50 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



DESCRIPTION OF THE GENUS AND SPECIES. 

 CENCHRUS L. 



Spikelets sessile, one to several together, permanently inclosed in a bristly 

 or spiny involucre or bur, composed of more or less coalesced sterile branchlets ; 

 burs sessile or nearly so on a slender, compressed or angled axis, its apex 

 produced into a short point beyond the uppermost bur, the burs falling entire, 

 the grains germinating within them; involucre (especially in our species) 

 somewhat oblique, its body irregularly cleft, the lobes rigid, in most species 

 resembling the spines, the cleft on the side of the bur next to the axis reaching 

 to the tapering, abruptly narrowed or truncate base, the bristles or spines 

 barbed, at least toward the summit; spikelets mostly glabrous or nearly so; 

 first glume 1-nerved, usually narrow, sometimes wanting; second glume and 

 sterile lemma 3 to 5-nerved, the lemma inclosing a well-developed palea and 

 usually a staminate flower ; fruit usually turgid, indurate, the lemma acuminate, 

 the nerves visible toward the summit, the margins thin, fiat, a prominent 

 U-shaped ridge on the back just above the base, the radicle at germination 

 breaking through its outer margin ; stamens 3 ; styles 2, the stigmas plumose ; 

 grain dorsally compressed, with a punctiform hilum, free within the lemma 

 and palea. 



Annuals or perennials, mostly of sandy or arid soils. The burs at 

 maturity are readily attached by their barbed spines to passing animals, the 

 seed thus being widely distributed. In the Caribbean Islands sandburs have 

 been found attached to the feet and plumage of water birds. 



In America the species are found from Massachusetts to Oregon and south to 

 Argentina and Chile. In the United States they are commonly called sandburs. 

 Other names are burgrass, sand spur, hedgehog grass, and devil's burs. The 

 species have some forage value, especially in the Southwestern States, where, 

 starting growth in early spring, they produce an abundance of leafy forage 

 which is readily grazed until the burs ripen. On the whole, however, the species 

 are troublesome weeds in fields and waste ground. 



About 25 species are known, 15 in the western hemisphere, the others in arid 

 parts of southwestern Asia, eastern Africa, Australia, New Zealand, Tasmania, 

 and Hawaii. 



In Cenchrus is found the extreme specialization of sterile branchlets of the 

 inflorescence, the simplest form of which is found in Pancium, subgenus 

 Paurochaetium (Panicum chapmani Vasey and its allies) 1 , in which the ulti- 

 mate branchlet of the narrow panicle is produced beyond the uppermost spikelet 

 as a minute bristle, persistent on the axis, the spikelet falling without it. In 

 the West Indian genus Paratheria and the Australian Chamaer aphis, with a 

 single sterile branchlet below the spikelet, is found the simplest form of the 

 series in which the articulation is at the base of the spikelet-bearing branch, 

 the sterile branchlets falling attached to it. In Pennisetum the sterile branch- 

 lets are few to many, usually very slender, not rigid, free or rarely united at 

 the very base. In Cenchrus the sterile branchlets are rigid and united below. 

 This specialization reaches its extreme development in our North American 

 species, in all but one of which the united branchlets form a cuplike receptacle 

 in which the spikelets are partly hidden. The immense burs of C. palmeri are 

 the utmost known development of the specialization of sterile branchlets. Sev- 

 eral species of the eastern hemisphere are more like the introduced C. cathar- 

 ticus. In C. pilosus the bristles are antrorsely scabrous. In C. australis of 

 Australia, with plumose, less rigid involucre, the genus approaches Pennisetum. 



1 Hitchcock & Chase, Contr. U. S. Nat. Herb. 15: 22. 1910. 



