30 



RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



on good soils, it should be tried also on sandy or stony slopes, since 

 such lands would be less expensive and the natural habitat indicates 

 that growth can be made under such conditions. As in the case of 

 Asclepias calif omica, the tendency of the plants to spread out close 

 to the ground may interfere somewhat with mowing, although close 

 planting might correct this tendency. Field observations indicate 

 that there is considerable variation in the habit, the stems in some 

 localities tending to spread, in others to assume a strictly upright 

 position. The best-formed plants seen during the present study were 

 found south of Pueblo, Colorado. Here each individual formed 

 a dense leafy clump 3 feet high and about 4 feet across. The excep- 

 tionally large size of the roots of this species augurs well for its ability 

 to send up new shoots after a crop has been gathered. 



Rubber-content.— In considering table 5, Nos. 342 and 914 may be 

 ignored, since the former includes very old leaves and the latter only 

 stems. This leaves 2 to 3.8 per cent for the rubber-content as far as 

 known. A study of the weight of the foliage and of probable yield of 

 rubber per acre has been made on the basis of material gathered at 

 Ashfork, Arizona, under Nos. 928 and 953, as follows : average number 

 of leaves per stem, 24; average dry weight of single leaves, 0.037 

 ounce; average number of stems per square foot, 4; computed weight 

 of dry leaves per acre, 9,392 pounds. On the basis of a 3 per cent 

 analysis this would yield 281 pounds of rubber per acre. It is evident 

 that a yield so low as this would not justify one in growing this plant 

 for rubber even on cheap land, and that other species, notably Asclepias 

 subulata, are much more promising as rubber producers, based upon 

 our present knowledge. The figures indicate, however, only the yield 

 from the unimproved form. Attempts to increase the yield should 

 aim at the development of a more erect and taller grower, as well as 

 of a strain with increased rubber-content. 



Table 5. — Chemical analyses of Asclepias latifolia. 



* Medium material. Leaves of this number from several plants. 



' Very old. 



