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The families of flowering plants

L. Watson and M.J. Dallwitz

Elatinaceae Dum.

Including Isomeraceae Dulac

Habit and leaf form. (Sub-) shrubs, or herbs. Plants non-succulent. Annual, or perennial. Hydrophytic and helophytic (mostly tolerant of changing water levels); rooted. Leaves submerged and emergent. Leaves opposite (and decussate), or whorled (rarely); petiolate to sessile; simple; epulvinate. Lamina entire; linear to ovate, or obovate; pinnately veined; attenuate to the base. Leaves stipulate. Stipules interpetiolar. Lamina margins entire, or crenate, or serrate. Leaf development not ‘graminaceous’.

Leaf anatomy. The leaf lamina dorsiventral, or bifacial. Mucilaginous epidermis present, or absent. Stomata present; on both surfaces; anomocytic, or tetracytic. Hairs often entirely absent, or present; when present, eglandular, or glandular (sometimes exclusively so), or eglandular and glandular. Lamina with secretory cavities, or without secretory cavities. Minor leaf veins without phloem transfer cells (Elatine).

Axial (stem, wood) anatomy. Young stems often with hollow internodes (then with pith lacking). Primary vascular tissues in a cylinder, without separate bundles, or comprising a ring of bundles (?). Secondary thickening absent (?), or developing from a conventional cambial ring.

The wood exhibiting growth rings in old stems of Bergia. The vessels somewhat quadrangular; in radial multiples. The vessel end-walls simple. The vessels without vestured pits. The axial xylem with tracheids.

Reproductive type, pollination. Plants hermaphrodite.

Inflorescence, floral, fruit and seed morphology. Flowers solitary, or aggregated in ‘inflorescences’; when solitary, axillary; when aggregated, in cymes. The ultimate inflorescence units cymose. Inflorescences axillary. Flowers small; regular; 2–5(–6) merous; cyclic; tetracyclic, or pentacyclic. Free hypanthium absent.

Perianth with distinct calyx and corolla; 4–10(–12); 2 whorled; isomerous. Calyx 2–5(–6); 1 whorled; polysepalous, or gamosepalous; regular; imbricate. Corolla 2–5(–6) (the same number as K); 1 whorled; polypetalous; imbricate; regular; persistent.

Androecium 2–5(–6), or 4–10(–12) (i.e. the same number as or twice C). Androecial members free of the perianth; all equal; free of one another; 2 whorled, or 1 whorled (the inner whorl sometimes aborted). Androecium exclusively of fertile stamens. Stamens 2–5(–6), or 4–10(–12); isomerous with the perianth, or diplostemonous; when 1-whorled oppositisepalous. Anthers dorsifixed; versatile; dehiscing via longitudinal slits; introrse; tetrasporangiate. Endothecium developing fibrous thickenings. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or isobilateral. Anther wall initially with more than one middle layer (2); of the ‘basic’ type. Tapetum glandular. Pollen grains aperturate; 3 aperturate; colporate; 2-celled (Bergia), or 3-celled (Elatine).

Gynoecium 2–5(–6) carpelled. Carpels isomerous with the perianth. The pistil 2–5(–6) celled. Gynoecium syncarpous; synovarious; superior. Ovary 2–5(–6) locular (but the septa sometimes not reaching the ovary apex). The ‘odd’ carpel (when G3) anterior. Gynoecium stylate. Styles 2–5(–6) (same number as locules); free; apical. Stigmas capitate. Placentation axile. Ovules 15–50 per locule (i.e. ‘many’); horizontal to ascending; with lateral or superior raphe; non-arillate; anatropous; bitegmic; crassinucellate. Outer integument contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; ephemeral. Synergids pear-shaped (?), or hooked (Bergia). Endosperm formation nuclear. Embryogeny solanad.

Fruit non-fleshy; dehiscent; a capsule. Capsules valvular (septifragal). Seeds non-endospermic. Cotyledons 2 (relatively short). Embryo straight to curved, or bent. Micropyle zigzag (sometimes), or not zigzag.

Seedling. Germination phanerocotylar.

Physiology, phytochemistry. Alkaloids absent (one species). Iridoids not detected. Saponins/sapogenins absent. Proanthocyanidins present. Ellagic acid at least sometimes present. Aluminium accumulation not found.

Geography, cytology. Temperate to tropical. Widespread. X = 6, 9.

Taxonomy. Subclass Dicotyledonae; Crassinucelli (probably, despite ‘nuclear endosperm’). Dahlgren’s Superorder Theiflorae (?); Theales (?). Cronquist’s Subclass Dilleniidae; Theales. APG III core angiosperms; core eudicot; Superorder Rosanae; fabid. APG IV Order Malpighiales.

Species 40. Genera 2; Bergia, Elatine.

General remarks. Better anatomical information required.

Illustrations. • Bergia suffruticosa: Thonner. • Le Maout and Decaisne: Merimea (=Bergia), Elatine hexandra, E. hydropiper, E. triandra. • Elatine hexandra and E. hydropiper: Eng. Bot. 262 and 263, 1864. • Bergia ammannioides and Elatine ambigua: R. Wight (1840).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or 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). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 5th March 2018.’.