Callitriche L.

Description as for the family. The leaf shape and arrangement vary considerably with the habitat of the plant. Submerged plants often have lanceolate or linear leaves and long internodes, while in plants where the shoots reach the surface the internodes tend to be shorter and have rosettes of elliptic or spathulate leaves. Terrestrial plants tend to have ovate or suborbicular leaves. The adaptation of the genus to aquatic life has resulted in the reduction of several morphological features, so that only a few reliable characters remain which can be used for identification. The most useful character for identification is that of ripe fruits. Other taxonomically useful characters, albeit unsuitable for use in species’ keys, are the morphology of pollen and the number of chromosomes. In pollen, the exine surface shows the Croton pattern, which is a reticulate pattern with processes (clavae) at the intersections of the walls (muri) (Erdtman, G., 1952, Pollen morphology and plant taxonomy: Angiosperms, p. 173). This pattern is present in all tropical African species. Chromosome counts from European material shows the somatic numbers between 6 and 40. The four tropical African species fall well within the range of variation. All tropical African material was earlier classified as C. stagnalis Scop., but recent taxonomic work has shown that several species can be recognized based on morphological, pollen and chromosomal differences.

Stagnant or slowly moving fresh water, or, in humid localities, terrestrial, often gregarious.