Family Hydrostachydaceae
Hydrostachydaceae Engl.Alternatively Hydrostachyaceae Habit and leaf form. Aquatic herbs. Perennial; with neither basal nor terminal aggregations of leaves; tuberous. Hydrophytic; rooted (with a holdfast and fibrous roots). Leaves submerged. Leaves simple (elongate), or compound; when divided, bipinnate, or multiply compound (bi- or tripinnatisect, the segments flattened, vesicular or filiform, the rachis beset with numerous small, scalelike or fringed appendages). Lamina dissected, or entire; if considered simple/ dissected, much-divided. Leaves ligulate (if the ‘stipules’ be so described), or eligulate; inconspicuously stipulate, or exstipulate (if considered ‘ligulate’). Stipules if recognised as such, intrapetiolar; concrescent. Leaves becoming compound from primordial lobes. Leaf anatomy. Stomata absent. Vessels absent. Stem anatomy. Secondary thickening absent (the vascular system much reduced). Xylem without vessels. Root anatomy. Root xylem without vessels. Reproductive type, pollination. Fertile flowers functionally male, or functionally female, or functionally male and functionally female. Plants dioecious (usually), or monoecious. Inflorescence, floral, fruit and seed morphology. Flowers densely aggregated in ‘inflorescences’; in spikes. The ultimate inflorescence unit racemose. Inflorescences scapiflorous. Flowers bracteate (the bracts accrescent); small (inconspicuous). Hypogynous disk absent. Perianth absent. Androecium in the male flowers, 1; exclusively of fertile stamens. Stamens 1; shortly filantherous. Anthers dehiscing via longitudinal slits; extrorse; bilocular (the locules well separated); tetrasporangiate. Pollen shed in aggregates; in tetrads. Pollen grains nonaperturate; 2-celled. Gynoecium 2 carpelled. The pistil 1 celled. Gynoecium syncarpous; synovarious to synstylovarious; superior. Ovary 1 locular. Gynoecium median; stylate. Styles 2 (long, slender, persistent); free to partially joined; apical. Placentation parietal. Ovules in the single cavity 30–100 (‘many’); anatropous; unitegmic; tenuinucellate. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells not formed (the three nuclei soon degenerating). Synergids pear-shaped. Endosperm formation cellular. Endosperm haustoria present; micropylar. Fruit non-fleshy; dehiscent; a capsule. Capsules septicidal. Fruit many seeded. Seeds non-endospermic. Seedling. Germination phanerocotylar. Physiology, biochemistry. Aluminium accumulation not found. Geography, cytology. Paleotropical and Cape. Temperate to tropical. Southern Africa, Madagascar. Taxonomy. Subclass Dicotyledonae; Tenuinucelli. Dahlgren’s Superorder Lamiiflorae; Hydrostachyales. Cronquist’s Subclass Asteridae; Callitrichales. APG 3 core angiosperms; core eudicot; Superorder Asteranae; Order Cornales. Species 30. Genera 1; only genus, Hydrostachys. 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 provided preliminary insights from chloroplast gene sequencing. Illustrations. |
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