5. Cyperus diandrus Torr.

Pl. 69 d–f; Map 249

Pycreus diander (Torr.) C.B. Clarke

Plants annual, tufted, lacking rhizomes and tubers. Aerial stems 3–35 cm long, erect to spreading, bluntly trigonous, smooth. Leaf blades 1–25 cm long, 1–3 mm wide (sometimes folded longitudinally and thus appearing narrower), shorter than to about as long as the stems. Inflorescences of 1–3 sessile spikes or irregular umbels with 1–3 sessile spikes and 1–4 rays, each ray smooth, ending in a cluster of 1 to few sessile spikes. Inflorescence bracts 2–4, mostly much longer than the rays, ascending. Spikes 7–20 mm long, with 3–10 spikelets, broadly ovoid, headlike, open, usually somewhat flattened, the spikelets spreading to ascending, attached near the tip of the axis, the spikelet bases usually visible. Spikelets 8–15 mm long, 2.0–3.5 mm wide, oblong to narrowly elliptic, usually pointed at the tip, strongly flattened in cross-section, with 6–25 florets, the fruits and scales shed successively from the base to the tip, leaving the persistent axis. Spikelet axis not winged. Spikelet scales 2.5–3.0 mm long, strongly overlapping, appressed to ascending, oblong-ovate, bluntly to fairly sharply angled along the back, rounded to bluntly pointed and slightly incurved at the tip, with 1–3 nerves, reddish brown, the reddish purple pigmentation best developed near the base, often tapered to a band near each margin toward the tip of the scale, the midrib usually green. Stamens 2(–3), the anthers 0.3–0.4 mm long. Stigmas 2, the styles (including stigmas) divided at least 2/3 of the way and often nearly to the base, noticeably extended past the subtending scale (often breaking off prior to fruiting). Fruits 0.9–1.3 mm long, oblong-elliptic to narrowly obovate in outline, biconvex and somewhat flattened in cross-section, the surface pebbled with a fine pattern of 4–5-sided cells that are about as long as wide (visible under magnification), brown to dark brown at maturity, more or less shiny, sometimes somewhat iridescent. June–October.

Uncommon and widely scattered in southern and central Missouri, mostly in the Missouri and Mississippi River floodplains (northeastern U.S. and adjacent Canada west to North Dakota and Missouri; also in New Mexico and possibly Arkansas). Margins of rivers, streams, and sloughs; also in ditches and moist, open areas.

The status of C. diandrus in Missouri is unclear. Most of the collections from the state were made prior to 1940. Possibly, it has been overlooked by collectors, because of the species’ great morphological similarity to the much more common C. bipartitus. In addition to the characters in the key, the exserted stigmas of C. diandrus give the maturing spikelets a shaggy or fringed appearance along the margins, in contrast to the smooth-margined spikelets of C. bipartitus.