Plants annual or perennial; habit various. Culms 3-800 cm, not woody, sometimes branching above the base; internodes solid or hollow. Ligules membranous and ciliate, or of hairs, rarely completely membranous; blades sometimes pseudopetiolate. Inflorescences spicate panicles with highly reduced branches termed fascicles; panicles 1-many per plant, terminal on the culms or on both the culms and the secondary branches, or terminal and axillary, or only axillary, usually completely exposed at maturity; rachises usually terete, with (1)5-many fascicles; fascicle axes 0.2-7.5(28) mm, with (1)3-130+ bristles and 1-12 spikelets. Bristles free or fused at the base, disarticulating with the spikelets at maturity; of 3 kinds, outer, inner, and primary, in some species with all 3 kinds present below each spikelet, in others 1 or more kinds missing from some or all of the spikelets; outer (lower) bristles antrorsely scabrous, terete; inner (upper) bristles antrorsely scabrous or long-ciliate, usually flatter and wider than the outer bristles; primary (terminal) bristles located immediately below the spikelets, solitary, antrorsely scabrous or long-ciliate, often longer than the other bristles associated with the spikelet; disarticulation usually at the base of the fascicles, sometimes also beneath the upper florets. Spikelets with 2 florets; lower glumes absent or present, 0-5-veined; upper glumes longer, 0-11-veined; lower florets sterile or staminate; lower lemmas usually as long as the spikelets, membranous, 3-15-veined, margins usually glabrous; lower paleas present or absent; upper lemmas membranous to coriaceous, 5-12-veined; upper paleas shorter than the lemmas but similar in texture; lodicules 0 or 2, glabrous; anthers 3, if present. x = 5, 7, 8, 9 (usually 9). Name from the Latin penna, feather, and seta, bristle, an allusion to the plumose bristles of some species.
Spikelets lance-oblong, usually dorsally compressed, sessile, with 1 perfect terminal fl above a neuter or staminate floret; glumes membranous or hyaline, the first 1- or
3-veined and equaling to much shorter than the 1-7-veined second one, or the first glume obsolete; sterile lemma 3-7-veined, about as long as the fertile one, with a well developed palea; fertile lemma about as long as the spikelet, membranous, 5- or 7-veined, equaling and partly enclosing its palea; 1- several spikelets collectively enclosed (or the tips exserted) in a bur of concrescent, barbed or scabrous spines or bristles, the burs sessile or nearly so on a slender rachis, forming a solitary, spike-like panicle with the axis usually prolonged into a short point; grasses with solid culms, compressed-keeled sheaths, and mostly flat lvs, the ligule reduced to a ciliate rim. 20, widespread, mostly in warm reg. Two spp. of Tragus are rarely found in our range, especially on ballast and about wool mills. The bur, instead of being composed of concrescent branchlets, as in Cenchrus, is composed of the enlarged second glumes, which are uncinate-spiny. T. racemosus (L.) All. has the second glume 3.6-4.5 mm, whereas T. berteronianus Schult. has it 2.3-3 mm.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Spikelets with a single perfect terminal fl and a lateral fl represented only by a thin, sterile, glume-like lemma; first glume short or wanting; second glume about as long as the sterile lemma; fertile lemma chartaceous, narrowly lanceolate, awnless; panicles dense, spike-like, with short-pediceled, involucrate groups of 1-few spikelets, the group and its invol falling together, the invol of several-many distinct or basally connate, often plumose bristles (short sterile branches). 100+, warm Old World.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.