Flora of New Zealand Series

Aquatic perennials of fresh or salt water, partly or completely submersed; monoecious or dioecious. Stems long, slender, simple or branched, or 0. Leaves basal or cauline, usually sessile, alternate or whorled, variable in shape and size, submersed or floating. Flowers actinomorphic, commonly unisexual, sometimes bisexual. Inflorescence (or flower) sessile or long-pedunculate, in a tubular bifid spathe or within 2 opposite bracts; male flowers usually many within spathe; female flowers solitary; bisexual flowers solitary or several together. Perianth-segments usually 6 or fewer, in 1-2 whorls, 3 (rarely 2) in each whorl; tube often elongate. Stamens (1)-3-many, anthers 2-celled, opening by longitudinal slits (staminodia in female flowers). Ovary inferior, syncarpous, 1-locular, with 3-6 (rarely more) parietal placentae (rudimentary ovaries in male flowers); style-branches as many as placentae; stigmas entire or bifid; ovules many. Fruit submersed, often fleshy, indehiscent, occasionally rupturing irregularly. Seeds many. Genera c.16, in warmer regions of the world, a few in temperate regions.

It is convenient to treat the so-called "oxygen weeds" - Egeria densa, Elodea canadensis and Lagarosiphon major -collectively in so far as their common features are concerned.

All are dioecious, submersed, herbaceous perennials, represented here, virtually, by plants of one sex only and therefore unable to form seed; all reproduce vegetatively from portions of stem which produce adventitious roots; all are periodically available commercially and grown in aquaria and garden ponds and all have escaped, either as fragments in overflow drains, or as surplus material tossed into drains and other watery places. In nature, portions of stem are dispersed by water, by man on boat shafts and propellors, cradles and boat trailers (new occurrences have been detected near boat ramps on lakes and rivers) and possibly by water fowl. All three oxygen weeds cause drainage problems and adversely affect electric power generation, boating, fishing and aquatic sports.