Eleocharis (sect. Eleocharis) ser. Tenuissimae
Rhodora 31: 129. 1929.
Plants perennial or annual, some species often proliferating from spikelets or stoloniferous and sometimes reproducing entirely asexually. Rhizomes absent or present, creeping or ascending and caudexlike, 0.2–1 mm thick. Culms rarely compressed, 0.1–0.75 mm wide. Leaves: distal leaf sheaths persistent or disintegrating, closely sheathing, thinly membranous to sometimes papery, apex usually acute to acuminate. Spikelets terete or laterally compressed in some species, tooth absent; floral scales spiraled or distichous in some species, membranous or papery, or sometimes cartilaginous; basal spikelets present in some species. Flowers: styles 3-fid, rarely 2-fid. Achenes colorless, whitish, gray, greenish, stramineous or brown, trigonous, very rarely biconvex, smooth to honeycomb-reticulate or with longitudinal rows of large depressions at 10–20X. Tubercles distinct from achene apex in color, texture, and form.
Temperate to mostly tropical regions worldwide.
Species ca. 70+ (13 in the flora).
Eleocharis ser. Tenuissimae is probably polyphyletic and includes several difficult complexes which have not been the subjects of recent monographic studies. The dark spots on the achenes of some species apparently are the dark periclinal walls of the epidermis contrasting with the pale anticlinal walls. Seven species of ser. Tenuissimae often proliferate from their spikelets. Six of these may form entirely vegetative mats or aquatic colonies without achenes or normal spikelets. They cannot be identified to species. They are often mistaken for Websteria confervoides. (See discussion under description, p. 121.)