Plants with C4 photosynthesis, annual or
perennial, forming tufts or clumps. Flowering stems spreading or ascending from
spreading bases, somewhat flattened (circular in cross‑section
elsewhere), glabrous. Leaf sheaths glabrous or sparsely hairy, especially along
the margins, keeled, the ligule 0.5–1.5 mm long, a minute membrane with a hairy
margin. Leaf blades flat, roughened and sometimes also sparsely hairy.
Inflorescences dense, narrow spikes (actually spikelike panicles) the spikelets
appearing as clusters along the main axis. Clusters (short branches) with
(1)2–4 spikelets, subtended by numerous retrorsely barbed bristles and
sometimes also flattened spines, these fused only at the very base or to above
the middle and forming a spiny, burlike cluster, the cluster shed intact as a
unit, including the bristles and spines. Spikelets without a cuplike ring or
knoblike disk at the base. Lower glume lanceolate to ovate, sharply pointed at
the tip, awnless, 1‑nerved. Upper glume shorter than the rest of the
spikelet, ovate, not inflated, sharply pointed at the tip, awnless, 1–5‑nerved,
glabrous. Lowermost floret sterile or staminate, the lemma shorter than to
about as long as the rest of the spikelet, lanceolate to ovate, tapered to a
beaklike, sharply pointed tip, awnless, 3–7‑nerved, glabrous. Fertile
(perfect) floret with the lemma lanceolate to ovate, tapered to a beaklike,
sharply pointed tip, awnless, 3‑ or 5‑nerved, glabrous, relatively
thin, but firm and somewhat thicker in texture than the glumes and sterile
lemma, glabrous, the margins relatively flat and only slightly wrapped around
the palea and fruit. Paleas glabrous or roughened, about as long as and similar
in texture to the lemma, 2‑nerved. Fruits broadly oblong‑ovate in
outline, yellowish brown. About 20 species, nearly worldwide, mostly in
tropical and subtropical regions, but most diverse in the Americas.