HITCHCOCK AND CHASE NORTH AMERICAN PANICUM. 45 



Panicum arizonicum Scribn. & Merr. U. S. Dept. Agr. Div. Agrost. Circ. 32: 2. 

 1901. Based on "(Panicum (sine nomine) Scribn. Bui. Torr. Bot. Club, 9: 76. 1882; 

 P. fasciculatum dissitiflorum Vasey, in herb. Not P. dissitifiorum Steud. 1841)." 

 The authors also cite, "Type specimen collected on mesas near Camp Lowell, Santa 

 Cruz Valley, Arizona, 465 C. G. Pringle, 1881." As indicated above this species 

 was first mentioned as "Panicum (Virgaria) sp." where the specimen referred to is 

 Pringle 465. The same specimen, which is in the National Herbarium, was marked 

 by Doctor Vasey, "Panicum fasciculatum var. dissitiflorum, " and later by Scribner and 

 Merrill as the type of P. arizonicum. It is about 60 cm. high, but more slender than 

 Palmer's specimen mentioned above; the sheaths and blades bear only a few scattered 

 papillae, mostly without hairs. 



Panicum fasciculatum dissitifiorum Vasey; Scribn. & Merr. U. S. Dept. Agr. Div. 

 Agrost. Circ. 32:2. 1901. This herbarium name is given as a synonym of P. 

 arizonicum of which it is a typonym. 



Panicum arizonicum tenue Scribn. & Merr. U. S. Dept. Agr. Div. Agrost. Circ. 32 : 

 3. 1901. "Type specimen collected at Fort Huachuca, Arizona, by T. E. Wilcox in 

 1894." The type, in the National Herbarium, is the small form common in sterile 

 soil. The largest specimen is 17 cm. high. Some of the sheaths are sparsely papillose- 

 hispid, some glabrous. 



Panicum arizonicum laeviglume Scribn. & Merr. U. S. Dept. Agr. Div. Agrost. Circ. 

 32: 3. 1901. "Type specimen collected at Mescal, Arizona, 1810 David Griffiths, 

 October, 1900." The type, in the National Herbarium, is a plant about 20 cm. high, 

 with glabrous spikelets and glabrous to sparsely papillose-hispid sheaths. 



Panicum arizonicum major[us] Scribn. & Merr. U. S. Dept. Agr. Div. Agrost. Circ. 

 32: 3. 1901. Based on P.fuscum majus Vasey. 



DESCRIPTION. 



Plants erect or ascending, sometimes decumbent at base and rooting at the lower 

 nodes, branching at the base and lower nodes; culms 20 to 60 cm. high, glabrous except 

 below the panicle, the nodes sometimes slightly pubescent; sheaths shorter than the 

 internodes or the upper often overlapping, rather loose, glabrous to strongly papillose- 

 hispid; ligule a ring of hairs about 1 mm. long; blades rather thin, ascending or 

 spreading, 5 to 15 cm. long, 6 to 12 mm. wide, rounded at base, glabrous on both 

 surfaces, or scabrous to papillose-hispid beneath, the scabrous, thin, cartilaginous 

 margin usually papillose-ciliate at base; panicles usually long-exserted, 7 to 20 cm. 

 long, the solitary, ascending, slender branches loosely flowered, the spikelets borne on 

 very short, appressed branchlets, the pedicels and axes of branchlets, branches, and 

 the entire panicle finely pubescent and also copiously papillose-hirsute; spikelets 3.5 

 to 3.8 mm. long, obovate-elliptic, abruptly pointed, attenuate at base as in P. molle, 

 densely hirsute to glabrous; first glume clasping, half the length of the spikelet, acute, 

 5-nerved ; second glume and sterile lemma pointed beyond the fruit, 5-nerved, the 

 nerves sometimes anastomosing as in P. molle; fruit 2.9 to 3 mm. long, 1.5 to 1.6 

 mm. wide, obovate-elliptic, apiculate. 



This species is variable in size and in the amount of pubescence. In cultivated or 

 moist soil it is robust as in the type of the species or of Scribner and Merrill's subspecies 

 majus. The commoner form is smaller, more like the type of Scribner and Merrill's sub- 

 species tenue. The form separated by Scribner and Merrill as subspecies laeviglume, 

 because of the glabrous spikelets, appears to have no other distinguishing characters. 

 The following specimens are this form, though in some cases the spikelets are sparsely 

 pubescent or some of the spikelets are glabrous and some are pubescent: Canby 8, 

 Griffiths 1913, 6152, 6168, 6929, 6938, 6939, 6990, Griffiths & Thornier 75, 230, 239, 

 Merton 1694, Metcalfe 768, Pringle 487, Smith in 1896, Wilcox in 1894. 



