HITCHCOCK AND CHASE—-NORTH AMERICAN PANICUM. 45 
Panicum arizonicum Scribn. & Merr. U. 8. Dept. Agr. Div. Agrost. Circ. 32: 2. 
1901. Based on ‘‘(Panicum (sine nomine) Scribn. Bul. Torr. Bot. Club, 9: 76. 1882; 
P. fasciculatum dissitiflorum Vasey, in herb. Not P. dissitiflorum 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 Scribnerfind . 
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 
papille, mostly without hairs. 
Panicum fasciculatum dissitiforum 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 17cm. high. Some of the sheaths are sparsely papillose- 
hispid, some glabrous. 
Panicum arizonicum laeviglume Scribn. & Merr. U. 8. 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. 
82: 3.1901. Based on P. fuscwm 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 15cm. 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 isrobust as in the type of the species or of Scribner and Merrill’s subspecies 
majus. ‘The commoner form issmaller, more like the type of Scribner and Merrill’s sub- 
species ftenue. 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 & Thornber 75, 230, 239, 
Merton 1694, Metcalfe 768, Pringle 487, Smith in 1896, Wilcox in 1894. 
