1. Section Acrocystis Dumort.

(Rettig, 1988)

Plants monoecious (rarely dioecious in C. umbellata), with short- to long-creeping rhizomes, forming dense tufts or loose colonies of tufts. Vegetative growth consisting of tufts of basal leaves at the tips of rhizome branches. Flowering stems glabrous, usually reddish purple tinged at the base. Leaves basal or nearly so, often longer than the stems, glabrous, the margins and midrib often minutely roughened or toothed, the leaf sheaths usually reddish purple tinged at the base. Spikes 1–5 per stem; if 1 per stem then the stems with staminate spikes generally longer than those with pistillate spikes, which are often very short and hidden among the leaf bases; if 2–5 per stem, then the terminal spike staminate and the sessile lateral and long-stalked basal spikes pistillate. Lowermost (or only) pistillate spike often with the bract reduced to a tubular sheath, the blade absent to short-triangular, or short and leaflike in a few species, and then lacking a sheath. Perigynia usually with 2 prominent, longitudinal ribs on opposite sides, otherwise lacking nerves, somewhat inflated and nearly circular in cross-section, tapered to a short beak at the tip and to a stalklike base of spongy tissue below the fruit, usually hairy, the beak usually somewhat flattened, with short, soft, inconspicuous teeth at the tip. Styles withering during fruit development, jointed to the main body of the fruit, which is beakless or short-beaked at maturity. Stigmas 3. About 30 species, nearly worldwide, nearly all in the Northern Hemisphere.

Some species of section Acrocystis can be quite difficult to differentiate. The fruits of C. albicans and C. nigromarginata, as well as perhaps some other species of this section, are dispersed in part by ants.