THE MOST IMPORTANT SPECIES. 29 



ASCLEPIAS LATIFOLIA (BROAD-LEAF MlLKWEED). 

 Synonym: Asclepias jamesi. 



Description. Plant a stout, very leafy perennial herb, usually 1 

 to 2 but sometimes 3 feet high; roots coarse and woody; stems several, 

 sometimes much crowded and forming bush-like plants, unbranched, 

 either erect or spreading, not hairy; leaves ample, commonly 12 to 20 

 but often 30 on each stem, oval to orbicular, somewhat heart-shaped at 

 base, 4 to 6 inches long and nearly as wide, very thick, minutely hairy when 

 young but soon becoming smooth and green; flowers greenish, appear- 

 ing from June to August, borne in dense, short-stalked clusters from 

 the axils of the upper leaves; pods erect on curved stalks, ovoid, 2 to 3 

 inches long, about 1 inch thick, maturing in September and October. 



References. Gray, Syn. Fl., 2^92, 1878 (as A. jamesi). Britton and Brown, 111. Fl.' 

 ed. 2, 27: fig. 3391, 1913. 



Distribution and ecology. The broad-leaf milkweed is an inhabitant 

 of the plains and lower foothills from Nebraska and Colorado south and 

 west to Kansas, Texas, and northwestern Arizona. It belongs to the 

 pinon-juniper association and always grows, so far as we have been 

 able to observe, in small, widely separated clumps, these occupying 

 several square feet and with stems probably all from one root. Along 

 the eastern base of the Rocky Mountains it usually occupies the 

 summits of low, rounded hills, or grows scatteringly over warm, dry 

 southerly slopes where the soil is well drained. Specific localities 

 include hills south of Hays, Kansas; Ashfork and near Williams, 

 Arizona; and the following, all in Colorado: Canyon City, low hills 

 north of Walsenberg, and south slopes above the Purgatoire River 

 just west of Trinidad. Records of rainfall and temperature at all of 

 these stations are not at hand, but at Trinidad, which may be taken 

 as representing the slopes along the easterly base of the Colorado 

 Rockies, where the species is abundant, the normal annual precipita- 

 tion is 17 inches, the lowest recorded temperature is 26 F., and the 

 highest recorded temperature 101 F. At Pueblo, in the same general 

 district, the temperatures are about the same, but the normal precipi- 

 tation is slightly under 12 inches. 



In this species we have a form which is not exacting in its require- 

 ments. To all appearances it has been driven to the habitats just 

 described because of competition with other plants where the soil 

 and moisture conditions are more favorable. If this has been the 

 case, then if brought under cultivation on good soils and competition 

 removed through proper tillage the plants would doubtless respond 

 with a greatly increased vigor and growth. While this would increase 

 the tonnage, the percentage of rubber in the plant might also be affected 

 and perhaps disastrously. Only by experiment can this point be 

 positively determined. Aside from experimenting with this species 



