Flora of North America species comparison

Species ca. 75 (13 in the flora).

Callitriche has been included in Haloragaceae, a family now included in Saxifragales (Angiosperm Phylogeny Group 2016), and in the monogeneric Callitrichaceae, which is nested within Plantaginaceae (D. C. Albach et al. 2005).

C. F. Hegelmaier (1864) divided the genus into two sections: Callitriche (as Eucallitriche Hegelmaier) and Pseudocallitriche Hegelmaier. N. C. Fassett (1951b) emended the diagnosis and established sect. Microcallitriche Fassett. H. D. Schotsman (1967) divided the genus into five groups; one conforms to Pseudocallitriche, three to Callitriche, and one to Microcallitriche. In a comprehensive study of taxa from North America, South America, and the Mediterranean, including cytological analysis, C. T. Philbrick and D. H. Les (2000) found support for only sections Callitriche and Pseudocallitriche. All these infrageneric separations are based on a small proportion of the described taxa and almost exclusively cover taxa from Europe and the Americas.

Review of the full range of described taxa of Callitriche shows that there may be other means of subdividing the genus. For example, three Australasian terrestrial species (C. capricorni R. Mason, C. muelleri Sonder, and C. sonderi Hegelmaier) have trilocular anthers; in these species and C. cycloptera Schotsman, the filament arises from the pedicel (H. D. Schotsman 1985). Callitriche fassettii, C. hermaphroditica, C. stenoptera, and C. transvolgensis Tzvelev form a closely related group relatively distinct from their nearest relatives. Current concepts of both C. heterophylla and C. palustris include taxa that have been recognized at subspecific rank. It is likely that a comprehensive review of morphological data combined with molecular study would enable recognition of more useful groupings; to date, intermediates have been found between all subgeneric groupings.

Descriptions here refer to aquatic or amphibious forms except for Callitriche pedunculosa, C. peploides, and C. terrestris, which occur only as terrestrial forms. Reliable characters by which to distinguish Callitriche taxa are few. Useful characters include wings on mericarps and numbers and distributions of flowers. The presence, absence, or extent of wings on mericarps is important. Here, wing applies to a clearly differentiated structure arising from the exocarp of a mericarp. All species may have solitary flowers in pairs of axils; some species may have various combinations of pistillate and staminate flowers.

Callitriche is of considerable secondary importance in freshwater wetland systems providing habitat for many kinds of animals.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Two varieties have been recognized in Callitriche heterophylla, with var. bolanderi distinguished on the basis of its schizocarps being larger (for example, N. C. Fassett 1951b); however, the size of the schizocarps overlaps widely, and measurements of schizocarps from throughout North America show that there is no clear geographical or ecological separation between plants with smaller fruit and those with larger fruit. Therefore, there is no justification for maintaining the distinction.

Populations of small plants with linear leaves from parts of Canada and Alaska have been separated as Callitriche anceps. However, these appear to represent the lower extremes of variation in C. heterophylla, and morphological characters do not justify maintaining the distinction.

(Discussion copyrighted by Flora of North America; reprinted with permission.)