The families of flowering plants.
Habit and leaf form. Aquatic herbs.‘Normal’ plants. Annual, or perennial; with neither basal nor terminal aggregations of leaves. Hydrophytic; non-marine; rooted. Leaves submerged. Leaves (sub-) opposite to whorled, or alternate (‘subopposite or subverticillate’); when in verticils, 3 per whorl; sessile; sheathing (more completely so in the lower members of each sub-pair or verticil). Leaf sheaths not tubular; with free margins. Leaves simple. Lamina entire; linear; one-veined; without cross-venules. Leaves somewhat stipulate (at the sides of the sheath), or exstipulate. Axillary scales often present (two, small). Lamina margins serrate to dentate (usually toothed), or entire.
General anatomy. Accumulated starch exclusively ‘pteridophyte type’.
Leaf anatomy. Stomata absent. Hairs absent.
The mesophyll without calcium oxalate crystals. Minor leaf veins without phloem transfer cells (Najas). Vessels absent.
Stem anatomy. Secondary thickening absent. Xylem without vessels.
Root anatomy. Root xylem without vessels.
Reproductive type, pollination. Plants monoecious, or dioecious. Pollination by water.
Inflorescence, floral, fruit and seed morphology. Flowers solitary; axillary; with (male) or without (female) a spathelike set of scales and a flask-shaped or two-lipped inner involucre; small. Perigone tube present, or absent (depending on interpretation). Hypogynous disk absent.
Perianth of ‘tepals’, or vestigial, or absent (depending on interpretation of the ‘involucre’, which in female flowers is absent or inconspicuous and adnate to the gynoecium); free, or joined; if interpreted as such, sepaloid.
Androecium 1; exclusively of fertile stamens. Stamens 1; with sessile anthers. Anthers dehiscing irregularly; unilocular, or four locular; tetrasporangiate. Endothecium not developing fibrous thickenings. Anther epidermis persistent. Microsporogenesis successive. The initial microspore tetrads tetrahedral, or isobilateral, or T-shaped, or linear. Anther wall of the ‘reduced’ type. Tapetum amoeboid. Pollen shed in aggregates. Pollen grains nonaperturate; 3-celled.
Gynoecium 1 carpelled. The pistil 1 celled. Gynoecium ostensibly monomerous; ostensibly of one carpel; superior. Carpel 1 ovuled. Placentation basal. Ovules ascending; non-arillate; anatropous; bitegmic; crassinucellate. Outer integument not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing only after one has been fertilized, or fusing simultaneously with the male gamete (?). Antipodal cells formed; 3; not proliferating; persistent. Synergids pear-shaped. Endosperm formation nuclear. Embryogeny caryophyllad.
Fruit non-fleshy. The fruiting carpel indehiscent; an achene (pericarp thin). Seeds non-endospermic. Seeds with starch. Embryo well differentiated. Cotyledons 1. Embryo chlorophyllous; straight. Testa without phytomelan; very thin.
Seedling.Germination phanerocotylar. Hypocotyl internode present (long). Seedling collar not conspicuous. Cotyledon hyperphyll elongated; assimilatory; more or less circular in t.s. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.
Physiology, biochemistry. Not cyanogenic. Proanthocyanidins present; cyanidin. Flavonols absent. Ellagic acid absent. C3. Anatomy non-C4 type (Najas).
Geography, cytology. Frigid zone to tropical. Cosmopolitan. X = 6, 7.
Taxonomy.Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG 3 core angiosperms; Superorder Lilianae; non-commelinid Monocot; Order Alismatales (as a synonym of Hydrocharitaceae).
Species 50. Genera 1; Najas.