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~ Hydrocharitaceae.
Habit and leaf form. Aquatic herbs. Annual, or perennial; without conspicuous 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.
Leaf anatomy. Stomata absent. Hairs absent. The mesophyll without crystals. Foliar vessels absent. Minor leaf veins without phloem transfer cells (Najas).
Axial (stem, wood) anatomy. Secondary thickening absent. The axial xylem without vessels.
Root anatomy. Root xylem without vessels.
Reproductive type, pollination. Unisexual flowers present. 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, phytochemistry. C3. Anatomy non-C4 type (Najas). Accumulated starch exclusively ‘pteridophyte type’. Not cyanogenic. Proanthocyanidins present; cyanidin. Flavonols absent. Ellagic acid absent.
Geography, cytology. Frigid zone to tropical. Cosmopolitan. X = 6, 7.
Taxonomy. Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG III core angiosperms; Superorder Lilianae; non-commelinid Monocot. APG IV Order Alismatales (as a synonym of Hydrocharitaceae).
Species 50. Genera 1; Najas.
General remarks. Differing conspicuously from Hydrocharitaceae in the androecium (a single stamen with sessile anther and irregular dehiscence) and the monomerous, superior gynoecium; and the compiled data also depict differences in anther wall development, embryology, and seed details.
Illustrations. • Le Maout and Decaisne: Najas, Caulinia (= Najas). • Najas flexilis: as Naias, Eng. Bot. 1432 (1869).
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: 25th April 2024. delta-intkey.com’.
