LEGUMINOS&. SILVA OF NORTH AMERICA. 107 
PROSOPIS PUBESCENS. 
Screw Bean. Screw Pod Mesquite. 
Lrecume thick, spirally twisted. Pinne 10 to 16-foliolate. 
Prosopis pubescens, Bentham, Lond. Jour. Bot. v. 82; PP. odorata, Torrey, Frémont’s Rep. 313, t.1 (excl. flowers). 
Trans. Linn. Soc. xxx. 380 (Rev. Mim.). — Walpers, P. Emoryi, Torrey, Emory’s Rep. 139. 
Ann. i. 259.— Watson, King’s Rep. v. 420.— Brewer & Strombocarpa pubescens, Gray, Smithsonian Contrib. iii. 
Watson, Bot. Cal. i. 163. — Rothrock, Wheeler’s Rep. 60; v. 51 (Pl. Wright. i., ii.) ; Ives’ Rep. 9.— Torrey & 
vi. 42, 107. — Hemsley, Bot. Biol. Am. Cent. i. 344. — Gray, Pacific R. R. Rep. ii. 263.— Torrey, Pacific R. 
Sargent, Forest Trees N. Am. 10th Census U. S. ix. R. Rep. iv. 11, 20, 82; v. 360, t. 4; vii. pt. iii. 10; Bot. 
62. — Coulter, Contrib. U. S. Nat. Herb. ii. 95 (Man. Pl. Mex. Bound. Surv. 60. 
W. Texas). Strombocarpa odorata, Torrey, Sitgreaves’ Rep. 158. 
A small tree, rarely twenty-five or thirty feet in height, with a slender trunk sometimes a foot in 
diameter ; or more often a tall many-branched shrub. The bark of the trunk is thick and light brown 
tinged with red, and exfoliates in long thin persistent ribbon-like scales." The branches are terete, 
armed with stout rigid geminate supra-axillary spines; they are canescently pubescent or glabrate 
when they first appear, but glabrous in their third year, when they are covered with light red-brown 
bark. The leaves are alternate on the young shoots and fascicled in the axils of the leaves of the 
previous years, canescently pubescent, and deciduous; they are two or three inches long, each with a 
slender petiole from a third to two thirds of an inch in length bearing a minute gland near the apex, 
which is tipped with a small spinescent rachis, and two pinne each an inch and a half or two inches 
long with five or eight pairs of oblong or somewhat falcate acute sessile or shortly petiolulate leaflets, 
often apiculate, conspicuously reticulate-veined, and from a third to two thirds of an inch in length 
and an eighth of an inch in breadth. The stipules are spinescent and deciduous. The flowers, 
which begin to open in early spring, are produced in successive crops, and are greenish white; they 
are sessile from the axils of minute scarious deciduous bracts, and are borne in dense or interrupted 
cylindrical pedunculate spikes two or three inches in length. The calyx is obscurely five-lobed, pubes- 
cent on the outer surface, one third or one fourth as long as the narrow acute petals coated on the 
inner surface near their apex with thick white tomentum, and slightly puberulous on the outer surface. 
The ovary and young fruit are covered with pale tomentum. The legumes, which are borne in dense 
racemes, are sessile and are twisted with from twelve to twenty turns into a narrow straight spiral one 
or two inches in length. The outer coat of the legume is thin and woody and of a pale straw color ; 
it incloses the thick sweet pulp of the mesocarp in which are the seeds, wrapped in separate envelopes 
and flattened by mutual pressure. The seeds are obovate, a sixteenth of an inch long, with a thick 
very hard pale brown testa and thin horny albumen. The fruit ripens throughout the summer, and falls 
in the autumn. 
Prosopis pubescens is common in the valley of the Rio Grande in western Texas, from the mouth 
of the Devil’s River to El Paso; it extends westward through New Mexico and Arizona, and in Cali- 
fornia through the arid region of the Colorado basin to San Diego County; it reaches the southern 
borders of Utah and Nevada, and extends southward into northern Mexico. It occupies sandy or gravelly 
bottom-lands, and attains its greatest size in the United States in the valleys of the lower Colorado and 
Gila rivers. 
1 The bark on an old trunk of Prosopis pubescens has a shaggy appearance like that of a very old Grape-vine or of a large-stemmed 
Cowania Mexicana. 
