Family Zosteraceae
Zosteraceae Dum.Habit and leaf form. Aquatic herbs. Perennial; with a basal aggregation of leaves, or with neither basal nor terminal aggregations of leaves; rhizomatous (monopodial). Hydrophytic; marine; rooted. Leaves submerged. Leaves alternate; distichous; flat; sessile; sheathing. Leaf sheaths with free margins, or with joined margins. Leaves edgewise to the stem, or with ‘normal’ orientation, or edgewise to the stem and with ‘normal’ orientation (???); simple. Lamina entire; linear (ribbonlike); parallel-veined; without cross-venules. Leaves ligulate. Axillary scales present. Lamina margins entire. Leaves with a persistent basal meristem, and basipetal development. Leaf anatomy. Stomata absent. The mesophyll without calcium oxalate crystals. Minor leaf veins with phloem transfer cells (3 species of Zostera). Vessels absent. Stem anatomy. Secondary thickening absent. Xylem without vessels. Root anatomy. Root xylem without vessels. Reproductive type, pollination. Fertile flowers functionally male and functionally female, or functionally male, or functionally female. Plants monoecious, or dioecious (Phyllospadix). Pollination by water. Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in spikes. The ultimate inflorescence unit racemose. Inflorescences axillary (enfolded in the subtending sheath, exserted only when fruiting); flattened spikes or spadices, the flowers on one side — males and females alternating in two ranks in the monoecious species. Flowers ebracteate; ebracteolate; small. Perigone tube absent. Perianth vestigial (if the marginal outgrowths from the axis — retincacula, which sometimes enclose the stamens — are interpreted as perianth), or absent. Androecium 1 (though interpretable as two fused members); exclusively of fertile stamens. Stamens 1; with sessile anthers. Anthers dehiscing via longitudinal slits; unilocular; tetrasporangiate (representing fusion of two bisporangiate members?). Microsporogenesis successive. Tapetum amoeboid. Pollen shed as single grains (becoming filamentous and up to 2 mm long when released). Pollen grains lacking exine, and dispersed in the sea as long filaments. Pollen grains nonaperturate; 3-celled. Gynoecium ostensibly 1 carpelled (but subject to fanciful alternative interpretations). The pistil 1 celled. Gynoecium (?pseudo) monomerous; ostensibly of one carpel; superior. Carpel apically stigmatic; 1 ovuled. Placentation apical. Ovules sessile; pendulous; non-arillate; orthotropous; bitegmic; pseudocrassinucellate. Outer integument not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating. Synergids pear-shaped. Hypostase present. Endosperm formation nuclear. Embryogeny caryophyllad. Fruit non-fleshy. The fruiting carpel indehiscent; an achene. Fruit 1 seeded. Seeds non-endospermic. Cotyledons 1. Embryo curved. Testa without phytomelan; membranous. Seedling. Germination phanerocotylar. Hypocotyl internode present (long, and developing below into a large disk bearing rhizoids). Seedling collar conspicuous (if the hypocotyl disk be so interpreted). Cotyledon hyperphyll elongated; assimilatory; more or less circular in t.s. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral (or rather, virtually absent). Physiology, biochemistry. Proanthocyanidins absent. Flavonols absent. Ellagic acid absent. Geography, cytology. Frigid zone to tropical. Widespread coastal, but mainly extra-tropical, and lacking from West Africa, India, Malaysia and South America. X = 6, 10. Taxonomy. Subclass Monocotyledonae. Dahlgren et al. Superorder Alismatiflorae; Zosterales. APG 3 core angiosperms; Superorder Lilianae; non-commelinid Monocot; Order Alismatales. Species 18. Genera 3; Heterozostera, Phyllospadix, Zostera. Illustrations. |
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