21 HOUGH'S AMERICAN WOODS. 



furnished with minute glands. Flowers regular, small, greenish, and usually 

 sessile in cylindrical or globose, axillary, pedunculate spikes or heads; calyx 

 campanulate, with 5 very short teeth valvate in aestivation; petals 5, connate at 

 first below, at length free, distinct, totnentose within (in our species), hypogynous, 

 valvate in aestivation; stamens 10, free, exserted, those opposite the calyx-lobes 

 rather the longer, with oblong, versatile, introse, 2-cellecl anthers, dehiscent by 

 lateral longitudinal slits, and connective usually tipped with a minute deciduous 

 gland; pistil with tiliform style, minute stigma and villose (in the American species) 

 ovary containing many anatropous, suspended ovules in 2 ranks, from the inner 

 angle of the ovary. Fruit a linear coriaceous legume, compressed or nearly terete, 

 straight, falcate or twisted into a spiral, indehiscent, with usually thick spongy 

 mesocarp and partitions between the numerous compressed ovate-oblong seeds, 

 which have a crustaceous testa and contain horny albumen, an embryo with short, 

 straight radicle and flat cotyledons. 



Genus represented in the United States by small trees and shrubs in the arid 

 regions of the Southwest. (The name Prosopix is the ancient Greek name of the 

 burdock and is of obscure application here.) 



205. PROSOPIS ODORATA, TORR. & FREM.* 



SCREWBEAN, SCREW-POD MESQUITE. 



Ger., Scfiraubenhulse; Fr., Cosse de vis; Sp., Tornillo. 



SPECIFIC CHARACTERS: Leaves alternate on the new shoots and in fascicles in 

 the axils of the leaves of previous years, canescently puberulent, deciduous, 2-3 

 in. long, with slender petiole about $ in. long terminating in a slender spine, and 

 a pair of pinnaa, each furnished with 5-8 pairs of oblong, acute, subsessile leaflets 

 about in. long by in. broad and conspicuously recticulate-veined ; stipules 

 spinescent and deciduous; branchlets terete, somewhat pubescent the first year, 

 armed with rigid supra-axillary spines arranged in pairs. Flowers (in early spring 

 and continuing later), greenish white about | in. long in dense cylindrical pedun- 

 culate spikes 2-3 in. in length; calyx obscurely 5-lobed, pubescent outside, about 

 one quarter as long as the petals, which are narrow, acute, puberulous outside 

 aud white tomentose within near apex; stamens exserted; ovary very villous. 

 Fruit a narrow yellowish pod which is twisted in to a close spiral of 12-20 turns, 

 from 1-2 in. in length and ^ in. in diameter and subsessile in racemose clusters. 

 The pod contains a sweet pulp (mesocarp) which invests the small obovate seeds 

 about Yg- in. long with horney albumen. 



(The specific name, odorata, the Latin for fragrant, alludes to, the fragrance of 

 the flowers.) 



The Screw-pod is a small tree occasionally attaining the height of 

 30 ft. (9 m.) with wide top and a trunk occasionally 1 ft. (0.30 m.) in 

 diameter, .vested in a rather thin cinnamon -brown bark which exfoli- 

 ates in long, thin, papery scales and strips, giving to old trunks a 

 shaggy appearance similar to that of an' old grape-vine. It is often 

 shrubby in habit of growth, but is always quickly recognizable by its 

 curious ringlet-like pods. 



HABITAT. The valley of the Rio Grande in western Texas and 

 westward through New Mexico and Arizona to the Colorado desert 

 in southeastern California, occupying mainly moist bottom-lands and 

 attaining its best development in the vallies of the Gila and lower 

 Colorado rivers. 



* Prosopis pubescens. Benth. 



