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The families of flowering plants

L. Watson and M. J. Dallwitz

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 one (Wolffioideae) or two (Lemnoïdeae) budding pouches, raphides present with mucilage cells in Lemnoïdeae only, xylem without vesses, phloem transfer cells lacking at least in Lemna). Leaves absent. Plants with roots (usually, in Lemnoïdeae), or rootless (Wolffioïdeae). Annual. Hydrophytic; free floating.

Axial (stem, wood) anatomy. Secondary thickening absent.

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, these developed in one of the budding pouches in Lemnoïdeae but from a dorsal cavity in Woffioïdeae). Inflorescences spatheate (Lemnoïdeae), or espatheate (Wolffioïdeae). Flowers minute.

Perianth absent.

Androecium 1; exclusively of fertile stamens. Stamens 1. Anthers dehiscing transversely (Lemnoïdeae), or dehiscing via pores to dehiscing via longitudinal slits (Wolffioïdeae); unilocular (Wolffioïdeae), or bilocular (Lemnoïdeae); 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, phytochemistry. C3. C3 physiology recorded directly in Lemna. Anatomy non-C4 type (Lemna, Spirodela). Accumulated starch exclusively ‘pteridophyte type’. Not cyanogenic. Alkaloids absent (one species). Arbutin absent. Proanthocyanidins absent. Flavonols absent. Ellagic acid absent.

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 (as a synonym of Araceae).

Species 30. Genera 4–6; Lemnoïodeae: Lemna, Spirodela (including Landoltia); Wolffioïdeae: Pseudowolffia, Wolffia, Wolffiella, Wolffiopsis.

General remarks. The data compiled for the present descriptions suggest that these genera, comprising two rather distinct subfamilies (see above) and credibly interpreted as derived from Araceae, are inappropriately sunk therein by dogmatic application of cladistic hypotheses.

Economic uses, etc. Besides being a likely future source of clean biofuel, Duckweeds have an important rôle in removing excess nutrients and other pollutants from artificial and naturally occurring bodies of water.

Illustrations. • Technical details: Lemna. • Technical details: Wolffia (including Grantia, Telmatophace). • Lemna gibba: Eng. Bot. 1396 (1869). • Lemna minor: Eng. Bot. 1395 (1869). • Lemna polyrrhiza: Eng. Bot. 1397 (1869). • Lemna trisulca: Eng. Bot. 1394 (1869). • Wolffia arrhiza: as Lemna, Eng. Bot. 1398 (1869).


. . . The green mantle of the stagnant pool
(‘King Lear’, iii., 4 - Lemna minor)

The descriptions are offered for casual browsing only. We strongly advise against extracting comparative information from them. This is much more easily achieved using the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG).

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 22nd August 2016.’.