Family Lemnaceae
Lemnaceae S.F. Gray~ Araceae Including Wolffiaceae (Engl.) Nak. Habit and leaf form. Much reduced, aquatic herbs. Plants of very peculiar vegetative form; thalloid (thallus small to minute, globular, flat or linear, with or without raphides). Leaves absent. Plants with roots, or rootless. Annual. Hydrophytic; free floating. General anatomy. Accumulated starch exclusively ‘pteridophyte type’. Leaf anatomy. The mesophyll inapplicable, but the thallus contains mucilage cells. Minor leaf veins inapplicable, but the thallus of Lemna minor is without phloem transfer cells. Stem anatomy. Secondary thickening absent. Xylem (thallus) without vessels. Root anatomy. Root xylem without vessels. Reproductive type, pollination. Plants monoecious (usually), or dioecious (rarely). Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’ (in groups of 2—3). Inflorescences spatheate, or espatheate. Flowers minute. Perianth absent. Androecium 1; exclusively of fertile stamens. Stamens 1. Anthers dehiscing transversely, or dehiscing via pores to dehiscing via longitudinal slits (sometimes dehiscing apically via pores or short slits); bisporangiate (Wolffia), or tetrasporangiate (Lemna). Endothecium developing fibrous thickenings. The endothecial thickenings spiral. Microsporogenesis successive. The initial microspore tetrads tetrahedral, or isobilateral. Anther wall with no middle layer; of the ‘reduced’ type. Tapetum amoeboid. Pollen grains aperturate; 1 aperturate; ulcerate; 3-celled. Gynoecium ostensibly 1 carpelled. The pistil 1 celled. Gynoecium ostensibly monomerous (imaginatively interpretable as pseudomonomerous?); ostensibly of one carpel; superior. Carpel 1–7 ovuled. Placentation basal. Ovules ascending; orthotropous, or anatropous, or hemianatropous; bitegmic; crassinucellate. Outer integument contributing to the micropyle, or not contributing to the micropyle. Embryo-sac development Polygonum-type, or Allium-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral, or persistent (Spirodela). Synergids pear-shaped. Endosperm formation cellular. Endosperm haustoria present; chalazal. Embryogeny onagrad (or irregular). Fruit non-fleshy. The fruiting carpel indehiscent; a 1—4-seeded utricle. Seeds endospermic, or non-endospermic. Endosperm oily, or not oily. Seeds with starch. Embryo weakly differentiated to well differentiated (sometimes lacking a radicle). Cotyledons 1. Embryo straight. Testa without phytomelan; thick. Seedling. Seedling collar not conspicuous. Coleoptile absent. Seedling cataphylls absent. Primary root ephemeral (absent). Physiology, biochemistry. Not cyanogenic. Alkaloids absent (one species). Proanthocyanidins absent (as a synonym of Araceae). Flavonols absent. Ellagic acid absent. Arbutin absent. C3. C3 physiology recorded directly in Lemna. Anatomy non-C4 type (Lemna, Spirodela). Geography, cytology. Frigid zone to tropical. Cosmopolitan. X = 5, 8, 10, 11, 21. Supposed basic chromosome number of family: 5 (?). Taxonomy. Subclass Monocotyledonae. Dahlgren et al. Superorder Ariflorae; Arales. APG 3 core angiosperms; Superorder Lilianae; non-commelinid Monocot; Order Alismatales. Species 30. Genera 5; Lemna, Spirodela, Wolffia, Wolffiella, Wolffiopsis. Illustrations. Quotations . . . The green mantle of the stagnant pool |
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