parallel divisons; leaves spiral, ± unifacial, (2-ranked, ventralized isobifacial [oriented edge on to the stem] - Tetroncium), (ligules - Triglochin); (plant dioecious - Tetroncium), (flowers polygamous); (flowers sessile), bracts 0 (+); flowers 1-4-merous, (monosymmetric), P 0-4, 6; A 3-8; (pollen bicellular); G 1 [3-10], weakly (more strongly) connate (± free - Cycnogeton), fertile carpels opposite inner P, (opposite P - Tetroncium), (alternating with an outer whorl of sterile carpels), (styluli long - some Lilaea s. str., Tetroncium), stigma penicillate; ovules 1-few/carpel, basal, outer integument ³3 cells across, parietal tissue 4-6 cells across; fruit schizocarpic, drupaceous, achenial, indehiscent, (hooked, winged), T persistent or not; exotesta and entegmen with cuticle, otherwise crushed; (endosperm +), embryo ?colour, with short thick hypocotyl, primary root lateral, exogenous; n = 6, 8, 15, etc., chromosomes 0.6-1.1 µm long.

3[list]/30. Cosmopolitan, largely coastal (map: see Hultèn 1961; Meusel et al. 1965; Hultén & Fries 1986; Fl. N. Am. 22: 2000; FloraBase 2004; Köcke et al. 2010). [Photo - Habit, Fruit.]

Age. Crown-group ages for Juncaginaceae are ca 51.7 m.y. (Les et al. 2003) and (80.4-)70.2, 44.1(-27.4) m.y. (von Mering & Kadereit 2014).

• Juncaginaceae are herbs often growing in saline marshy places. They have more or less unifacial leaves and a scapose racemose or spicate inflorescence; all parts of the flower appear to be free.

Evolution: Divergence & Distribution. Movement of the family in the southern hemisphere may have been facilitated by the proximity of ex-Gondwana fragments; there is a fair amount of habitat-linked diversification in Triglochin (von Mering & Kadereit 2014).

Chemistry, Morphology, etc. Imperfect flowers may lack a perianth (carpellate flowers of Lilaea) and have a single stamen and carpel; to a certain extent the number of parts in the flower is connected with flower size (Buzgo et al. 2006). The abaxial median tepal is somewhat bract-like (Buzgo 2001; Buzgo et al. 2006; Remizowa et al. 2010b). Stamens of the outer tepal-stamen unit may be outside tepals of the inner tepal-stamen unit (Dahlgren et al. 1985; Endress 1995b). There is no evidence of pseudanthia; terminal flowers are close to being peloric (Buzgo et al. 2006). Seedlings of Triglochin have two-ranked leaves.

Some information is taken from Arber (1925: general), Agrawal (1952: embryology), Haynes et al. (1998b: general), and von Mering (2013) and Sokoloff et al. (2015c), both Tetroncium; for alternative interpretations of the gynoecium, see Igersheim et al. (2001).

Phylogeny. Relationships are [Tetroncium [Cycnogeton + Triglochin]]; Lilaea is embedded in Triglochin (von Mering & Kadereit 2008, 2014).

Classification. For a classification of the family, see von Mering and Kadereit (2008). Trias-Blasi et al. (2015) still include Maundia in the family.

Synonymy: Heterostylaceae Hutchinson, Lilaeaceae Dumortier, Triglochinaceae Berchtold & J. Presl

[Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]: vessels 0; carpels ascidiate [sampling!], ovule 1/carpel, apical, pendulous, straight.

Age. Janssen and Bremer (2004) suggest that the age of this node is ca 75 m. years.

Phylogeny. For discussion of the position of Maundiaceae, see Les and Tippery (2013).

MAUNDIACEAE Nakai   Back to Alismatales

Rosette herb; rhizome endodermis +, outer vascular bundles of scape inverted; abaxial peripheral foliar vascular bundles inverted, no fibre bundles; stomata brachyparacytic; leaves 2-ranked, 2 expanded leaves/module, ± triangular, isobifacial, ligule 0, base narrow [ca 1/2 surrounding the stem], cataphylls +, sheath closed, branching extravaginal; inflorescence scapose, spicate, flowers sessile; flowers monosymmetric by reduction; bracts/bracteoles 0; P 2, tranverse-abaxial, (-4 - terminal flowers), clawed, (with three traces); A (4-)6, with separated thecae; G [(3-)4], laterally ± free, cruciform, styluli marginal, recurved; outer integument 3-4 cell layers across, with air canals, inner integument ca 2 cells across, parietal tissue 4(?+) cells across, nucellar coenocyte + [cell walls of tissue immediately distal to vascular bundle break down]; fruit a schizocarp, T persistent; testa obliterated; n = ?

1[list]/1: Maundia triglochinoides. East Australia (map: from Australia's Virtual Herbarium, viii.2009).

Evolution. Vegetative Variation. Platonova et al. (2016) drew attention to the remarkable similarity in anatomy of the inflorescence axis and leaves, also describing the distinctive vegetative construction of the plant in detail.

Chemistry, Morphology, etc. There appear to be twice as many stamens as tepals, but this is because the anther thecae are separated. See von Mering and Kadereit (2010) for the interpretation of the androecium; as they note, similar stamens are found in Posidoniaceae and Zosteraceae. Sokoloff et al. (2013c) described the morphology of Maundia in detail, noting i.a. variation in floral diagrams, while Platonov et al. (2016) looked at its vegetative anatomy..

[[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]: submerged marine aquatics; rhizome with central vascular tissue; peripheral foliar vascular bundles 0; epidermis chlorophyllous, stomata 0; hydrophily + [water pollination]; flowers imperfect; carpellate flowers: G free; fruit ± drupaceous; embryo with massive elongated hypocotyl, also prominent in seedling, base of hypocotyl [collar] much enlarged.

Age. Janssen and Bremer (2004: c.f. topology; see also L.-Y. Chen et al. 2014b) suggest that the first split within this clade can be dated to ca 73 m.y., and they also give other divergence dates within it; see also Coyer et al. (2013: again, c.f. topology).

Evolution: Divergence & Distribution. Whether vessels and stomata are lost in parallel several times, and/or are lost and then regained, is unclear (see Zosteraceae for more discussion). Thus genes involved in stomatal differentiation have been lost in Zostera (Olsen et al. 2016; H. T. Lee et al. 2016), while the stomata of Potamogetonaceae have a rather odd development, perhaps suggesting that there they have been reacquired.

Ecology & Physiology. Les et al. (1997b) observed that halophily might have evolved at this node, and was subsequently lost, or it evolved twice within this clade, depending on how optimisation was carried out; Australia figures prominently in scenarios for the evolution of halophily (e.g. Les et al. 1997b; L.-Y. Chen et al. 2013). For the ecology of seagrasses, see discussion above.

Pollination Biology & Seed Dispersal. Underwater pollination, hypohydrophily, is particularly common here and pollination mechanisms throughout the group have been much studied (e.g. Pettit et al. 1980; Cox 1988; Cox et al. 1991; Ackerman 2006; Remizowa et al. 2012b; Du & Wang 2014; see also above).

Chemistry, Morphology, etc. There are many questions about plant growth, for which, see Tomlinson (1974b); information about the sympodial/monopodial growth habit is taken from Hartog and Kuo (2006). Inflorescence and flower morphology in this clade can also be difficult to interpret. For anatomy, see Sauvageau (1891, 1892), and for details of morphology, anatomy, etc., although not of the non-marine members, see Larkum et al. (2006); stomata are absent even on the carpels (Sokoloff et al. 2013c).

Phylogeny. This clade is only poorly supported in some molecular studies (Les et al. 1997b), but the relationships above were e.g. found by Les and Tippery (2013), Ross et al. (2015: strong support for all families and their relationships), Du and Wang (2014) and Du et al. (2016). However, Cymodoceaceae was found to be sister to the other families of this group by Nauheimer et al. (2012b). L.-Y. Chen et al. 2013) did not recover a [Posidoniaceae [Ruppiaceae + Cymodoceaceae] clade, rather, Cymodoceaceae were either very paraphyletic or polyphyletic, depending on one's interpretation of the tree, and they are paraphyletic in Isles et al. (2013) and unresolved in J.-M. Chen et al. (2004a). Janssen and Bremer (2004) found a clade [Ruppiaceae [Zosteraceae + Potamogetonaceae]. Ruppiaceae are sister to [Posidioniaceae + Cymodoceaceae] in a rbcL analysis of Y. Kato et al. (2003), in the analysis of Petersen et al. (2015), dominated by mitochondrial genes, but relationships in this area were generally unclear in the studies of Liu and Li (2010) and Les and Tippery (2013). A [Zosteraceae + Potamogetonaceae] clade was recovered by J.-M. Chen et al. (2004a).

For phylogenies of several of the genera included here, see Waycott et al. (2006) and Les and Tippery (2013).

[Posidoniaceae [Ruppiaceae + Cymodoceaceae]]: plant monopodial; sulphated phenolic acids +; leaves two-ranked; P 0; anthers with apical development of the connective; pollen filiform.

Age. For a possible date for this node of ca 27 m.y., see Coyer et al. (2013: c.f. topology).

POSIDONIACEAE Vines, nom. cons.   Back to Alismatales

Rhizome cortex with fibre strands, strands from leaf sheath persistent; inflorescence branched, branches with bracts, flowers sessile [ultimate units spicate]; flowers usu. perfect, bracteate, bracts unvascularized; A 3, thecae more or less separate, deciduous, connective broad, shield-like; pollen filiform, smooth, exine 0 [pollen not resistant to acetolysis]; G 1, stylulus 0, stigma complex; compitum necessarily 0; ovule sessile, campylotropous, with an outgrowth of fused integuments opposite the micropyle, outer integument ca 6 cells across, inner integument ca 4 cells across, parietal tissue ca 10+ cells across; fruit a fleshy follicle, surrounded by persistent connective; seed coat photosynthetic; first cleavage of zygote vertical; chloroplast ndh genes lost/subfunctionalized; n = 10, dimorphic; chloroplast ndh genes lost; seedling with primary root, root hairs few, or root 0.

1[list]/9. Mediterranean, temperate Australia (map: see den Hartog 1970).

• Posidoniaceae are marine "grasses" that are readily recognisable by the fibrous strands, remains of the leaf sheaths, that persist along the monopodial rhizome. These fibres form grape-fruit size balls by wave action and can be found along the beach. The inflorescence is branched.

Evolution. Ecology & Physiology. Clones of Posidonia oceanica in the Mediterranean may be up to 15 km across and thousands to tens of thousands of years old (estimates were up to 200,000 years; dissemination by fragments of plants was taken into consideration (Arnaud-Haon et al. 2012). Carbon in dense Posidonia oceanica meadows in the Mediterranean may be more than 3,000 years old, and deposits, whether as rhizomes in sediment or leaves washed up along the shore, may be massive (Mateo et al. 2006; Gobert et al. 2006).

Evolution. Seed Dispersal. The fleshy fruits can float (Ackerman 2006).

Chemistry, Morphology, etc. The morphology of the ovules is distinctive and basically uncategorizable (see also Remizowa et al. 2012b). Ma et al. (2012) described embryo sac development; it is monosporic and 4-nucleate; two of the nuclei fuse and form a diploid polar nucleus. This should be confirmed, as should the plane of division of the zygote, which was described as being vertical but looks almost oblique (Ma et al. 2012).

Additional information is taken from Kuo and McComb (1998), Gobert et al. (2006) and den Hartog and Kuo (2006), all general; for chemistry, see Heglmeier and Zidorn (2010), for germination, see Kuo and Kirkman (1997).

[Ruppiaceae + Cymodoceaceae]: leaves serrulate; flowers monosymmetric by reduction; A 2; compitum 0.

RUPPIACEAE Horaninow, nom. cons.   Back to Alismatales

Plant often in brackish or fresh water; roots unbranched; sulphates?; rhizome ?vascular tissue, ?endodermis; ?fibre bundles in leaf;; leaves 1-veined, sheath not ligulate, ± auriculate [= "stipule"]; inflorescence densely spicate, peduncle long [to 1 m] (not); plant monoecious; pollen elongate-arcuate, triaperturate; G 2-15(-many), long-stipitate, stylulus 0, stigma ± peltate/funneliform; ovules also lateral, micropyle bistomal, parietal tissue ca 7 cells across; fruit an operculate drupelet; testa 2-layered, exotegmen cells large with branched protuberances from the walls, all becoming crushed; endosperm helobial, primary root lateral, exogenous; n = 8-12, 15, dimorphic, chromosomes 0.7-4.4 µm long.

1[list]/1-10. More or less world-wide, apparently quite frequently gowing well away from the sea in all continents (map: see Hultén 1961; Fl. N. Am. 22: 2000; Heywood 1978 [for some of the southern hemisphere]; Ito et al. 2010; Fl. Austral. vol. 39: 2011; Trop. Afr. Fl. Pl. Ecol. Distr. 7. 2012).

• Ruppiaceae are submerged aquatic plants growing in saline to fresh water that may be recognised by their two-ranked, serrulate leaves with an obvious midrib and a sheathing base; the internodes are well developed.

Evolution. Seed Dispersal. The fruits are eaten by water birds; long distance dispersal is likely (Ito et al. 2010).

Chemistry, Morphology, etc. Although Haynes (1978; see also Haynes et al. 1998a) described the ovules as being campylotropous, they are shown as being straight by Gamerro (1968) and Posluszny and Sattler (1974). I follow the latter for the interpretation of stamen morphology; they mention that parietal tissue is about 1 cell across - but c.f. Haynes et al. (1998a). Ovule morphology needs to be cleared up.

Phylogeny. Ito et al. (2010) look at the biosystematics of this difficult group in which cytological variation is considerable and hybridisation is likely; Ito et al. (2015) focussed on southern African taxa.

Classification. Ruppiaceae are only doubtfully distinct from Cymodoceaceae (Les et al. 1997b).

CYMODOCEACEAE Vines, nom. cons.   Back to Alismatales

(Plant sympodial - Thalassodendron), stems erect; (roots branched), (root hairs 0); distinctive cyclitols; rhizome with two or a ring of vascular bundles; (sieve tubes with thick nacreous walls - Halodule); (adaxial peripheral foliar vascular bundles +, inverted - Syringodium), leaves two ranked, (blade terete - Syringodium), (apex serrulate); plant dioecious (monoecious); flowers in cymose groups enclosed by bracts, or solitary, terminal; staminate flowers: A (2), ± connate, (apical appendages 0 - Syringodium), (filaments 0); (microsporogenesis simultaneous - Thalassodendron); pollen filiform, (smooth), exine 0/?+; carpellate flowers: G 2, stylulus +, stigmatic branches 2, 3, long; fruit achene or drupelet, pericarp/endocarp stony, 1 (2) seeded; testa 0, (seed coat with flattened cells that have annular thickenings - Halodule); n = 7, 8, 10, 12, 14-16, chromosomes 0.22-16.3µm long; (chloroplast ndh genes lost/subfunctionalized - Amphibolis); (seed viviparous), seedling with tuft of root hairs.

5[list]/16. More or less tropical (to warm temperate), Australia in particular (map: see den Hartog 1970; van Baloogy 1975; Australia's Virtual Herbarium xii.2013). [Photo - Habit.]

Evolution: Divergence & Distribution. Both Thalassodendron and Cymodocea are known fossil from late Middle Eocene deposits from Florida although their current distribution is entirely Old World (Lumbert et al. 1984; Ivany et al. 1990).

Pollination and Seed Dispersal. For pollination, see Cox and Humphries (1993). Amphibolis and Thalassodendron are viviparous (Ackerman 2006).

Chemistry, Morphology, etc. For Remizowa et al. (2011) the flowers of Cymodoceaceae represented racemose partial inflorescences. Tomlinson (1982) described Thalassodendron as having a basal, anatropous ovule, but Takhtajan (1985) and Tomlinson and Posluszny (1978) described the ovules of Syringodium as being apical and straight. Pollen of Amphibolis is up to 5 mm long. Cymodocea is viviparous, and the cotyledon is at most small (e.g. Arber 1925: C. antarctica).

Additional information is taken from Kuo and McComb (1989, 1998: general); for cyclitols, see Drew (1983) and for chromosomes, see Kuo (2013: ?Thalassodendron) and Vanitha et al. (2016) and for floral morphology see Kay (1971) and McConchie et al. (1982).

This family needs work.

Phylogeny. For relationships in Cymodoceaeae, see Ross et al. (2014); Halodule is well supported as sister to the rest of the clade (see also Petersen et al. 2015) and Cymodocea is paraphyletic.

Classification. Trias-Blasi et al. (2015) included Rupiaceae in Cymodoceaceae.

[Zosteraceae + Potamogetonaceae]: roots unbranched; leaf with apical pore, (sheath closed); plant mono- or dioecious; compitum 0.

Age. Estimates of the time these two clades diverged range from ca 100 m.y. (Y. Kato et al. 2003) to ca 47 m.y. (Janssen & Bremer 2004); Wilf and Escapa (2014) date the Patagonian Babiancarpus, stem Potamogetonaceae, to 56-42 m.y. ago.

ZOSTERACEAE Dumortier, nom. cons.   Back to Alismatales

Plant monopodial (sympodial - Heterozostera); sulphated phenolic acids and flavonoids + (0), fructan sugars accumulated; roots in two groups/rows; rhizome cortex with two vascular bundles (several - Heterozostera), unlignified fibrous strands (not Phyllospadix); sieve tubes with thick nacreous walls; ?fibre bundles in leaf; foliar vascular bundles with xylem and phloem separated; leaves two-ranked, ligule +, (sheath closed); plant ± dioecious; inflorescence leaf opposed, beanched, with spathe and spadix, spadix axis flattened, flowers in two ranks, alternating on adaxial surface; flowers monosymmetric by reduction, bracts 0; (T 0); staminate flowers: P 1/0; A 1, anther thecae separate, joined by connective, filament 0; pollen filiform, (bicellular), smooth, exine 0 [pollen not resistant to acetolysis]; pistillode 0; carpellate flowers: staminode +, G 1, ± asymmetrical, stylulus +, stigmatic branches 2, long, ± fimbriate; ovule with outer integument to 7 cells across, parietal tissue none, 2 nucellar layers laterally, supra-chalazal area massive, postament +; fruit an achene, ribbed; exotestal cells ± anticlinally and periclinally elongate, other cells persist, ± thickened or not, tegmen degenerates; n = 6, 9, 10, chromosomes 0.9-1.6 µm long; no primary root.

2[list]/14. Temperate to subtropical (map: see den Hartog 1970; van Balgooy 1975).

Age. Divergence may have started within Zosteraceae ca 33 m.y.a. (Y. Kato et al. 2003), ca 17 m.y.a. (Janssen & Bremer 2004), or ca 23.3. m.y.a. (Coyer et al. 2013).

• Zosteraceae are sea grasses with leaf-opposed branches and flattened inflorescence axes (spadices) with flowers on the adaxial surface; the inflorescences are enclosed by a spathe. Most taxa have small tepals opposite the stamens.

Evolution. Ecology & Physiology. The genome of Zostera marina shows many changes that can be linked with adaptations to problems of living the submerged life - genes for u.v. protection, exine production, terpenoid synthesis and stomatal differentiation have all been lost, while the cell wall pectin has a distinctive composition and there are sulphated polysaccharides (galactans) involved in osmotic balance (Olsen et al. 2016; H. T. Lee et al. 2016). The ability to synthesize ethylene, an important plant hormone, seems to have been lost (see also Golicz et al. 2015) and the jasmonate and gibberlin pathways have also been affected. However, it is unclear to what extent these features might restricted to Zosteraceae, to this immediate group of largely marine Alismatales (q. v.), or to marine Alismatales in general (see above).

There appears to be significant nitrogen fixation by bacteria on the root surfaces in Zostera marina (Marbà et al. 2006).

The age of a clone of Zostera marina in the Baltic was estimated to be more than 1,000 years (Reusch et al. 1999).

Pollination Biology & Seed Dispersal. De Cock (1980) describes what goes on in Zostera marina.

Bacterial/Fungal Associations. The heterokont Labrinthula zosterae causes the wasting disease that has severely affected Zostera marina and may affect other species of the genus (K. A. Moore & Short 2006).

Chemistry, Morphology, etc. All leaves on a plant are similar in morphology.

Tomlinson (1982) suggested that the staminate flowers had two bisporangiate/monothecal anthers. The course of endosperm development is unclear.

General information is taken from Kuo and McComb (1998), Tomlinson and Posluszny (2001) and K. A. Moore and Short (2006: Zostera), reproductive morphology from Soros-Pottruff and Posluszny (1994: Phyllospadix).

Phylogeny. For relationships in Zosteraceae see Y. Kato et al. (2003), Les et al. (2001) and in particular Coyer et al. (2013); Phyllospadix is sister to the rest of the family (see also J.-M. Chen et al. 2004a).

Classification. For generic limits, see Les et al. (2001).

POTAMOGETONACEAE Berchtold & J. Presl, nom. cons.   Back to Alismatales

Often freshwater plants; vessels +; ?rhizome anatomy; (stomata +, development odd); leaves spiral, 2-ranked or opposite, with petiole and blade, blade vernation involute, primary veins merge with each other, ligule basal, sheathing, auricles 0; flowers sessile [inflorescence spicate], (inflorescence bracts + - "subtending spathe"); (plant monoecious); flowers (perfect), (2-)4-merous; P clawed, adnate to A; A 1-4; (pollen vestigial sulcate?); G (1-)4(-8), alternating with P, ± stipitate, partly ascidiate, stigma ± expanded; ; ovule becoming campylotropous, parietal tissue 4-6 cells across, (nucellar cap 2-5 cells across), (hypostase +), obturator 0; fruit a drupelet (berrylet), T persistent; seed exotestal, or coat crushed [some Potamogeton]; embryo coiled, white; n = 12-18, chromosomes 0.5-2.3 µm long.

4[list]/102: Potamogeton (60). Worldwide, esp. temperate (map: see Hultén 1961; Meusel et al. 1965; Haynes & Holm-Nielsen 2003; Kaplan 2008; Trop. Afr. Fl. Pl. Ecol. Distr. 7. 2012). [Photo - Habit, Potamogeton Inflorescence.]

Age. Divergence within Potamogetonaceae may have begun ca 25 m.y.a. (Janssen & Bremer 2004) or ca 38.3 m.y.a. (Les et al. 2003: [Zannichellia + Lepilaena]).

• Potamogetonaceae-Potamogetoneae are aquatic plants which have more or less petiolate leaves with midrib and cross veins; there is usually a conspicuous basal ligule. The inflorescence is densely spicate and the flowers have small tepals appearing to be borne on the backs of the stamens. The carpels are stipitate, as is particularly clear in Zannichellia, although that genus does not have a spicate inflorescence.

Zannichellieae Dumortier

Plants ± marine, submerged; flavone sulphates +; apical meristems of vegetative axes bifurcating; leaves ± linear, with apical pore, (margin serrulate), sheath/ligule adnate to leaf base, free ligule + (bifid); plant monoecious (dioecious), inflorescence sympodial, ± fasciculate; flowers pedicillate (sessile); staminate flowers terminal: P 0 or 3; A 1, 2-12-sporangiate; carpellate flowers: P tubular, or 3-4; G 1-8, when 3, opposite P, when 4, diagonal or cruciform, stipitate, stylulus +, stigma enlarged, peltate or infundibular with ± feathery margin; seed coat crushed.

3/13. Europe and North Africa, South Africa, the Antipodes, Zannichellia palustris almost cosmopolitan.

Evolution. For character evolution in Zannichellieae, see Ito et al. (2016); there has been some pretty serious long distance dispersal in this clade.

Pollination Biology & Seed Dispersal. Cross-pollination is by wind, or by pollen floating on the surface of the water. Pereira Nunes et al. (2012) suggested that Potomageton illinoensis might have a hyperstigma.

Potamogeton in particular is a very important source of food for ducks in North America; the fruit floats and is photosynthetic.

Chemistry, Morphology, etc. There is great variation in the leaf base, including the ligules (often called stipules), and in leaf shape both within and between species; some taxa of Potamogeton are heterophyllous, with submerged and floating leaves differing greatly in form. Potamogeton tends to have trilacunar nodes, and there has been some debate as to whether the ligule is "really" a stipule (Colomb 1887; Sinnott & Bailey 1914).

Remizowa et al. (2011) thought that the flowers of Zannicellia might represent racemose partial inflorescences. Posluszny and Tomlinson (1977) suggested that staminate flowers of Zannichellia had a single anther with up to 12 sporangia, although other interpretations seem possible; the anthers are sessile. There has also been debate as to the nature of the ovule, which is often more or less campylotropous, sometimes because of an ingrowth of the carpel wall (Takaso & Bouman 1984; Pereira Nunes et al. 2010).

Much general information is taken from from Haynes (1978) and Haynes et al. (1998b); see also Posluszny (1981) and Charlton and Posluszny (1991) for floral morphology, Pereira Nunes et al. (2009) for pollen development (esp. tetrad shape) and Kaplan et al. (2013) for chromosome numbers (not Zannichellia).

Phylogeny. The morphologically very distinctive Zannichellia and relatives, which alone in the family commonly have flavone sulphates, is rather weakly embedded within Potamogetonaceae (Les et al. 1997) or sister to the rest of the family (Les & Tippery 2013), however, the old Zannichelliaceae are quite well supported as sister to Groenlandia in the plastid phylogenomic study of Ross et al. (2015) while in Du and Wang (2014) and Du et al. (2016) Groenlandia was sister to the rest of the family. For relationships around Zannichellia, see Ito et al. (2016). Potamogeton itself is para- or polyphyletic (Les & Haynes 1995); for the phylogeny and evolution of Potamogeton in particular, see Lindqvist et al. (2006).

Synonymy: Hydrogetonaceae Link [status?], Zannichelliaceae Chevallier, nom. cons.