36 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



simple, straight, covered with a fine down which is deciduous, leaving 

 the old bark smooth; leaves numerous to the top, 10 to 20 or perhaps 

 more to each stem, either all in pairs or some of the upper ones in 

 whorls of 3, broadly lanceolate or oblong-ovate, 4 to 6 inches long, 2 to 3 

 inches wide, thick, finely gray-tomentose on both sides, or greener and 

 nearly smooth above; flowers flesh-color, large, and showy, appearing 

 from June to August, in stout-stalked umbels from the upper leaf- 

 axils and terminal; pods on deflexed stalks, either erect or pendent, 

 with an ovoid base but prolonged into a stout beak, 3.5 to 4 inches 

 long, 1 to 1.25 inches thick, covered with a gray felt-like tomentum, 

 through which project numerous short, soft processes, maturing in 

 August and September. 



References Curtis, Bot. Mag., pi. 4413, 1848. Gray, Syn. Fl., 2 1 : 91, 1878. Britton 

 and Brown, 111. Fl., ed. 2: 30, fig. 3399, 1913. 



Distribution and ecology. This is the most widely distributed of all 

 of the milkweeds of western North America. It ranges from Alberta 

 and Minnesota to Iowa, Texas, Arizona, California, and British 

 Columbia. The vertical distribution is also remarkable, since it grows 

 from the hot, low, interior valleys, as at St. George, Utah, and Caliente, 

 southern Nevada, to the moderately cool mountain meadows in the 

 lower part of the pine belt (Transition Zone), where it is more abundant, 

 reaching altitudes of over 8,000 feet in New Mexico. It is always 

 partial to moist soil and is therefore found in seepage areas along 

 ditches and creeks and in moist meadows. 



It is readily seen from the facts of distribution and habitat that the 

 showy milkweed is a species which could be grown under a wider 

 range of temperature conditions than any other. It is also to be 

 noted, on the other hand, that it is very exacting in its moisture 

 requirements and that probably it could not be grown to perfection, 

 except in moist and fairly good soil. Sandy soil would doubtless be 

 satisfactory if kept moist and free from excessive alkali. These condi- 

 tions, together with the usually small percentage of rubber carried 

 by the plant, do not indicate this as a promising species for further 

 trial. 



Rubber-content. The earlier analyses by Widtsoe and Hirst have 

 been already referred to (p. 10). The discouragingly low yields 

 obtained by these workers are practically duplicated by most of the 

 analyses reported upon in table 9. The amount of rubber in the 

 leaves reached as high as 3 per cent in only 2 samples out of the 7 

 examined. It is possible that the analysis of a large series from widely 

 separated localities will disclose some high-percentage plants, but 

 Asclepias spedosa does not hold out as much promise in this respect as 

 some other species of similar habit. 



