The families of flowering plants.
Habit and leaf form. Aquatic herbs. Annual; with neither basal nor terminal aggregations of leaves, or with terminal aggregations of leaves. Hydrophytic, or helophytic; rooted. Leaves submerged, or emergent, or submerged and emergent. Heterophyllous (usually), or not heterophyllous. Leaves opposite (decussate, to rosetted towards branch tips); not gland-dotted; simple, or simple and compound (linear and bifid when submerged); epulvinate. Lamina dissected, or entire; when dissected, finely dichotomously dissected. Leaves exstipulate.
Leaf anatomy. Stomata present (sporadically, on submerged leaves); mainly confined to one surface (adaxial, when floating), or on both surfaces.
Minor leaf veins without phloem transfer cells.
Stem anatomy. Secondary thickening absent. Xylem with vessels, or without vessels (?).
Reproductive type, pollination. Plants monoecious, or dioecious. Pollination sometimes by water.
Inflorescence, floral, fruit and seed morphology. Flowers solitary (usually), or aggregated in ‘inflorescences’ (rarely, then paired). Inflorescences and flowers axillary. Flowers commonly bracteolate (the bracteoles hornlike); minute.
Androecium in male flowers 1; exclusively of fertile stamens. Stamens 1. Anthers dehiscing via longitudinal slits. Endothecium developing fibrous thickenings (or undifferentiated). Anther epidermis persistent. Microsporogenesis simultaneous. Tapetum glandular. Pollen grains aperturate, or nonaperturate; when aperturate, 3–4 aperturate; colpate; 3-celled.
Gynoecium in female flowers 2 carpelled. The pistil 4 celled. Gynoecium syncarpous; synovarious; superior. Ovary 2 locular (but ostensibly four, cf. Labiatae). Locules secondarily divided by ‘false septa’. Ovary sessile. Styles 2; free; from a depression at the top of the ovary; apical. Stigmas 2. Placentation axile to apical. Ovules 2 per locule (i.e. 1 per locellus); pendulous; anatropous; unitegmic; tenuinucellate. Endothelium differentiated. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral (small). Synergids pear-shaped. Endosperm formation cellular. Endosperm haustoria present; chalazal and micropylar (the latter the more aggressive). Embryogeny onagrad.
Fruit non-fleshy; a schizocarp. Mericarps 4; comprising nutlets. Seeds endospermic (the endosperm fleshy). Cotyledons 2. Embryo straight.
Physiology, biochemistry. Not cyanogenic. Alkaloids absent (on species). Iridoids detected; ‘Route II’ type (decarb.). Verbascosides detected. Proanthocyanidins absent. Ellagic acid absent. Betalains absent. Saponins/sapogenins absent. C3. C3 physiology recorded directly in Callitriche. Anatomy non-C4 type (Callitriche).
Geography, cytology. Frigid zone, temperate, sub-tropical, and tropical. Cosmopolitan, except South Africa.X = 3 or 5(+).
Taxonomy.Subclass Dicotyledonae; Tenuinucelli. Dahlgren’s Superorder Lamiiflorae; Lamiales. Cronquist’s Subclass Asteridae; Callitrichales. APG 3 core angiosperms; core eudicot; Superorder Asteranae; lamiid; Order Lamiales (as a synonym of Plantaginaceae).
Species 25. Genera 1; only genus, Callitriche.
General remarks. For discussion of classificatory problems posed by Scrophulariaceae, impinging on Bignoniaceae, Buddlejaceae, Callitrichaceae, Plantaginaceae,Hippuridaceae, Lentibulariaceae, and Hydrostachydaceae, and such problem genera as Paulownia and Schlegelia, see Olmstead and Reeves (1995), who provide preliminary insights from chloroplast gene sequencing.