The families of flowering plants

L. Watson and M.J. Dallwitz

Hydrocharitaceae Juss.

Including Blyxaceae Nak., Elodeaceae Dum., Enhalaceae Nak., Halophilaceae J.G. Agardh, Stratioteae (Stratiotaceae) Link, Thalassiaceae (Aschers and Gurke) Nak., Vallisneriaceae Dum.

Habit and leaf form. Vegetatively diverse aquatic herbs. Plants green and photosynthesizing. Mostly perennial; with a basal aggregation of leaves, or without conspicuous aggregations of leaves. Hydrophytic, or helophytic; marine (Thalassioideae, Halophiloideae), or non-marine (Hydrocharitoideae); free floating, or rooted. Leaves submerged, or emergent, or floating, or submerged and emergent, or submerged and floating. Heterophyllous (usually), or not heterophyllous. Leaves alternate, or opposite, or whorled; spiral, or distichous; membranous, or ‘herbaceous’; petiolate, or sessile, or petiolate and sessile; sheathing to non-sheathing. Leaf sheaths with free margins. Leaves simple; epulvinate. Lamina entire; linear, or oblong to orbicular (usually with ribbonlike submerged leaves); one-veined, or pinnately veined, or palmately veined, or parallel-veined; cross-venulate, or without cross-venules. Leaves stipulate, or exstipulate. Axillary scales (and sometimes serial axillary buds) present. Lamina margins often with thick-walled prickle-hairs. Leaf development ‘graminaceous’.

Leaf anatomy. Stomata present, or absent; when present, paracytic. Hairs absent. Lamina without secretory cavities. The mesophyll without crystals. Foliar vessels absent. Minor leaf veins without phloem transfer cells (5 genera).

Axial (stem, wood) anatomy. Secondary thickening absent. The axial xylem without vessels.

Root anatomy. Root xylem with vessels, or without vessels; vessel end-walls scalariform.

Reproductive type, pollination. Plants monoecious, or dioecious, or polygamomonoecious (rarely). Floral nectaries present, or absent. Nectar secretion (when manifest) from the androecium (from staminodial nectaries). Pollination entomophilous (some Hydrocharitoideae), or by water (mostly, sometimes from free-floating male flowers).

Inflorescence, floral, fruit and seed morphology. Flowers solitary, or aggregated in ‘inflorescences’; when aggregated, in cymes. The ultimate inflorescence units cymose. Inflorescences scapiflorous, or not scapiflorous; axillary; few flowered cymes; spatheate (the spathe formed of (1-)2 connate bracts). Flowers small; regular, or somewhat irregular (Vallisneria). The floral irregularity involving the perianth. Flowers 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present, or absent.

Perianth with distinct calyx and corolla (usually), or of ‘tepals’; 6, or (2–)3; free, or joined; 2 whorled; isomerous; if not resolvable into calyx and corolla, sepaloid, or petaloid; similar in the two whorls (then semipetaloid), or different in the two whorls; white, or yellow, or red, or purple, or blue. Calyx (2–)3; 1 whorled; polysepalous (sometimes from a hypanthium); regular. Corolla when present, 3; 1 whorled; polypetalous (attached to the gynoecium or to the perigone tube). Petals clawed, or sessile.

Androecium 2–3 (rarely), or 4–100 (i.e. to ‘many’). Androecial members unbranched, or branched (the members opposite the calyx sometimes paired); usually maturing centripetally; free of the perianth; free of one another; 1–10 whorled (the whorls trimerous, but sometimes with pairs opposite the calyx). Androecium exclusively of fertile stamens, or including staminodes (the innermost or outermost members often constituting staminodal nectaries). Staminodes external to the fertile stamens, or internal to the fertile stamens. Stamens 2–25; reduced in number relative to the adjacent perianth to polystemonous; alterniperianthial, or oppositiperianthial. Anthers dehiscing via short slits; generally extrorse; bisporangiate, or tetrasporangiate. Endothecium developing fibrous thickenings, or not developing fibrous thickenings. The endothecial thickenings spiral. Anther epidermis persistent. Microsporogenesis successive. The initial microspore tetrads isobilateral, or decussate, or linear. Anther wall initially with one middle layer, or initially with more than one middle layer; of the ‘monocot’ type. Tapetum amoeboid. Pollen shed in aggregates (as threadlike chains, in the marine forms), or shed as single grains. Pollen grains nonaperturate; 3-celled (in 5 genera).

Gynoecium (2–)3–6(–20) carpelled. The pistil 1–6(–20) celled. Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary 1 locular (but often with deeply intruding partial partitions). Locules partially secondarily divided by ‘false septa’, or without ‘false septa’. Styles (2–)3–6(–20) (but often individually bifurcated, and then seeming to be twice as many as the carpels); partially joined; apical. Stigmas dry type; non-papillate; Group II type. Placentation laminar-dispersed, or basal (e.g. Elodea). Ovules in the single cavity 12–100 (i.e. ‘many’); pendulous to ascending; non-arillate; orthotropous (rarely), or hemianatropous to anatropous; bitegmic; crassinucellate. Outer integument contributing to the micropyle (rarely), or not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; persistent (into early embryogeny). Synergids pear-shaped (sometimes with filiform apparatus), or hooked. Endosperm formation helobial. Embryogeny caryophyllad.

Fruit fleshy, or non-fleshy; dehiscent, or indehiscent (often opening by decay); a capsule, or capsular-indehiscent. Capsules splitting irregularly (underwater). Dispersal by water. Seeds scantily endospermic (Otelia), or non-endospermic (usually). Seeds with starch. Cotyledons 1 (bifacial). Embryo achlorophyllous (1/1); straight. Testa without phytomelan.

Seedling. Germination phanerocotylar (if applicable). Hypocotyl internode present (well developed). Seedling collar not conspicuous. Cotyledon hyperphyll elongated (at least, quite large); assimilatory; dorsiventrally flattened. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral (or even absent altogether, e.g. Stratiotes).

Physiology, phytochemistry. Disputably C3, C4, CAM, and C3-C4 intermediate. C4 physiology recorded directly in Vallisneria spiralis and Hydrilla verticillata (or interpretable as such: see Sage et al (1998). CAM recorded directly in Vallisneria americana (aquatic CAM only). C3-C4 intermediacy in Vallisneria spiralis. Anatomy non-C4 type (Elodea, Hydrilla, Vallisneria). Accumulated starch other than exclusively ‘pteridophyte type’. Cyanogenic (seldom), or not cyanogenic. Alkaloids absent (one species). Iridoids not detected. Saponins/sapogenins absent. Proanthocyanidins present (Hydrocharis), or absent (3 genera); in Hydrocharis, cyanidin. Flavonols absent. Ellagic acid absent. Sieve-tube plastids P-type; type II.

Geography, cytology. Temperate to tropical. Cosmopolitan, except frigid zones. X = 7–12.

Taxonomy. Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Hydrocharitales. APG III core angiosperms; Superorder Lilianae; non-commelinid Monocot. APG IV Order Alismatales.

Species 80. Genera 16; Apalanthe, Appertiella, Blyxa, Egeria, Elodea, Enhalus, Halophila, Hydrilla, Hydrocharis, Lagarosiphon, Limnobium, Maidenia, Nechamandra, Ottelia, Stratiotes, Thalassia, Vallisneria.

Economic uses, etc. Including some important aquarium and watergarden ornamentals.

Illustrations. • Le Maout and Decaisne: Hydrocharis morsus-ranae. • Le Maout and Decaisne: Hydrocharis. • Ottelia alismoides: Thonner. • Elodea canadensis: Eng. Bot. 1446 (1869). • Hydrocharis morsus-ranae: Eng. Bot. 1444 (1869). • Hydrocharis morsus-ranae (B. Ent.). • Stratiotes aloides: Eng. Bot. 1445 (1869). • Stratiotes aloides (B. Ent.). • Thalassia hemprichii, as Schizotheca: Hemprich & Ehrenberg (1820–25).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 15th April 2018.’.