THE MOST IMPORTANT SPECIES. 31 



ASCLEPIAS MEXICANA. 



Description. — ^A perennial erect herb, woody at the base, 2 to 4 

 feet or in moist situations to over 6 feet high, usually narrow and 

 slender-stemmed, but up to 8 feet in diameter in vigorous forms, 

 especially in alkaline soil, sometimes spreading by the growth of the 

 roots to form beds 15 feet or more across; roots connected by deep- 

 seated undergroimd horizontal branches; stems usually few at a place, 

 but imder favorable conditions as many as 80 to 100 are crowded into 

 a single clmnp, straight, erect, mostly unbranched, green and glabrous; 

 leaves 40 to 80 on each stem, arranged in whorls of 3 or 6 each, linear, 

 tapering to the apex, 2.5 to 6 inches long, 0.25 to 0.5 inch wide, green 

 and glabrous like the stems; flowers appearing in summer, dull white, 

 smaller than in most other species, in small roimded clusters on erect 

 pedimcles from axils of upper leaves; pods on curved or straight stalks, 

 narrowly ovoid, tapering to apex, 2.5 to 3.5 inches long, about 0.5 inch 

 thick, minutely pubescent, the seeds ripening from late July to October. 



Ke/ercnces.—Cavanilles, Icon., 1:42, pi. 58, 1791. Gray, Syn. FI., 2M 96, 1878. Hall 

 and Yates, Calif. Agr. Exp. Sta. Bull. 249, fig. 5, 1915. 



Distribution and ecology. — The range of this species extends from 

 Mexico, whence came the original specimens, to Arizona, Nevada, 

 California, northern Idaho, and southern Washington. It grows in 

 greatest abimdance and to the largest size in the hot interior valleys 

 of California, but plenty of vigorous plants are also to be found in the 

 more elevated valleys of western Nevada, such as those of the Carson 

 and Truckee Rivers. The largest plants are always foimd in moder- 

 ately alkaline soil and are often associated with such halophytes as 

 Sporobolus airoides and Distichlis spicata. These facts suggest that 

 the proper place for the cultivation of the plant on a large scale would 

 be the vast expanses of territory in the San Joaquin Valley of California 

 and the valleys of western Nevada which are now uncultivated either 

 because of alkaline conditions or the lack of water for irrigation. 



An ecologic feature especially noticeable in this species is the prompt- 

 ness with which new stems are sent up after the old ones have been 

 removed. It is certain that xmder ordinary conditions two crops 

 could be harvested in a year, and this probably without any reduction 

 in the percentage content of rubber (see p. 52). Even a third crop 

 seems not impossible if the roots receive a reasonable amoimt of 

 moisture. In the single instance where this was attempted the 

 locality and the season were so unfavorable that only a sparse growth 

 resulted. The experiment should be repeated in one of the moist, 

 alkaline valleys where the plants exhibit their maximum vigor. It 

 must be remembered, however, that excessive cropping might so 

 weaken the roots as to diminish the growth of the year following. 



Rubber-content. — The chemical analyses of Asclepias mexicana 

 indicate that there is a wide range of variation in rubber-content 



