Dioecious, submerged freshwater perennial or annual herb, attached to substrate; roots fibrous; stolons usually present. Leaves basal or cauline, alternate or scattered, with sheathing bases, submerged or floating, sessile, strap-like. Male plants with numerous minute subsessile flowers enclosed by 2-lobed, reflexed united floral bracts; perianth of 2 whorls, the outer 3-lobed, the inner rudimentary; stamens 2 (sometimes united); staminodes 1; male flowers released, floating to the surface. Female plants with inflorescence 1-flowered (sometimes with several flowers), with a long peduncle; flowers subtended by 2-lobed, united floral bracts; perianth usually of 2 whorls of 3 segments each, the inner rudimentary; ovary unilocular; styles (short or reduced), each with a 2-lobed stigma; female flowers floating, peduncle coiling at maturity, drawing developing fruit below the surface. Fruit elongate, ellipsoidal, indehiscent. Seeds numerous.
Cosmopolitan genus of c. 10 species; 8 in Australia; 1 in Victoria.
Since several features are difficult to observe in herbarium material, it is preferable, when collecting this genus, to thoroughly examine and describe the flower, as far as is possible. Flowers placed in 70% ethyl alcohol preserve the delicate features for later examination under higher magnification than is possible in the field. Male plants with immature male flowers are extremely difficult to identify because the stamens are often insufficiently developed to determine if they are free or fused. The distribution of the species in Australia (including Victoria) is inadequately known. There is a large amount of herbarium material at MEL which can not be determined to species.
Jacobs, S.W.L. & McColl, K.A. (2011). Hydrocharitaceae, 14-44, in: Wilson, A., Flora of Australia, Vol. 39, Alismatales to Arales, CSIRO Publishing, Collingwood.