Callitrichaceae

This small family comprises a single genus, Callitriche, which is almost cosmopolitan but most diverse at high latitudes or at high altitudes in the tropics. Most species in Australia are found in the temperate south-east and south-west, although some extend north into Queensland and into the eastern arid zone.

Characteristic features of the family Callitrichaceae in Australia include:

• small, often creeping or floating herbs found in wet places
  
• leaves opposite, thin and parallel-veined, surface ones often rosetted and rounded or spathulate, submerged ones sometimes narrower
  
• flowers minute, unisexual, solitary in the leaf axils, the males comprised of a single stamen, the female a naked ovary with 2 style branches
  
• fruit a small, usually winged schizocarp with 4 nutlets

Description

Annual or perennial aquatic herbs rooted in the substrate with their leaves mostly submerged, floating or emergent, perennating by creeping stems. Vegetative reproduction absent or by rhizomes or stolons. Internal secretions not obvious. Plants glabrous or with peltate scales. Leaves well developed or much reduced (i.e. to scales, etc.), opposite, cauline, petiolate, or subsessile. Stipules absent. Lamina simple, symmetric, filiform, acicular, subulate, linear, lanceolate, ovate, elliptic, oblanceolate, ovate, spathulate or orbicular; base cuneate or attenuate; margins entire, ±flat; one-veined, or the venation parallel, with the midrib inconspicuous, and the tertiary venation not reticulate; surfaces not punctate; herbaceous. Male and female flowers mostly occurring on the same plant, or rarely occurring on separate plants. Inflorescences axillary, consisting of solitary flowers. Bracts absent. Bracteoles present or absent. Pollination by wind. Flowers odourless; sessile. Floral disc absent; nectaries absent. Perianth absent. Male flower of a solitary stamen. Anther basifixed, not versatile, opening sideways by longitudinal slits; 2-celled. Female flower of a sessile naked ovary. Carpels 2, fused; ovary with 2 or apparently 4 locules. Style terminal, single and branched above or from the base. Ovules 1 per locule, stalked; placentation axile. Fruit a dry schizocarp forming mericarps. Disseminule macro-surface featureless or winged; micro-surface ±smooth, white, green, brown or grey, dull. Seeds 2 or 4 per fruit. Aril absent. Cotyledons 2. Embryo straight or curved.
(Note: this description has been generated from the coded data compiled for the key. Any errors in the key data will be reflected in the descriptions.)

A treatment of the family Callitrichaceae has not yet been published in the Flora of Australia. It will appear in Volume 32.

Australian genera of Callitrichaceae (as recognised for the Flora of Australia)

† = some species native, others introduced

†Callitriche