|Michael T. Stieber|
Plants annual or perennial. Culms
5-200 cm, erect or decumbent, usually geniculate; nodes and internodes
usually glabrous. Sheaths open, usually glabrous; ligules membranous,
ciliate, cilia as long as or longer than the basal membrane; blades flat
or folded, margins cartilaginous, scabridulous. Inflorescences terminal,
spikelike panicles of highly reduced branches termed fascicles ("burs");
fascicles consisting of 1-2 series of many, stiff, partially fused, usually
retrorsely scabridulous to strigose, sharp bristles surrounding, sometimes almost
concealing, 1-4 spikelets; outer (lower) bristles, if present, in
1 or more whorls, terete or flattened; inner (upper) bristles usually
strongly flattened, fused at least at the base and forming a disk, frequently
to more than 1/2 their length and forming a cupule; disarticulation at
the base of the fascicles. Spikelets sessile, with 2 florets; lower
florets usually sterile; upper florets bisexual. Lower glumes
ovate, scarious, glabrous, 1-veined, acute to acuminate; upper glumes and
lower lemmas ovate, 3-9-veined; lower paleas equaling the lemmas,
tawny or purplish; upper lemmas and paleas subequal, indurate, ovate,
obscurely veined, acuminate. Caryopses obtrulloid. x = 17. Name from
the Greek kengchros, millet.
Cenchrus has about 16, primarily tropical species, most of which are readily (and painfully) recognized by their spiny fascicles. Most of its species differ from those of Pennisetumin having retrorsely scabrous or strigose inner bristles that are fused to well above their bases. The species are generally considered to be undesirable weeds.
Seven species of Cenchrus are native to the Flora region. The eighth species in this treatment was collected once in Westchester County, New York, but does not appear to have become established in the Flora region.
All bristles terete, fused only at the base; fascicles not burlike ..... 7. C. myosuroides
Inner bristles flattened, variously fused, forming a shallow disk or distinct cupule; fascicles burlike (2)
Fascicles having 1 whorl of fused, flattened inner bristles, subtended by 5-25 free, terete, outer bristles (3)
Fascicles having more than 1 whorl of flattened inner bristles, these originating at irregular intervals throughout the body of the cupule, sometimes subtended by terete outer bristles (5)
Inner bristles fused only at the base and forming a shallow disk, their abaxial surfaces with 1-3 grooves ..... 8. C. biflorus
Inner bristles fused for 1/3-1/2 their length or more, forming a globose cupule, their abaxial surfaces not grooved (4)
Rachis internodes 0.8-1.7 mm long; the majority of the outer bristles equaling or slightly exceeding the inner, flattened bristles ..... 1. C. brownii
Rachis internodes 2-4 mm long; the majority of the outer bristles about 1/2 as long as the inner, flattened bristles ..... 2. C. echinatus
Plants perennial, long-lived; fascicles not imbricate, usually glabrous; leaf blades 1-3.5 mm wide ..... 3. C. gracillimus
Plants annual or perennial but short-lived; fascicles imbricate, usually pubescent; leaf blades (1)3-14.2 mm wide (6)
Inner bristles 0.5-0.9(1.4) mm wide at the base; fascicles with 45-75 bristles ..... 5. C. longispinus
Inner bristles 1-3 mm wide at the base; fascicles with 8-43 bristles (7)
Fascicles densely pubescent, 9-16 mm long, with 1(2) spikelets; inner bristles 4-8 mm long ..... 6. C. tribuloides
Fascicles glabrous or sparsely to moderately pubescent, 5.5-10.2 mm long, with 2-4 spikelets; inner bristles 2-5.8 mm long ..... 4. C. spinifex
1. Cenchrus brownii Roem. & Schult.
Slimbristle Sandbur, Green Sandbur
Plants annual. Culms 25-100 cm, erect or decumbent. Sheaths slightly compressed; ligules 0.6-1.3 mm; blades 6-30 cm long, 0.4-1.1 cm wide, adaxial surfaces glabrous or sparsely pilose. Panicles 4-15 cm; rachis internodes 0.8-1.7 mm; fascicles 5-8 mm long, 2-4.5 mm wide, imbricate, globose, villous at the base, tawny; outer bristles 5-25, the majority equaling or slightly exceeding the inner bristles but narrower and terete, arising in a whorl at the base of the fascicles; inner bristles 4-10, 2-4 mm long, 0.6-1.8 mm wide at the base, flattened, not grooved, erect or interlocking at maturity, fused for 1/3 their length or more, forming a globose cupule. Spikelets 2-3 per fascicle, 3-6 mm. Lower glumes 0.5-2.5 mm; upper glumes 2.2-4.9 mm, 3-5-veined; lower lemmas 3-5.5 mm; upper florets 3.6-5.4 mm; anthers 0.8-2.3 mm. Caryopses 1.9-2.6 mm long, 0.8-1.9 mm wide, ovoid. 2n = 34.
Cenchrus brownii is native to sandy waste places and forest borders. It occurs infrequently on the coastal plain of the southeastern United States, but is common through the Caribbean, Central America, and the northern coast of South America. It has also been introduced to other parts of the world. The record from Texas may represent an introduction; only one specimen is known from the state.
2. Cenchrus echinatus L.
Plants annual. Culms 20-100 cm, ascending from a geniculate base. Sheaths from shorter than to equaling the internodes, compressed; ligules 0.7-1.7 mm; blades 4-18(35) cm long, 2-10(14.2) mm wide, adaxial surfaces sparsely pilose, hairs papillose-based. Panicles 2.5-12 cm; rachis internodes 2-4 mm; fascicles 5-10 mm long, 3.5-6(6.3) mm wide, imbricate; outer bristles 10-20, terete, the majority no more than 1/2 as long as the inner bristles; inner bristles 2-5 mm long, 0.6-1.5 mm wide, flattened, not grooved, mostly erect, fused for at least 1/2 their length into a globose cupule, sometimes interlocking at maturity, shortly pubescent, often purple at maturity. Spikelets 2-3(4) per fascicle, 4.8-7 mm. Lower glumes 1.3-3.4 mm; upper glumes 3.8-5.7 mm, 3-7-veined; lower lemmas 4.5-6.5 mm; upper florets 4.7-7 mm; anthers 0.8-2.4 mm. Caryopses ovoid, 1.2-3.2 mm long, 1.3-2.2 mm wide. 2n = (34), 68.
Cenchrus echinatus grows in disturbed areas throughout the coastal plain and piedmont of the southern United States, Mexico, Central and South America, and, as an unwelcome introduction, elsewhere.
3. Cenchrus gracillimus Nash
Plants perennial; sometimes forming dense clumps. Culms 20-80 cm, wiry. Sheaths shorter than the internodes, keeled, usually glabrous, rarely sparsely pilose; ligules 0.2-0.6 mm; blades 5-25 cm long, 1-3.5 mm wide, stiff, adaxial surfaces usually glabrous, smooth or scabrous. Panicles 2-6(6.8) cm; rachis internodes 2-4 mm; fascicles 5-13 mm long, 2-4 mm wide, not imbricate, ovoid, glabrous; outer bristles sometimes present, flattened; inner bristles less than 30, 3.2-6 mm long, 0.2-1 mm wide at the base, in more than 1 whorl, fused for at least 1/2 their length into a distinct cupule, diverging at irregular intervals from the cupule, somewhat flattened, spreading, purple-tipped at maturity. Spikelets 1-3 per fascicle, 4-7 mm. Lower glumes 1.4-3.1 mm; upper glumes 3.2-5.4 mm, 3-5-veined; upper lemmas 4-6 mm, 3-5-veined; upper florets 3.9-6.5 mm; anthers 0.9-1.9 mm. Caryopses 1.8-3 mm long, 1-1.5 mm wide, ovoid-elliptic. 2n = 34.
Cenchrus gracillimus grows in sandy soils of open pinelands, wet prairies, and river flatsof the southeastern United States and the West Indies.
4. Cenchrus spinifex Cav.
Coastal Sandbur, Common Sandbur
Plants annual or perennial but short-lived; tufted. Culms 30-100 cm, geniculate. Sheaths compressed, glabrous or sparsely pilose; ligules 0.5-1.4 mm; blades 3-28 cm long, (1)3-7.2 mm wide, glabrous or sparsely long-pilose adaxially. Panicles 3-5(8.5) cm; fascicles 5.5-10.2 mm long, 2.5-5 mm wide, imbricate, ovoid to globose, glabrous or sparsely to moderately pubescent; outer bristles, when present, mostly flattened; inner bristles 8-40 (rarely more), 2-5.8 mm long, 1-2 mm wide, fused at least 1/2 their length, forming a distinct cupule, the distal portions usually diverging from the cupule at multiple, irregular intervals, sometimes diverging at more or less the same level, ciliate at the base, pubescent, stramineous to mauve or purple, flattened. Spikelets 2-4 per fascicle, 3.5-5.9 mm, glabrous. Lower glumes 1-3.3 mm; upper glumes (2.8)3.5-5 mm, 5-7-veined; lower florets sometimes staminate; lower lemmas 3-5(5.9) mm, 5-7-veined; lower paleas sometimes reduced or absent; anthers 1.3-1.6 mm; upper lemmas 3.5-5(5.8) mm; anthers 0.5-1.2 mm. Caryopses about 2.5 mm long, 1-2 mm wide, ovoid. 2n = 34 (32).
Cenchrus spinifex is common in sandy woods, fields, and waste places throughout the southern United States and southwards into South America. It may be more widespread than shown in the northern portion of the contiguous United States because it has often been confused with C. tribuloides. Cenchrus spinifex differs from C. tribuloides in its glabrous or less densely pubescent fascicles, narrower inner bristles, and larger number of bristles. It has also been confused with C. longispinus, but differs in having shorter spikelets, fewer bristles overall, wider inner bristles, and outer bristles that are usually flattened rather than usually terete.
5. Cenchrus longispinus (Hack.) Fernald
Mat Sandbur, Longspine Sandbur, Cenchrus à Épines Longues
Plants annual; tufted. Culms 20-90 cm, sometimes decumbent, often with many branches arising from the base. Sheaths strongly compressed-keeled; ligules 0.6-1.8 mm; blades 4-27 cm long, 1.5-5(7.5) mm wide, adaxial surfaces scabrous or sparsely pilose. Panicles 1.5-8(10) cm; fascicles 8.3-11.9 mm long, 3.5-6 mm wide, somewhat globose, medium- to short-pubescent; bristles 45-75; outer bristles numerous, shorter and thinner than the inner bristles, imbricate, mostly terete, reflexed; inner bristles 3.5-7 mm long, 0.5-0.9(1.4) mm wide at the base, irregularly placed, fused for 1/2 their length or more, forming a distinct cupule, the distal portions diverging at irregular intervals from the cupule, often grooved along the margins, purple-tinged. Spikelets 2-3(4) per fascicle, (4)5.8-7.8 mm. Lower glumes 0.8-3 mm; upper glumes 4-6 mm, 3-5-veined; lower florets often staminate; lower lemmas 4-6.5 mm, 3-7-veined; anthers 1.5-2 mm; upper lemmas 4-7(7.6) mm; anthers 0.7-1 mm, seemingly not well-developed at anthesis. Caryopses 2-3.8 mm long, 1.5-2.6 mm wide, ovoid. 2n = 34 (38).
Cenchrus longispinus grows in sandy woods, fields, and waste ground in southern Canada and the contiguous United States. Its range extends southwards to Venezuela. It is often confused with C. spinifex and C. tribuloides; see discussion under those species.
6. Cenchrus tribuloides L.
Sanddune Sandbur, Dune Sandbur
Plants annual. Culms 10-70 cm, decumbent, branching and rooting at the lower nodes. Sheaths compressed, glabrous or pubescent; ligules 1-2.1 mm; blades 2-14 cm long, 3-14.2 mm wide. Panicles 2-8.2 cm; fascicles 9-16 mm long, 4-8 mm wide, imbricate, ovoid, densely pubescent; bristles 15-43; outer bristles usually present, flattened or terete; inner bristles 4-8 mm long, 1.2-3 mm wide, fused for at least 1/2 their length, forming a distinct cupule, the distal portions diverging at irregular intervals from the cupule, stramineous or purple. Spikelets 1(2) per fascicle, 6-8.8 mm. Lower glumes 1-4 mm; upper glumes 4.9-6.8 mm, 3-7-veined; lower lemmas 5.5-7.5 mm, 3-7-veined, enclosing the palea; upper lemmas 6-8.7 mm; anthers 0.8-2.8 mm. Caryopses 2.6-4 mm long, 2.2-3.1 mm wide, ovoid-elliptic. 2n = 34.
Cenchrus tribuloides grows in moist, sandy dunes and is restricted to the eastern United States. It differs from C. spinifex in its larger spikelets and smaller number of spikelets per fascicle, and from C. longispinus in its densely pubescent fascicles, fewer bristles, and wider inner bristles.
7. Cenchrus myosuroides Kunth
Plants perennial. Culms 5-200 cm, stout, glaucous. Sheaths from shorter than to equaling the internodes; ligules 1.5-2(3.4) mm; blades 12-40 cm long, 4-13 mm wide, glabrous or sparsely pilose adaxially. Panicles 4-23 cm; fascicles 3.8-8 mm long, 1.2-2.6 mm wide, composed of several whorls of bristles, not burlike; bristles 3-5.8 mm long, 0.2-0.6 mm wide, fused only at the base, not forming a cupule, terete, increasing in size inwards, inner bristles pubescent on the lower 1/2-2/3. Spikelets 1(2-3) per fascicle, 3.8-4.8(5.6) mm. Lower glumes 1.5-3 mm; upper glumes 3-5 mm, 3-5-veined; lower lemmas 3-5.5 mm; upper lemmas 3.8-5.4 mm; anthers 0.8-2.2 mm. Caryopses 1.5-2.6 mm long, 1-1.5 mm wide, ovoid. 2n = (54), 70.
Cenchrus myosuroides grows mostly along roadsides and in other waste places. Its native range extends through the Caribbean and Central America to northern South America.
8. Cenchrus biflorus Roxb.
Plants annual. Culms 5-150 cm, erect. Sheaths keeled, glabrous, scabrous, or slightly pubescent; ligules 1.3-2 mm; blades 2-35 cm long, 2-7 mm wide, flat, glabrous or scabrous (sparsely pilose). Panicles 2-15 cm; fascicles 4-11 mm long, 2-4.5 mm wide; bristles 30-60; outer bristles numerous, less than 1/2 as long as the inner bristles, terete; inner bristles 2.9-7 mm long, 0.2-1.1 mm wide, flattened, with 1-3 grooves abaxially, fused only at the base, forming a shallow disk, retrorsely scabrous, inner margins long-ciliate. Spikelets 1-3(4) per fascicle, 3.5-6 mm long, 1.2-1.9 mm wide; lower glumes 0.5-2.5 mm; upper glumes 2.5-4.9 mm, 3-5-veined; lower lemmas 3.2-5.5 mm, 4-5-veined; upper lemmas 3.4-5.9 mm; anthers about 1.5 mm. Caryopses 2-3.4 mm long, 1-3.5 mm wide, ovoid. 2n = 34.
Cenchrus biflorus is widely distributed from Africa to India. It was collected once in Westchester County, New York, but has not become established in the Flora region.