Including Wolffiaceae (Engl.) Nak.
Habit and leaf form. Much reduced, aquatic herbs. Plants of very peculiar 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. 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. 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 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; 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.
Physiology, biochemistry. Not cyanogenic. Alkaloids absent (one species). Proanthocyanidins absent. Flavonols absent. Ellagic acid absent. Arbutin absent. C3. C3 recorded 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. Superorder Ariflorae; Arales. Species 30. Genera 5; Lemna, Spirodela, Wolffia, Wolffiella, Wolffiopsis.
Illustrations. lemna839.gif
