EXTANT SEED PLANTS

Plant woody, evergreen; nicotinic acid metabolised to trigonelline, (cyanogenesis via tyrosine pathway); primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignins rich in guaiacyl units; true roots present, apex multicellular, xylem exarch, branching endogenous; arbuscular mycorrhizae +; shoot apical meristem multicellular, interface specific plasmodesmatal network; stem with ectophloic eustele, endodermis 0, xylem endarch, branching exogenous; vascular tissue in t.s. discontinuous by interfascicular regions; vascular cambium + [xylem ("wood") differentiating internally, phloem externally]; wood homoxylous, tracheids +; tracheid/tracheid pits circular, bordered; sieve tube/cell plastids with starch grains; phloem fibers +; stem cork cambium superficial, root cork cambium deep seated; nodes ?; stomata ?; leaf vascular bundles collateral; leaves megaphyllous [determinancy evolved first, then ad/abaxial symmetry], spiral, simple, axillary buds +[?], prophylls [including bracteoles] two, lateral, veins -5 mm/mm² [mean for all non-angiosperms 1.8]; plant heterosporous, sporangia eusporangiate, on sporophylls, sporophylls aggregated in indeterminate cones/strobili; true pollen [microspores, i.e. no distal pore for release of gametes] +, grains mono[ana]sulcate, exine and intine homogeneous; ovules unitegmic, crassinucellate, megaspore tetrad tetrahedral, only one megaspore develops, megasporangium indehiscent; male gametophyte development first endo- then exosporic, tube developing from distal end of grain, to ca 2 mm from receptive surface to egg, gametes two, with cell walls, with many flagellae; female gametophyte endosporic, initially syncytial, walls then surrounding individual nuclei; seeds "large", first cell wall of zygote transverse, embryo straight, endoscopic [suspensor +], short-minute, with morphological dormancy, white, cotyledons 2; plastid transmission maternal; two copies of LEAFY gene, PHY gene duplication [N/O//A/C and P//BE lines], mitochondrial nad1 intron 2 and coxIIi3 intron present.

MAGNOLIOPHYTA

Plant woody, evergreen; lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, non-hydrolysable tannins, quercetin and/or kaempferol +, apigenin and/or luteolin scattered, [cyanogenesis in ANITA grade?], lignins derived from both coniferyl and sinapyl alcohols, containing syringaldehyde [in positive Maüle reaction, syringyl:guaiacyl ratio less than 2-2.5:1], and hemicelluloses as xyloglucans; root apical meristem intermediate-open; root vascular tissue oligarch [di- to pentarch], lateral roots arise opposite or immediately to the side of [when diarch] xylem poles; origin of epidermis with no clear pattern [probably from inner layer of root cap], trichoblasts [differentiated root hair-forming cells] 0; stem with 2-layered tunica-corpus construction; wood fibers and wood parenchyma +; reaction wood ?, with gelatinous fibres; starch grains simple; primary cell wall mostly with pectic polysaccharides; tracheids +; sieve tubes eunucleate, with a sieve plate and cytoplasm with P-proteins, companion cells from same mother cell that gave rise to the sieve tube; nodes unilacunar [1:?]; stomata with ends of guard cells level with pore, paracytic, outer stomatal ledges producing vestibule; leaves with petiole and lamina [the latter formed from the primordial leaf apex], development of venation acropetal, 2ndary veins pinnate, fine venation reticulate, veins (1.7-)4.1(-5.7) mm/mm², endings free; flowers perfect, polysymmetric, parts spiral [esp. the A], free, development in general centripetal, numbers unstable; P not sharply differentiated, outer members not enclosing the rest of the bud, smaller than inner members; A many, with a single trace, introrse, filaments stout, anther ± embedded in the filament, tetrasporangiate, dithecal, with at least outer secondary parietal cells dividing, each theca dehiscing longitudinally by action of hypodermal endothecium, endothecial cells elongated at right angles to long axis of anther; tapetum glandular, binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, binucleate at dispersal, trinucleate eventually, tectum continuous or microperforate, ektexine columellar, endexine thin, compact, lamellate only in the apertural regions; nectary 0; G free, several, ascidiate, with postgenital occlusion by secretion, few [?1] ovules/carpel, ovules marginal, anatropous, bitegmic, [outer integument often largely subdermal in origin, inner integument dermal], micropyle endostomal, integuments 2-3 cells thick, nucellus at apex of ovule 1-3 cells thick, megasporocyte single, megaspore lacking sporopollenin and cuticle, chalazal, female gametophyte four-celled [one-modular, nucleus of egg cell sister to one of the polar nuclei], stylulus short, hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, dry [not secretory]; P deciduous in fruit; seed exotestal; pollen germinating in less than 3 hours, siphonogamy, tube elongated, growing at 80-600 µm/hour, with callose plugs and callose-based walls, penetrating between cells, penetration of ovules within ca 18 hours, distance to first ovule 1.1.-2.1 mm; tube moves between nucellar cells, double fertilisation +, endosperm diploid, cellular [first division oblique, micropylar end initially with a single large cell, chalazal end more actively dividing], copious, oily and/or proteinaceous, embryo cellular ab initio, minute; germination hypogeal, seedlings/young plants sympodial; Arabidopsis-type telomeres [(TTTAGGG)_n]; whole genome duplication, single copy of LEAFY and RPB2 gene, knox genes extensively duplicated [A1-A4], AP1/FUL gene, paleo AP3 and PI genes [paralogous B-class genes] +, with "DEAER" motif, SEP3/LOFSEP and PHYA + C/PHYB + E gene pairs.

Evolution. Possible apomorphies for flowering plants are in bold. Note that the actual level to which many of these features, particularly the more cryptic ones, should be assigned is unclear, because some taxa basal to the [magnoliid + monocot + eudicot] group have been surprisingly little studied, there is considerable variation between families in particular for several of these characters, and also because details of relationships among gymnosperms will affect the level at which some of these characters are pegged. For example, if reticulate-perforate pollen is optimized to the next node on the tree (see Friis et al. 2009 for a discussion), it effectively makes the pollen morphology of the common ancestor of all angiosperms ambiguous... For other features such a a nucellus only one (Nymphaeales) to three cells thick above the embryo sac and a stylar canal lacking an epidermal layer, although plesiomorphous for basal grade angiosperms (Williams 2009), where on the tree a thicker nucellus and a stylar epidermal layer are acquired has not yet been indicated.

NYMPHAEALES [AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]: vessels + [one position], elements with elongated scalariform perforation plates; axial parenchyma diffuse or diffuse-in-aggregate; tectum reticulate-perforate [here?]; ?genome duplication; "DEAER" motif in AP3 and PI genes lost, gaps in these genes.

ALISMATALES [PETROSAVIALES [[DIOSCOREALES + PANDANALES] [LILIALES [ASPARAGALES + COMMELINIDS]]]]: ethereal oils 0; raphides + (druses 0); leaf ptyxis variants of supervolute-curved; endothecium develops directly from undivided outer secondary parietal cells, pollen boat-shaped, tectum reticulate with finer sculpture at the ends of the grain, endexine 0, (septal [epithelial] nectaries + [intercarpellary fusion postgenital]); endosperm nuclear/helobial.  Back to Main Tree

Evolution. Magallón and Castillo (2009, which consult for more details) suggest ca 162 million years for relaxed and 126.5 million years for constrained penalized likelihood datings of the divergence of Alismatales from other monocots - probably underestimates.

Chemistry, Morphology, etc. Although raphides occuring 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). For pollen morphology, see J. A. Doyle et al. (2008) for a critical evaluation; pollen of the type described above is found in the fossil record, the Liliiacidites pollen type.

ALISMATALES Dumortier  Main Tree, Synapomorphies.

Plant rhizomatous; (cyanogenesis +); mycorrhizae absent [?Tofieldiaceae; present in some Araceae]; starch grains pteridophyte-type, amylophilic; anthers extrorse, tapetum amoeboid, cells uninucleate; carpels with completely unfused canals, styles +, stigma dry [common]; endosperm helobial; embryo large, (chlorophyllous), cotyledon large; seedling with hypocotyl and root well developed. - 14 families, 166 genera, 4490 species.

Evolution. Stem-group Alismatales are dated to ca 131 million years before present, crown group Alismatales to ca 128 million years before present (Janssen & Bremer 2004; ca 133 and 103 million years before present respectively in Bremer 2000b); Magallón and Castillo (2009, which consult for more details) suggest ca 147 million years for relaxed and 126 million years for constrained penalized likelihood datings of the beginning of divergence withun Alismatales - these dates are probably underestimates. The oldest fossils assignable to this clade, only recently discovered, are from the Early Cretaceous, 110-120 million years ago (Araceae-Pothoideae-Monstereae: Friis et al. 2004, see Stockey 2006 for a review of the fossils that have been placed in Alismatales). Les et al. (2003) discuss the distributions of a number of the hydrophytic taxa of this clade; most of them are rather young.

Alismatales include the only marine angiosperms as well as many other aquatics and hydrophytes and plants of marshy habitats, and almost the only monocots with green embryos (for the latter, see also Amaryllidaceae - Amaryllideae). Cox (1988: review), Cox et al. (1991: computer simulation of underwater pollination), Cox and Humphries (1993: Cymodoceaceae), Les et al. (1997: phylogeny and hydrophily) and others discuss the remarkable pollination devices of water-pollinated Alismatales, both marine and freshwater. These include staminate flowers that detach from the parent plant and rise to the surface, the flowers themselves then floating on the water and transporting pollen to the stigma, pollen variously aggregated and forming masses especially on the water surface, and underwater pollination where the pollen is sometimes very much elongated or forms elongated strands, so increasing the chanes of pollination. These adaptations associated 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 Alismatales with more conventional life styles. The large number of small families in Alismatales is in large part because the adaptations associated with the aquatic habitat are so striking that the flowers and inflorescences in particular of these families are very different from one another.

The apparent absence of mycorrhizae in this group may be connected with the prevalence of the aquatic habitat its members prefer; mycorrhizae are usually absent in such situations. 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 and Haloragaceae, etc.

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).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. In some Alismatales there appears to be a merger between the bract subtending the flower and the abaxial tepal (Buzgo 2001). There have been suggestions that flowers in Alismatales are really pseudanthial (Soltis et al. 2005 for references). Is the pollen endexine ever lamellate (it is in Acorus)? It has been said that Alismatales, inc. Araceae, are the only monocots in which green (chlorophyllous) embryos have been reported (Seubert 1993), although they are also known from some Amaryllidaceae-Amaryllidae.

For seedling morphology, see Tillich (1985), for information on pollen, see Grayum (1992), for that on ovules, see Igersheim et al. (2001), and for carpel evolution, see Chen et al. (2004a).

Phylogeny. Tofieldiaceae were placed here with only moderate support (Källersjö et al. 1998; Chase et al. 2000a; Graham et al. 2006 - sister to the rest of the order, but limited sampling); they are rather different from other Alismatales, as can be seen by their numerous potential apomorphies. However, Tamura et al. (2004), Janssen and Bremer (2004), Givnish et al. (2006b) and Chase et al. (2006: strong support) all place Araceae as sister to rest of the order, and Tofieldiaceae then sister to the remaining taxa; this is the topology followed here. For relationships within core Alismatales, see Les et al. (1997 - on which the rest of the tree is largely based), also Kato et al. (2003), Chen et al. (2004b) and G. Petersen et al. (2006c: 2 mitchondrial and 1 chloroplast genes); a number of uncertainties over relationships remain and G. Petersen et al. (2006c) do not even recover a monophyletic Hydrocharitaceae + Alismataceae s.l. + Butomaceae. The tree in Janssen and Bremer (2004: see above) is largely similar to that below, but the positions of Aponogetonaceae and Scheuchzeriaceae are reversed, and Ruppiaceae are placed sister to Zosteraceae + Potamogetonaceae. Iles et al. (2009) and von Mering and Kadereit (2009) suggest that Juncaginaceae are paraphyletic; the position of Maundia separate from that family clarifies the gynoecial variation within Juncaginaceae s.l. However, von Mering and Kadereit (2009) were not sure of the exact position of Maundiaceae, and they found weak support for a clade [Araceae + Tofieldiaceae].

For a possible [Acorus + Alismatales] relationship, see Acorales.

Previous Relationships. Most of the taxa here included in Alismatales have been recognised as being related, thus most are in Cronquist's (1981) and Takhtajan's (1997) Alismatidae. However, Araceae have often been separated because of the supposed similarities of their inflorescence to that of some Arecaceae, etc. - thus the Spadiciflorae included those taxa with a spadix, i.e. Cyclanthaceae, Araceae and Arecaceae, three 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 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 - Alismatanae Takhtajan, Aranae Reveal, Butomanae Reveal, Najadanae Reveal, Zosteranae Doweld - Alismatidae Takhtajan, Aridae Takhtajan - Aropsida Bartling, Hydrocharitopsida Bartling, Najadiopsida Hoffmannsegg & Link

ARACEAE Jussieu, nom. cons.   Back to Alismatales

(Stem tuberous, or erect/climbing); cyanogenic glucoside triglochinin, flavone C-glycosides +; dimorphic root hypodermis +; sieve tube plastids also with starch; pseudopetiole bundles scattered; stomata unorientated, also anomo- ["basal" genera] and tetracytic; leaves usu. with pseudopetiole, lamina and (pseudo)midrib, tertiary venation cross to reticulate, base with lateral (auriculate) flanges, (ligule +); inflorescence scapose, inflorescence bract [spathe] well developed, (coloured), inflorescence densely spicate [spadix], floral bracts 0; flowers 2-3-merous, in latter case median member of outer whorl of T adaxial, T ± hooded, with single trace, free (connate), anthers extrorse; (microsporogenesis simultaneous); pollen (often starchy), ektexine +; septal nectaries 0; carpels (basally) ascidiate, fusion usu. congenital, loculus usu. with secretion, style at most short, stigma also wet; ovules tenuinucellate; fruit a berry; testa multiplicative, often parenchymatous, or with exotesta and/or endotesta and mesotesta lignified, tegmen collapsed; cotyledon not photosynthetic, (collar rhizoids or collar roots +).

106[list]/4025 - 8 subfamilies below. Mostly tropical.

Gymnostachydoideae + Orontioideae: tectum ± continuous, ovules straight, inner integument 3-4 cells thick, ± multiplicative; seedling with cataphylls.

1. Gymnostachydoideae Bogner & Nicolson

Foliar vascular bundles with fiber sheaths and girders; stomata parallel; leaves two-ranked, not petiolate, linear, venation parallel, margins minutely toothed; inflorescence complex, branched, spathe 0; flowers 2-merous [outer pair of T lateral]; A thecae forming tip above slit; G 1, ascidiate, loculus ?lacking secretion, 1 apical unitegmic ovule/carpel, micropyle 0, stigma dry; testa 0; endosperm copious, green, with starch, embryo green; n = 12; collar rhizoids 0.

1/1: Gymnostachys anceps. E. Australia (map: from Mayo et al. 1997).

2. Orontioideae Mayo, Bogner & Boyce

Flavonols +, glycoflavones 0; (collenchyma in cortical bands), bundle-associated fibre strands +/0; (laticifers + - Orontium); sieve tube plastids with no starch; leaves spiral, elliptic; (spathe 0); flowers 2-3-merous, (A also ventrifixed), ovary inferior (not - Orontium), 1-2 ± basal (hemianatropous - Orontium) crassinucellate [?: Symplocarpus] ovules/carpel, outer integument 22+ cells across, inner integument 5-10 cells across; endosperm ± 0; n = 13-15.

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

Buzgo (2001) suggests that from the point of view of floral development Orontium is more like core Araceae than is Lysichiton or Symplocarpus. Orontieae and Symplocarpeae are anatomically distinct (Keating 2003a); the former have a long internode (not always obvious) between the base of the spike and the subtending leaf or spathe and there may be commen A-C primordia (Buzgo 2001). The "cataphylls" of Orontium are relatively long, linear structures (Tillich 2003b).

Synonymy: Orontiaceae Bartling

Lemnoideae [[Pothoideae + Monsteroideae] [Lasioideae + Zamioculcadoideae + Aroideae]]: nucellar tissue disappears during ovule maturation.

3. Lemnoideae Engler

Thallus-like, stemless, floating aquatic herbs, (roots 0); collenchyma and bundle fibers 0; vessels 0; spathe + spadix inflorescence not discernable; P 0; A 1, monothecal, or 2, pollen ulcerate, spiny, trinucleate; G 1, 1(straight)-7 ovules/carpel, micropyle endostomal, embryo sac Allium-type, stigma funneliform; fruit an achene [sort of]; seed operculate; endosperm cellular, starchy, embryo undifferentiated; n = 10, extensive polyploidy and dysploidy; chalazal endosperm haustorium +; hypocotyl and primary root 0, cotyledonary sheath broad and photosynthetic.

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

Whether or not the endosperm is really cellular in Lemnoideae must be confirmed. For a general bibliography, see Landolt (1980), cytology, Urbanska-Worytkiewicz (1980), morphological details, see Landolt (1986, 1998), chemistry, etc., see Landolt and Kandeler (1987), for phylogeny in particular see Les et al. (2002) and Rothwell et al. (2004), and for speciation, see Crawford et al. (2006).

Synonymy: Lemnaceae Martynov, nom. cons., Woffiaceae Bubani

[Pothoideae + Monsteroideae] [Lasioideae + Zamioculcadoideae + Aroideae]: petiole clearly differentiated from sheathing base; (calcium oxalate crystals, inc. raphides, mixed with pollen); inner integument not multiplicative, endothelium +; seedling cataphylls +/0.

Pothoideae + Monsteroideae: stem usu. aerial; (separate stem cortical vascular system +); (vessels in stem); H- or T-shaped trichosclereids + (0), styloids +; leaves two-ranked or spiral, elliptical to complex, higher-order venation reticulate, pseudopetiole apically geniculate; crystals often surrounding the embryo.

The seeds of Pothoidaeae and Monsteroideae are described as frequently being ana-campylotropous (Seubert 1997).

4. Pothoideae Engler

(Plant monopodial; climbers); fibers ensheathing bundles; secondary and tertiary veins reticulate; spathe not enclosing spadix; flowers 2-3-merous; anther thecae often forming tip above slit, pollen forate (Anthurium), 1-2 basal/parietal apotropous ovules/carpel, outer and inner integuments 6-8 cells across, outer integument multiplicative; spathe persistent in fruit; endosperm with starch, 0 (copious - Anthurium), (embryo green); n = (10) 12 (14-15); (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.]

Pothos and relatives have monopodial shoots, distichous leaves the blades of which have two or more submarginal veins running at some distance from the margin, and flattened petioles; they are mostly climbers.

Keating (2000b) records collenchyma for a few members of Pothoideae, but for some of the same taxa, Gonçalves et al. (2004) fail to find it. The ovules of Pothos macrophyllus are shown by Buzgo (2001) as being anatropous and apotropous, although Pothos is described in the same paper as having straight ovules. The inner integument of Pothos is ca 6 cells across.

Synonymy: Pothaceae Rafinesque

5. Monsteroideae Schott

Fibers ensheathing or capping bundles; (laticifers +); flowers 2(-3 - Spathiphylleae)-merous; P often 0, pollen inaperturate, extended monosulcate or zonosulcate, (ectexine dissected - 0), 1-4(-many) often basal (hemianatropous) ovules/carpel; spathe soon deciduous; 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).

Lasioideae + Zamioculcadoideae + Aroideae: ?

6. Lasioideae Engler

Tuberous or rhizomatous, terrestrial or rooted aquatics; laticifers +; sieve tube plastids with little starch; fibers ensheathing or capping bundles; leaves ?spiral, pseudopetiole long, warty, aculeate, or strikingly coloured, ± geniculate apically; inflorescence flowering basipetally; anthers with oblique pore-like slits, pollen grains lacking starch, sulcus with ectexine lamella and thick bilayered endexine [outer: flakes or lamellae; inner: spongy]; G 1 [2-3], 1-2(-many - esp. Cyrtosperma) variously arranged apotropous ana-campylotropous ovules/loculus; seed surface usu. lamellate or warty, exotesta papillate, inner part of mesotesta and endotesta both lignified; endosperm thin (0), embryo green, curved; n = 13.

10/58. India and Southeast Asia to the Pacific (Africa - Lasimorpha) (map: from Mayo et al. 1997).

Note that although Keating (2000b) records collenchyma for a few members of this clade, for some of the same taxa, Gonçalves et al. (2004) again fail to find it. See Hesse (2002) for pollen morphology; the sulcus is unique among angiosperms. The subfamily is largely unknown embryologically, but Seubert (1997) described its seed morphology and anatomy.

Synonymy: Dracontiaceae Salisbury

Zamioculcadoideae + Aroideae: plants monoecious (dioecious); spathe differentiated into tube plus blade.

7. Zamioculcadoideae Bogner & Hesse

Biforine raphides 0; (leaves compound, petiole geniculate); staminate flowers: A (connate), introrse to extrorse, pollen in[omini]aperturate or zonasulcate with winding columellae forming a sort of internal tectum as well as the external tectum, endexine lamellate, intine thin, pistillode +; carpellate flowers: G [2], placentation axile, one ascending ovule/carpel; seeds almost lacking endosperm; n = 17.

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

8. Aroideae

Habit various; (anastomosing) laticifers +; collenchyma in cortical bands or bundle-associated strands (0); fibers associated with bundles various; (biforine raphides [wall thick, except for papillae at the two ends, lignified, cell contents mucilaginous] +); leaves spiral, very variable, (vein endings free); plants monoecious (dioecious); P 0, staminate flowers: A connate (not - Philodendron), connectives thick, (anthers introrse), pollen inaperturate, (trinucleate), (spiny), ektexine thin, often lacking sporopollenin, endexine thick, spongy, intine massive, pistillode +; pistillate flowers: staminodes +, (G 1, [2-4(-47], placentation parietal, apical, basal; many ovules/carpel; ovules orthtropous, outer integument ca 4 cells across [Pistia], crassinucellate - Calla); endosperm +/0 (with starch; storage cotyledons; embryo green); n = 7+ [esp. variable in Cryptocoryne], 13, 14, 17 common; (cotyledon sheath photosynthetic, bifacial [e.g. Colocasia, Philodendron, Xanthosoma], even leafy; collar rhizoids +).

68/2665: Philodendron (500: for a phylogeny, see Gauthier et al. 2008 - Homalomena may be part of this clade), Arisaema (170), Amorphophallus (150), Homalomena (140), Schismatoglottis (120), Alocasia (70), Xanthosoma (60), Cryptocoryne (50). Tropical and warm temperate (the latter - Arum and relatives) (map: from Mayo et al. 1997, distribution attributable to Calla alone in green). [Photo - Flowers.]

Synonymy: Arisaraceae Rafinesque, Caladiaceae Salisbury, Callaceae Bartling, 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 petaloid 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. Distinctive pollen that can be assigned to Pothooideae-Monstereae has been found in Early Cretaceous deposits of the late Barremian-early Aptian of some 110-120 million years 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). The site is now, alas, developed. Stem-group Araceae have been dated to ca 131 million years before present, crown group Araceae to ca 128 million years before present (Janssen & Bremer 2004). Araceae are not much liked 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 tranches in the leaves to interrup the latex flow and then eat out the portion of the leaf so isolated - it looks as if there are paper punch holes in the blade (Darling 2007); galerucines are known from other monocots and beetle herbivory in Araceae may be geographically more widespread.

Climbers and epiphytes are notably common in Pothoidaeae and Monsteroideae, particularly in Pothoideae. Many Araceae are plants of shaded conditions, and net veined leaves and fleshy fruits are associated with this habitat in monocots (Givnish et al. 2005).

A nectar-like but sometimes foul-tasting exudate may be produced by stigmatic hairs, etc., as in Anthurium (Daumann 1931; see also Fahn 1979) that attracts pollinators; Croat (1980) discussed pollination in this speciose neotropical genus. The spathe of Aroideae is usually differentiated into tubular and blade-like portions. The fertile flowers are restricted to the basal part of the spadix, hence being more or less enclosed by the sometimes inflated tubular portion of the spathe. Pollinators, attracted by the color of the blade, or the smell, or even the dangling apical portion of the spadix (e.g. some Arisaema) may be temporarily trapped inside the tubular portion by hairs, etc.; they are released when the staminate flowers open and they get covered with pollen. More or less unpleasant odours (to us), are common in Araceae, as is evident from common names like the marvelously-named dead horse arum (Helicodiceros [Dracunculus] muscivorus), and pollination by flies and beetles is common. Some neotropical Araceae are pollinated by euglossine bees (orchid bees; see Roubik & Hanson 2004). Thermogenesis has been detected in the flowers of a number of Araceae, and this is produced both by uncoupling proteins and the mediation of an alternative oxidase, the net result being that heat rather than energy in the form of adenosine triphosphate is produced by glycolysis (Gibernau et al. 2005; Watling et al. 2006; Onda et al. 2008; Barthlott et al. 2008 and references). The heat may volatilize compounds that attact pollinators, and/or provide a warm roost for them inside the spathe. In a number of Aroideae the pollen is extruded from the anthers in almost toothpaste-like threads; the anthers of a number of genera dehisce by pores. The arborescent South American Montrichardia (Aroideae) 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 pollinator, a hairless dynastid beetle (Weber & Halbritter 2007). For a discussion on the evolution of the distinctive pollen that characterises most Aroideae, see Hesse (2006b). Raphides and other crystals from the walls of the anther may become mixed with the pollen (Barabé et al. 2004b; Barabé & Lacroix 2008b and references). In Monsteroideae there are trichosclereids in the stylar tissue and the spathe is deciduous; the trichosclereids may protect the exposed ovary against insects (there may be a great variety of crystalline forms in differnt cells and tissues of the one plant - see Coté 2009).

Vegetative variation in Araceae is considerable. In addition to Gymnostachys, I have seen one taxon (unnamed, from Thailand) with softly dentate/spinulate leaf margins. A number of taxa have fenestrate or apparently compound leaves produced by localised cell death. The leaves of Monstera, the swiss cheese plant, are fenestrate (see Melville and Wrigley 1969), while compound-leaved taxa include Zamioculcas and Dracontium. Leaves of Zamioculcas appear to be truly compound, with localised development of the blastozone, the marginal leaf meristem, being responsible for the growth of the individual leaflets, while in Dracontium localised cell death results in what is clearly an initially simple leaf blade with entire margins separating into a complex structure with numerous "leaflets". Such leaves may be huge, thus in Dracontium gigas the dissected foliar part is up to 4 m in diameter and is born on a petiole ca 5 m tall (Bown 2000). Pulvini occur on the petiole of taxa like Dracontium. Other details of leaf development seem to be quite variable in Araceae. Scindapsus, but not Arisaema, Orontium, and Zamioculcas, develops in a "typical" monocot fashion (Troll & Meyer 1955; Bharathan 1996; Doyle 1998b); in the latter genera the blade develops from the upper part of the leaf primordium, i.e., they are similar in this to broad-leaved angiosperms. The leaves of Anthurium are notably variable, being entire to deeply lobed or even compound. The leaves of climbers are often strongly heteroblastic, the leaves of a plant in the climbing phase being simpler than when it becomes reproductive.

The highly reduced vegetative body of Lemnoideae is variously interpreted as being some combination of leaf and shoot, while the reproductive parts either represent a reduced but perfect flower or a very highly reduced inflorescence; given the phylogenetic position of Lemnoideae, the former is perhaps more likely. Wolffia and Wolffiella lack both roots and veins in the thallus, and the thallus of the former may be less than 2.5 mm across; it is the smallest flowering plant known (see Lemon & Posluszny 2000b for shoot development in Lemnoideae). The aquatic Pistia has a much less unconventional plant body, and its inflorescence, although reduced, is basically similar to that of other Aroideae; vegetative shoots are monopodial (Lemon & Posluszny 2000a). Both Lemnoideae and Pistia have supernumerary axillary buds which increases the complexity of their branching patterns. Although the early Tertiary fossil Limnobiophyllum seems "intermediate" between Lemnoideae and Pistia (Stockey et al. 1997), those two groups are not at all close in molecular phylogenies, and the fossil is to be assigned to Aroideae just as Pistia itself is to be; there are also other unrelated fossil floating aquatics in Araceae (Stockey et al. 2007).

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); Lemna, etc., also have such raphides. Raphides develop earlier than druses, at least in Amorphophallus, and may help protect young tissue, as well as aiding in regulating calcium (Prychid et al. 2008). Gonçalves et al. (2004) note 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 (cf. Keating e.g. 2000b). 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 different pathways in the evolution of imperfect flowers in Aroideae is unclear (cf. Barabé et al. 2004a). When flowers are 2-merous, the outer pair of tepals are lateral. Some taxa have binucleate tapetal cells (Wunderlich 1954). The pollen is quite often extruded in threads, but I do not know the distribution of this feature. For a discussion on the evolution of the distinctive pollen that characterises most Aroideae, see Hesse (2006b). Pistia has exostomal ovules, and in a number of taxa the ovules are tenuinucellate, in others straight, the micropyle bistomal, a nucellar cap and integumentary endothelium present, and so on. 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 but a single loculus. Gymnostachys has no micropyle since the integuments do not cover the nucellus. The megaspore that germinates is often micropylar in other than Aroideae and Calloideae, where it is chalazal, although not in e.g. Lysichiton (Grayum 1991). The uninucleate chalazal haustorium of Arum maculatum is reported to be 24,576 n (Werker 1997). Johri et al. (1992) describe the endosperm as being cellular, but it can be interpreted as being helobial. Variation in seedling morphology is extreme; in some taxa the roots are green, and in others they are always white (Tillich 1985, 2003b).

For floral morphology, see Barabé and Lacroix (2008a: Anthurium), Barabé and Lacroix (2008b: Anaphyllopsis), Fukai (2004: Arisaema).

I am grateful to Monica Carlsen and Richard Keating for discussions on Araceae and to Simon Mayo for comments. Much information is taken from Mayo et al. (1997, 1998); see also Gatin (1921: seedlings, unfortunately Gatin died before he could make more than this "première contribution"), Ertl (1932: venation and petiole anatomy, a more normal monocot venation may be common in the basal subfamilial pectinations), French (1986: endothecium), Ray (1988: shoot organisation), Tillich (1985, 2003b: seedlings), Seubert (1993: starch grains, seeds and seedlings, very variable), Dring et al. (1995: chemistry), Behnke (1995a: sieve tube plastids), French (1998: stem anatomy, extremely variable), Buzgo and Endress (1999: Gymnostachys), Bown (2000: general), Keating (2000b: collenchyma, 2003a: general anatomy, b: leaf anatomy, 2004 a: classification, b: raphides), Gonçalves et al. (2004: collenchyma), Barabé et al. (2004: anther crystals), Wilde at al. (2005), Bogner et al. (2007), and Herrera et al. (2008), all leaf fossils, Grayum (1991, 1992: pollen morphology, much variation in surface, etc.), Weber et al. (1999: pollen), Jayalakshmi (2004: pollen variation, phylogenetic framework inadequate), van der Ham et al. (2005: pollen of Amorphophallus and relatives, variable), Hesse (2006a, b: summary of pollen variation, phylogenetic framework reasonable), Bogner and Petersen (2007: chromosome numbers), Barabé and Lacroix (2008: development of Anthurium), and Tobe and Kadokawa (2008: good summary of embryological variation).

Phylogeny. 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) and a more recent tree based on the analysis of five plastid genes (Mayo et al. 2003) show somewhat less resolution. However, the clade [Gymostachydoideae + Orontioideae] remains as sister to all the rest of the family, and Lemnoideae are strongly supported as sister to the remainder; the groupings are largely those also evident in French et al. (1995). Barabé et al. (2004a) found that Calla was embedded in Aroideae, although without any strong support for this position; Lasioideae were not clearly separated from Aroideae, but again the support was weak (below 50% bootstrap in both cases). Although a recent 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 given here. The exact position of Zamioculcadoideae is still uncertain, but they can reasonably be excluded from Aroideae. There is no strong molecular support for a clade [Zamioculcadoideae + Aroideae], but the members of this larger clade do have some morphological features in common. In gross floral morphology Stylochaeton goes in Zamioculcadoideae, but it has the same pollen as most Aroideae and it has simple leaves (Hesse et al. 2001). Bogner and Hesse (2005) raised the group [Zamioculcas + Gonatopus] to subfamilial status as Zamioculcadoideae, and the subfamily as so circumscribed had numerous distinctive features. However, since Stylochaeton is sister to other Zamioculcadoideae, its reasonable (see Cabrera et al. 2008) inclusion makes that subfamily notably less distinctive morphologically. It may be of interest that the pollen of Lasioideae, at least, has a lamellate endexine rather like that of Zamioculcadoideae.

See Tam et al. (2004) for what is known about relationships within Pothoidaeae and Monsteroideae. Cabrera et al. (2008) offer a number of suggestions about tribal relationships in Aroideae. Gonçalves et al. (2007) discuss the phylogeny of the Andean Spathicarpeae, a clade in which the spathe is adnate to the spadix, many of which grow in very dry and/or high conditions. Calla palustris, in early (prior to ed. 8) versions of this site placed in a separate subfamily, seems best included in Aroideae, with which it agrees in lacking a perianth but differs in having biporate pollen and perfect flowers - and it has a more northerly distributioon (see Map), overlapping with that of other members of the subfamily only in Western Europe and N.E. North America. It is placed in a clade of Aroideae along with other rooted aquatics/marsh plants in molecular studies (see Cabrera et al. 2008), and it is possible its perfect flowers represent a reversal.

Classification. For a tribal classification, see Cabrera et al. (2008). For a checklist and bibliography, see Govaerts and Frodin (2002) and the World Checklist of Monocots, and for several keys and lots more, see CATE-Araceae.

Tofieldiaceae [Hydrocharitaceae + Scheuchzeriaceae groups]: carpels free.

TOFIELDIACEAE Takhtajan   Back to Alismatales

Steroidal saponins, chelidonic acid +; vessels?; fibers mixed with phloem; sieve tube plastids also with polygonal crystals; stomata anomocytic; also prismatic crystals +; leaves two-ranked, equitant and isobifacial [oriented edge on to the stem]; inflorescence a raceme, bracteate, (bract 0; flower single - Harperocallis), calyculus below individual flowers (some Tofieldia 0); T free (basally connate), with one trace [Tofieldia], median member of outer whorl adaxial [Tofieldia]; A (9-12 - Pleea; adnate to base of P; basally connate), introrse to latrorse, tapetum glandular, microsporogenesis simultaneous, pollen di(trichotomo)sulcate, septal or tepal nectaries +; G stipitate, carpels mostly plicate, periphery completely postgenitally fused, (placentation parietal), 5-many unitegmic thinly crassinucellate ana-campylotropous ovules/carpel, integumentary obturator +, (long chalazal extension +), (style + - Isidrogalvia); fruit a septicidal capsule or follicle ["ventricidal" capsule], (P persistent); seeds obliquely stacked, with appendages, phlobaphene +, tegmen thin; ?embryo; n = (14) 15 (16), chromosomes 0.9-2.5 µm long; radicle 0?, collar rhizoids + [Harperocallis].

3-5[list]/27. S.E. U.S.A., N.W. South America, N. temperate (map: see Hultèn 1961; Meusel et al. 1965; Hultén & Fries 1986; Fl. N. Am. 26: 2002). [Photo - Flowers] [Photo - Flowers]

• 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 stylodium. The fruit is usually a septicidal capsule.

Evolution. Stem-group Tofieldiaceae are dated to ca 124 million years before present, crown group Tofieldiaceae to ca 100 million years before present (Janssen & Bremer 2004: note their topology).

Chemistry, Morphology, etc. Branching in Tofieldiaceae needs study. Remizowa et al. (2005) suggest that the first two leaves of axillary shoots in Tofieldia (the adaxial prophyll and also the next leaf) are adaxial; this would be a very unusual arrangement, 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). The nectaries of Tofieldiaceae may be unique to the group; 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 a,d the nectary-less condition.

For Harperocallis, described from Florida in 1968, see McDaniel (1968), for general information, see Ambrose (1975, 1980), Zomlefer (1997b) and Tamura (1998 - as Nartheciaceae), for ovary, see Sterling (1979), for inflorescence and floral morphology, see Remizowa and Sokoloff (2003) and Remizowa et al. (2006a, b), for sieve tube plastid type, see Behnke (2000, 2003), and for ovule development, see Holloway and Friedman (2008).

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

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

Hydrocharitaceae + Scheuchzeriaceae groups: plant aquatic (with floating stems), leaves emergent; stem with lacunae; little oxalate accumulation; raphides and druses 0 (prismatic crystals +); hairs 0; bulliform cells 0 [?this level]; pollen grains trinucleate; carpel fusion partial at the periphery; endosperm 0; seedling collar and collar rhizoids +.

Evolution. The divergence of these two clades is ca 107 million years before present (Janssen & Bremer 2004).

All members of this clade grow in more or less marshy conditions, and all the marine angiosperms are to be found here. Cox and Humphries (1993) and Les et al. (1997) and references therein discuss the pollination devices of water-pollinated Alismatales, both marine and freshwater. Chen et al. (2004b) discuss the evolution of various life forms in the group - parallelisms are common. Sulphated phenolic compounds are common in seagrasses, including members of Hydrocharitaceae (McMillan et al. 1980), and probably arose in parallel; their function is unclear, although they may be involved in adaptation to life in the marine habitat.

Chemistry, Morphology, etc. Resting buds are produced sporadically throughout this group. Non-medullated roots are quite common, occuring in e.g. Butomaceae, Alismataceae, Limnocharitaceae (Stant 1964, 1967), Aponogeton, Triglochin, Potamogeton, although roots of e.g. Posidonia are medullated (von Guttenberg 1968). 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 these aquatic Alismatales (G. Petersen et al. 2006). Thickened (nacreous) walls occur in the sieve elements of a variety of seagrasses (Kuo. 1983).

Much general information is taken from Tomlinson (1982); see Zindler-Frank (1976) for oxalate accumulation and Kuo (1983) for sieve element walls in sea grasses. There have been extensive cytological studies in the group, see e.g. Harada (1956), Uchiyama (1989), Sharma and Chatterjee (1967), and Costa and Forni-Martins (2003). Wilder (1975) discussed vegetative branching, inflorescence morphology, etc.

Classification. Den Hartog (1970) gives a comprehensive taxonomic account of the marine Alismatales, while Green and Short (2003) provide a general ecological account, along with distribution maps, etc.

Alismataceae [Butomaceae + Hydrocharitaceae] : apical meristems of vegetative axes bifurcating; axillary squamules +; C-glycosyl flavones +; inflorescence scapose, determinate, bracteate; P = K + C, members of both whorls with many traces, (androecium with trunk bundles; stamen pairs +), carpels plicate; seeds exotestal; chromosomes (0.8-)2-13.6 µm long.

Chemistry, Morphology, etc. The prophylls of Limnocharis (Alismataceae) and Vallisneria (Hydrocharitaceae) may not be in the normal adaxial position (Wilder 1975).

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

ALISMATACEAE Ventenat, nom. cons.   Back to Alismatales

Plant with latex; (corms, stolons); (unicellular or stellate hairs); flavone and phenolic sulphates, tannins + (0); (vessels 0); stomata with parallel divisions; leaves two-ranked to spiral, involute, with pseudopetiole, midrib, cross veins and an apical subepidermal pore, primary veins merge or not with each other, (inverted vascular bundles); (plant mon- or dioecious); inflorescence branches whorled; C thin, more or less crumpled in bud, evanescent, nectar at base of C, A, or P, or from staminodes or carpel flanks, A 3-many, centrifugal or centripetal, pollen pantoporate (pollen 0-3 porate - Caldesia), spinose; G 2-many, free or connate basally, with residual floral apex, partly ascidiate, with one basal-lateral to many laminar ovules; ovules apotropous tenuinucellate, embryo sac tetrasporic (monosporic, 4-nucleate), basal cell of suspensor enlarged; fruit a follicle or achene; exotesta with outer wall thickened, (thin-walled, cells with upturned ends [Limnocharis]; with glandular hairs), tegmen ± obliterated or walls ± thickened; embryo strongly curved; n = (5-)7-8(-13), chromosomes 2.4-14.4 µm long.

15[list]/88. Pantropical, also temperate (map: see den Hartog 1957; Hultèn 1961; Meusel et al. 1965; Haynes & Holm-Nielsen 1997). [Photo - Flower, Echinodorus Flower, Fruit, Sagittaria Flower, Limnocharis vegetative, Hydrocleis flower]

• Alismataceae are laticiferous, aquatic herbs. Their floating or aerial leaves are petiolate and have a prominent midrib, parallel veins and also cross 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. In at least some species of Ranalisma and Sagittaria the pollen tubes may grow down the style into the floral axis and thence into adjacent carpels (Wang et al. 2002, 2006).

The individual fruitlets of Limnocharis separate from the axis and float; seeds that they contain may be dispersed by this means.

Chemistry, Morphology, etc. 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). Alisma and relatives have granular, not spinose pollen and nuclear not helobial endosperm. 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 the old Limnocharitaceae (e.g. Leins & Stadler 1973); the carpels are connate laterally, there are many ovules per carpel, and placentation is laminar.

General information is taken from Haynes, Les and Holm-Nielsen (1998: as Alismataceae and Limnocharitaceae) and Hooper and Symoens (1982: as Limnocharitaceae); for vegetative anatomy, see Stant (1964), and for floral development, see Sattler and Singh (1977), Wang and Chen (1997) and K.-M. Liu et al. (2002).

Phylogeny. Details of relationships between Alismataceae and Limnocharitaceae, as well as within Alismataceae, are still rather unclear (Soros & Les 2002; Kato et al. 2003; Chen et al. 2004a, b; von Mering & Kadereit 2009; 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 showing continuous variation).

Classification. Alismataceae include the "old" Limnocharitaceae (first recognized by Takhtajan in 1954!) here, and they certainly have much in common; if Limnocharitaceae were segregated, Alismataceae would have practically nothing by which they could be recognised..

Synonymy: Damasoniaceae Nakai, Limnocharitaceae Cronquist

Butomaceae + Hydrocharitaceae: ?

The divergence of these two clades is ca 88 million years before present (Janssen & Bremer 2004).

Butomaceae are embedded in Hydrocharitaceae in a rbcL analysis of Kato et al. (2003).

BUTOMACEAE Richard, nom. cons.   Back to Alismatales

Plant monopodial; flavonols?; leaves ± two-ranked; inflorescence umbellate, with subtending bracts, (floral bracts 0); P whorls both petaloid, not identical; A 9, some latrorse, nectar from carpel flanks; G 6, plicate, fusion postgenital, placentation laminar, many crassinucellate ovules/carpel, secretion covers placentae, stigma ± decurrent; fruit a follicle; outer walls of exotestal cells thickened and with encrustations, tegmen persists; embryo and color?; 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 petaloid; the fruit is a follicle.

Stant (1967) reports crystals "in the form of small rods" 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 Limnocharitaceae. 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).

Much information is taken from Cook (1998).

HYDROCHARITACEAE Jussieu, nom. cons.   Back to Alismatales

Branching?; flavone and phenolic sulphates +; vessels 0; endodermis obscure or thick-walled; (prophyll lateral); leaf base sheathing [?type] or not; inflorescence subtended by 2 often connate bracts; /androecium with trunk bundles?, (A introrse), pollen inaperturate, spinulate, trinucleate, nectaries 3, staminodial (0); G inferior, also basally ascidiate [Stratiotes, Limnobium], closure by secretion only, ovary loculi filled with secretion, placentae parietal, laminar, or subbasal, 1-many ovules/carpel, outer integument often ³3 cells across, micropyle bitegmic, style single, short, stigmas usu. divided; fruit often ± fleshy; stone cells in mesotesta and endotesta (all testal cells ± thickened except the outermost wall), endotegmen with tuberculate inner wall alone persisting; chalazal endosperm and suspensor haustoria +; n = notably variable, chromosomes (0.8-)2-10 µm long; cotyledon bifacial; extensive loss of mitochondrial genes.

18[list]/116 - 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 - incomplete: van Steenis & van Balgooy 1966; den Hartog 1970 for marine taxa).

1. Hydrocharitoideae Eaton

Leaves involute or convolute, broad, margins entire, vascular bundles inverted, pseudopetiole +, ligules basal, adaxial or paired lateral [totally enclosing young leaves]; plants monoecious; staminate flowers: K 3, C 3; A 1-6; carpellate flowers: K 3, C (0) 3; G [3-9], placentae strongly intruded, ovules orthotropous; exotestal cells much enlarged; n = 7-11, 13-15.

2/5. Temperate and subtropical. Also Limnobium.

Stratiotoideae [Anacharidoideae + Hydrilloideae]: roots unbranched; (ovules straight).

2. Stratiotoideae Luersson

Leaves tristichous, in rosettes, spiny; plant dioecious, K 3, C 3; staminate flowers: A many, adaxial 5-17 fertile; carpellate flowers: staminodes +; n = 10+.

1/1: Stratiotes aloides. Eurasian.

Synonymy: Stratiotaceae

Anacharidoideae + Hydrilloideae: leaves submerged.

3. Anacharidoideae Thomé

Root trichoblasts 0 [Blyxa]; leaves whorled to spiral, (usually opposite when scales), narrow, margins entire to spiny, pseudopetiole 0, (lamina broad, pseudopetiole +; fine veins reticulate - Ottelia); inflorescences axillary; plant dioecious (flowers perfect [Apalanthe, also Ottelia], the latter C large; A to 15); staminate flowers: released, usually as buds; P 3 + 3 (3); A 3 (+ 3 staminodes) or 6, (dorsifixed; pollen with discontinuous exine, little or no sculpturing); carpellate flowers: hypanthium +, usu. long; P 3 + 3; staminodes +; G [3(-20+)]; fruit a capsule, indehiscent or irregularly dehiscent; 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.

Synonymy: Blyxaceae Nakai, Elodeaceae Dumortier, Otteliaceae Chatin

4. Hydrilloideae Luersson

(Marine); root trichoblasts 0 [Vallisneria]; leaves (spiro)two-ranked or whorled, narrow, margins entire or toothed, pseudopetiole 0, (lamina broad, pseudopetiole + - Halophila); inflorescences axillary; perianth biseriate, undifferentiated, uniseriate, (0); staminate flowers: (released as buds); A 1-3, (1 staminode); carpellate flowers: hypanthium +; staminodes (2-)3; G [2-9]; fruit fleshy, capsular, or dehiscing irregularly; (exotegmic tuberculae +); n = 6-8, 10, 12, 15.

8/61: Naias (40). Tropical and subtropical, especially Old World; Naias 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 Link

• 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. Stem group Hydrocharitaceae are dated to ca 88 million years before present, crown group Hydrocharitaceae to ca 75 million years before present (Janssen & Bremer 2004).

There is much vegetative variation. Hydrocharitaceae such as Hydrilla and Egeria have C₄ photosynthesis with the metabolic compartmentalisation needed occuring within single cells (Bowes et al. 2002 for references). Marine taxa are rhizomatous, with leaf-bearing short shoots; taxa like Elodea have leaves borne all along the stem, while others have whorled leaves; Stratiotes aloides forms floating rosettes, and the plant sinks to the bottom of the water in winter, rising to the surface in the summer. Leaf shape and margin also vary a great deal.

There is a diversity of pollination mechanisms in Hydrocharitaceae, including entomophily, anemophily, epi- and hypohydrophily, and selfing, and these are associated with often quite considerable modifications of the flowers, hence the eleven family names available for the eighteen genera. Parallelism is pervasive. Hypohydrophily has evolved at least twice (e.g. Naias, 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 borne above the surface of the water on reflexed sepals can be 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 and the marine Enhalus; in the former genus, there is a maximum of a mere 24 pollen grains per staminate flower. In the marine Halophila the pollen is released in chains, and pollination is under the water. The insect-pollinated Blyxa has secondary pollen presentation. For more details, see Cook (1982, 1996, especially 1994-1995 [in german]).

Chemistry, Morphology, etc. The plants may be tanniniferous. Hydrocharis, apparently alone in the group, has a root epidermis that is of inner epidermal origin. Branching in Hydrocharitaceae needs more study; they produce only precocious axillary buds along their stems (Wilder 1975), and pseudodichotomous branching is quite common, and this is often interpreted as being the result of this precocious axillary branching (Tomlinson 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 considerable variation in bracteole number and position (lateral; paired, etc.: see Eichler 1875) in the family.

Elodea is shown as having its carpels opposite the inner perianth whorl (Eichler 1875). The carpels may be more or less free (Weberling 1989 for references). 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). Blyxa has notably short chromosomes (Uchiyama 1989). The mitochondrial nad1 intron 2 is absent in two representatives of this family (Gugerli et al. 2001), although just how widely this loss is spread is unclear.

Naias is particularly distinctive. Its leaves have an "auricle" at very base. Its staminate flowers have two envelopes and a single stamen, while the carpellate flowers have a single envelope, a single ascidiate carpel with one basal ovule, and the style is one- or two-branched. The fruit is an ?achene, sometimes with an exotegmen, the endosperm is nuclear, the embryo green; the cotyledon is unifacial. Najas has a well-developed root system, the roots being unbranched, but with many root hairs. Variation in chromosome length (1.6-11.7 µm) is considerable. For suggestions that the ovary of Najas, apparently superior, might really be inferior, see Miki (1937); floral development would repay re-examination.

General information is also taken from Cook (1998), Haynes, Holm-Nielsen et al. (1998), and Haynes and Holm-Nielsen (2001); for testa anatomy, see Shaffer-Fehre (1991a, b), and for pollination mechanisms, see Cook (1982, and especially 1994-1995), these correlate with pollen morphology and phylogeny, see Tanaka et al. (2004).

Phylogeny. Tanaka et al. (1997) suggest a series of quite well-supported nodes based on analysis of variation in two genes, the ultimate groupings recognised there are similar to those of Les et al. (1997). 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 not given because only a single outgroup was used, see above for composition of the clade), and then Stratiotoideae sister, but with only moderate support (72% bootstrap, all characters) to [Anacharidoideae + Hydrilloideae] (52%); however, all the subfamilies were individually strongly supported as being monophyletic. Within Hydrilloideae, Naias is 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. Despite the obvious morphological differences between Naias and other Hydrocharitaceae, Posluszny and Charlton (1999 and references) note that both branching and seed anatomy link them. Also within Hydrilloideae, Enhalus is grouped with [Halophila + Thalassia] - the three marine taxa form a single clade.

For a phylogeny of Valisneria, see Les et al. (2008).

Classification. Here I follow the classification suggested by Les et al. (2006).

Scheuchzeriaceae [Aponogetonaceae [Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]]]: primary root poorly developed; P members with a single trace; A usu. with short filaments; chromosomes (0.5-2.3(-4.5) µm long.

Evolution. The first branch in this clade (Aponogetonaceae versus the rest!) is dated to ca 98 million years before present (Janssen & Bremer 2004).

Chemistry, Morphology, etc. There has been discussion as to the nature of the small, perianth-like structures closely associated with the stamens that are found in many members of this clade. Members of the inner perianth whorl of Juncaginaceae may be borne internally to the outer stamens (Dahlgren et al. 1985). Sattler (1962) and Singh (1965) considered the perianth and androecium of Potamogetonaceae to be distinguishable although there is but a single common trace to the P/A pairs. However, this tepal-like structure is also called a retinaculum, and then considered not to be tepalline in origin; von Mering and Kadereit (2009) suggest that the clade [Maundiaceae + the rest] may even be characterized by a perianth-less flower, the apparent perianth members simply representing some kind of enation (several taxa do entirely lack a perianth of any sort). Here these structures are considered to be tepals, some flowers in this clade representing extreme examples of the close association between a tepal and the stamen opposite it that is common in monocots.

Phylogeny. Aponogetonaceae and Scheuchzeriaceae are sister taxa and in turn are sister to the other members of the group in a rbcL analysis of Kato et al. (2003); relationships between these two families were unclear in von Mering and Kadereit (2009).

Classification. There is a plethora of small families in this clade, in part because of the very distinctive floral and vegetative morphologies that have evolved in connection with the aquatic habitat its members favour. Maundiaceae are provisionally recognised below, further increasing the number, but rationalization may well be in order (see A.P.G. III 2009); Hydrocharitaceae s.l. might be the model to follow.

APONOGETONACEAE J. Agardh, nom. cons.   Back to Alismatales

Plant with a short rhizome or corm, apical meristems of vegetative axes bifurcating [?all]; vessels 0; articulated laticifers +; leaves spiral, involute, with pseudopetiole, midrib and cross veins, primary veins merge with each other, no tertiary veins, apex of old leaves with pore; plants usu. monoecious or dioecious; inflorescence spicate, bracts 0; (flowers monosymmetric); P 2-6, staminate flowers: (A -16; introrse; stamen pairs +;); microsporogenesis also simultaneous; pollen reticulum uniform; pistillode ?; carpellate flowers: (P 0); staminodes +; G 2-9, alt. P, plicate, 1-12 basal ovules/carpel, septal nectaries + (0); ovules (unitegmic), nucellar cap +; fruit a follicle; coat mucilaginous, exotesta protective or not, endotegmen tanniniferous, or undifferentiated and translucent; embryo green or not; n = ?12, 16, 19, etc., chromosomes 0.7-2.3 µm long; cotyledon bifacial.

1[list]/43. Old World, esp. South Africa, largely tropical and warm temperate, esp. in Southeast Asia-Malesia a number of localities (not on map) where it is under suspicion of being introduced (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 cross veins; the finest veins are reticulate. 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.

Chemistry, Morphology, etc. For cell death and the development of the fenestrate leaves of Aponogeton madagascariensis, see Wright et al. (2009). A bract may form an hybrid organ with a tepal, so making the flower slightly monosymmetric, but 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 (see Singh & Sattler 1976b).

Some information is taken from van Bruggen (1990, 1998); for hybridization between Madagascan and Indian taxa, etc., see Yadav (1995), for leaf development, see Gunawardena et al. (2004), for phylogeny, see Les et al. (2005).

Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]: sulphated phenolic acids +, leucanthocyanins, flavones 0; leaf ± linear, base auriculate, ligulate; P 0 or reduced ["abaxial outgrowth" of A], anthers sessile, pollen inaperturate, nectary 0, carpels with complete postgenital fusion [sampling!].

For distribution of sulphated compounds, see especially McMillan et al. (1980).

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

Plant irregularly sympodial; cyanogenic glucoside triglochinin +, flavonoids 0; stem endodermis +; stomata tetracytic; leaves two-ranked, auriculate, with apical pore; inflorescence a raceme, bracts +; pollen in dyads, inaperturate, nectary 0; G 3(-6), plicate, basally connate, plicate, fusion usually congenital, opposite outer P, (1) 2 subbasal crassinucellate ovules/carpel, outer integument 5-6 cells across, inner integument ca 4 cells across; fruit a follicle; testa smooth, anatomy?; embryo green; 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 distichous 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. Some information is taken from Haynes, Les et al. (1998).

Aponogetonaceae [Juncaginaceae [Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]]: fruit indehiscent.

Chemistry, Morphology, etc. Rudall (2003b, see also references) has suggested that the "flowers" of all or many of the taxa in this group are pseudanthia.

JUNCAGINACEAE Richard, nom. cons.   Back to Alismatales

Apical meristems of vegetative axes bifurcating [?all]; O- and C-glycosyl flavones, cyanogenic glucoside triglochinin +; stem endodermis + or 0; (laticifers - Lilaea); stomata also tetracytic, with parallel divisons; leaves spiral, ± unifacial (subequitant), (ligules - Triglochin); inflorescence ± scapose spike or raceme, (flowers polygamous; sessile), bracts 0 (+); flowers 1-3-merous, monosymmetric, "P" 0-4, 6; A subsessile, 3-8; G 1 [3-10], weakly connate, fertile carpels oposite inner P, plicate, 1-few basal crassinucellate ovules/carpel, outer integument ³3 cells across, (styles long - some Lilaea); fruit schizocarpic, achenial (hooked, winged); exotesta and entegmen with cuticle, otherwise crushed; endosperm 0, nuclear, basal cell of suspensor enlarged [Lilaea], embryo with short thick hypocotyl, color?; n = 6, 8, 15, etc., chromosomes 0.6-1.1 µm long; hypocotyl 0.

3[list]/15. Cosmopolitan, but largely coastal (map: see Hultèn 1961; Meusel et al. 1965; Hultén & Fries 1986; Fl. N. Am. 22: 2000; FloraBase 2004). [Photo - Habit, Fruit.]

• Juncaginaceae are herbs of marshy places with more or less unifacial leaves and a scapose racemose or spicate inflorescence; all parts of the flower appear free.

Evolution. Stem-group Juncaginaceae are dated to ca 82 million years before present, crown group Juncaginaceae to ca 52 million years before present (Janssen & Bremer 2004).

Chemistry, Morphology, etc. Imperfect flowers may lack a perianth (Lilaea) and either stamens or carpels; 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 bract-like (Buzgo 2001; Buzgo et al. 2006). There is no evidence of pseudanthia in the family; 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) and Haynes, Les et al. (1998); for alternative interpretations of the gynoecium, see Igersheim et al. (2001).

Synonymy: Heterostylaceae Hutchinson, Lilaeaceae Dumortier, Triglochinaceae Chevalier

[Maundiaceae [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]]: 1 apical pendulous straight ovule/carpel.

MAUNDIACEAE Nakai   Back to Alismatales

Leaves triangular; flowers sessile, bracts 0; P 2-4; A 4-6, thecae largely separate but adnate by the connective; G [3-4], styluli marginal, recurved; fruit a schizocarp.

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

Chemistry, Morphology, etc. See von Mering and Kadereit (2009) for the interpretation of the androecium; as they note, similar stamens are found in Posidoniaceae and Zosteraceae... However, Maundia is largely unknown.

[[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]]: aquatics; rhizome with endodermis; epidermis chlorophyllous; water pollination; carpels ascidiate [sampling!], fruit ± drupaceous; embryo with massive elongated hypocotyl, also prominent in seedling, collar or base of hypocotyl much enlarged.

Evolution. The first split within this clade can be dated to ca 73 million years before present (note that the topology from which this date was taken is slightly different from that used in the tree above), and other divergence dates are given for members of this group (Janssen & Bremer 2004).

Underwater pollination, hypohydrophily, is particularly common here and has been much studied (e.g. Pettit et al. 1980; Cox 1988; Cox et al. 1991).

Phylogeny. This group is only poorly supported in molecular studies (Les et al. 1997).

[Posidoniaceae [Ruppiaceae + Cymodoceaceae]: ± marine; vessels 0; stomata 0; leaves two-ranked; pollen filiform.

Posidoniaceae may have diverged from [Zosteraceae + Potamogetonaceae] ca 65 million years before present (Janssen & Bremer 2004).

Ruppiaceae are sister to [Posidioniaceae + Cymodoceaceae] in a rbcL analysis of Kato et al. (2003).

POSIDONIACEAE Hutchinson, nom. cons.   Back to Alismatales

Plant monopodial, with copious unlignified fiber strands; inflorescence pedunculate, branched-racemose, bracts 0; P 0; A 3, sessile, thecae more or less separate, opening basally, connective broad, shield-like, with prolongation [= P?]; G 1, stigma sessile, complex; fruit a fleshy follicle [?: pericarp fleshy]; n = 10, dimorphic; seedling?

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 leaves, that persist along the monopodial rhizome and by their branched-racemose inflorescences.

Chemistry, Morphology, etc. Information is taken from Kuo and McComb (1998).

Ruppiaceae + Cymodoceaceae: leaves serrulate; A 2.

RUPPIACEAE Horaninow, nom. cons.   Back to Alismatales

Plant monopodial, often in brackish or fresh water; sulphates?; vessels 0; endodermis?; leaves 1-veined, sheath not ligulate, ± auriculate [= "stipule"]; inflorescence terminal, spicate, bracts 0; A sessile, with abaxial outgrowth, pollen elongate-arcuate; G (2-)4(-16), stipitate, ovules also lateral, campylotropous, micropyle bistomal, stigma sessile, ± peltate; fruit an operculate drupelet; testa 2-layered, exotegmen cells large with branched protuberances from the walls, all becoming crushed; n = 8, 10-12, dimorphic, chromosomes 0.7-4.4 µm long.

1/1-10. 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 the southern hemisphere]).

• 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.

Chemistry, Morphology, etc. Some information is taken from Haynes, Holm-Nielsen et al. (1998); there are reports of nuclear endosperm (Johri et al. 1992).

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

CYMODOCEACEAE N. Taylor, nom. cons.   Back to Alismatales

Distinctive cyclitols; (sieve elements with thick nacreous walls - Halodule); leaves serrulate at apex; plant monoecious or dioecious; inflorescence cymose, flowers in groups enclosed by bracts or solitary; P 0; staminate flowers: A [2], filament present, (microsporogenesis simultaneous - Thalassodendron); carpellate flowers: G 2, styles long, often branched, stigma long; fruit an achene or drupelet; testa 0; endosperm nuclear; n = 7, 8, 10, 14, 15; ?seedling.

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

Chemistry, Morphology, etc. This family needs work. Note that Tomlinson (1982) described Thalassodendron as having a basal, anatropous ovule, while Takhtajan (1985) suggested that the ovules of Syringodium were apical and straight. Pollen of Amphibolis is up to 5 mm long. Cymodocea antarctica is viviparous, and the cotyledon is at most small (see Arber 1925).

Some information is taken from Kuo and McComb (1998: general); for cyclitols, see Drew (1983).

Zosteraceae + Potamogetonaceae: leaf with apical pore, (sheath closed); plant mono- or dioecious; P and A pair with single vascular trace.

Evolution. Estimates of the time these two clades diverged range from ca 100 million years before present (Y. Kato et al. 2003) to ca 47 million years before present (Janssen & Bremer 2004).

ZOSTERACEAE Dumortier, nom. cons.   Back to Alismatales

Marine, main stems monopodial, adventitous roots unbranched; also flavone sulphates +; vessels 0; rhizome cortex with fibrous strands (not Phyllospadix) and vascular bundles; sieve elements with thick nacreous walls; stomata 0; leaves two-ranked, apical pore developing; inflorescence with spathe and spadix, spadix axis flattened, flowers two ranked, alternating on adaxial surface, bracts 0; staminate flowers: P 1 ["retinaculum"]; A 1, anther thecae separate, ± dorsifixed, deciduous, joined by divided connective, pollen filiform, (binucleate); carpellate flowers: G [2], appearing to be attached tranversely on pedicel, one fertile, outer integument to 7 cells across, style short, styles long, stigmatic; fruit achenial, 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). [Photo - Zostera Inflorescence © D. Woodland]

• 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 "retinacules", perhaps tepals or bracts, opposite the stamens.

Evolution. Within Zosteraceae, divergence may have started ca 33 million years before present (Y. Kato et al. 2003).

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) and Tomlinson and Poslusny (2001), reproductive morphology from Soros-Potruff and Posluszny (1995).

Phylogeny. For relationships in Zosteraceae see Y. Kato et al. (2003) and Les et al. (2001).

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

POTAMOGETONACEAE Reichenbach, nom. cons.   Back to Alismatales

Freshwater plants, (apical meristems of vegetative axes bifurcating - Zannichellia); sulphated compounds 0, (flavone sulphates +, vessels 0 - Zannichellia); leaves spiral or opposite, involute, pseudopetiole, midrib and cross veins common, primary veins merge with each other, (margin serrulate), ligule basal, sheathing, (adnate to leaf base), auricules 0; inflorescence spicate, bracts 0 (+), (2-)4-merous; "P" clawed, adnate to A, (A with up to 12 loculi); G (1-)4(-8), alternating with P, ± stipitate, partly ascidiate - or [Zannichellia] inflorescence proliferating sympodially; staminate flowers: P 0 or 3; A 1, 2-8-sporangiate; carpellate flowers: P tubular, or 3-4; G 1-8, when 3, opposite P, ovule ± pendulous, apotropous, straight to campylotropous, stigma enlarged, peltate or infundibular with ± feathery margin; fruit a drupelet (berrylet); seed exotestal, or coat crushed [some Potamogeton, Zannichellia]; embryo coiled, white; n = 7, 12, 14-18, 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). [Photo - Habit, Potamogeton Inflorescence.]

• Potamogetonaceae are aquatic plants most of 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.

Evolution. Divergence wthin Potamogetonaceae may have begun ca 25 million years before present (Janssen & Bremer 2004).

Potamogeton in particular is a very important source of food for ducks in North America. Cross-pollination is by wind, or by pollen floating on the surface of the water.

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. In Potamogeton the fruit floats and is photosynthetic.

Much general information is taken from from Haynes (1978) and Haynes, Les et al. (1998); for the phylogeny and evolution of Potamogeton in particular, see Lindqvist et al. (2006).

Phylogeny. The morphologically very distinctive Zannichellia, alone commonly with flavone sulphates, is rather weakly embedded within Potamogetonaceae (Les et al. 1997). Potamogeton itself is para- or polyphyletic (Les & Haynes 1995).

Synonymy: Hydrogetonaceae Link, Zannichelliaceae Chevalier, nom. cons.