28 



RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



best development both of the species and of individuals is found on the 

 hot foothill slopes of southern California, whereas in the Coast Ranges 

 farther north the plants are more scattered and less well developed. 

 Irrigation would not be essential, although in some districts it might 

 be practiced during the winter or spring in order to stimulate growth. 

 On the other hand, an excess of moisture and low heavy soil, as well as 

 alkali, should probably be avoided. 



A feature of this species which might militate against its use is its 

 lack of an erect growing-habit. Since the stems spread more or less 

 from the base, each plant requires considerable space and perhaps would 

 be somewhat difficult to catch with ordinary mowing machinery. 

 However, the stems never lodge or lie directly on the ground, and 

 it is probable that they would approach a vertical position if grown 

 closely together. The abundant woolly hairiness of the stems and 

 leaves should also be taken into account, since this might interfere 

 with the use of certain types of machinery used in preparing the plants 

 for extraction. 



TABLE 4. Chemical analyses of Asclepias californica. 



1 Good condition. 2 Partly molded. 



6 Young; product granular, not rubber-like. 



3 Old and drying. 



4 Very old. 



Rubber-content. The results of the chemical examination of three 

 collections of this milkweed exhibit a wide range of variation. This 

 may be due to the inclusion of several strains in this one species, but 

 it may also be explained on other grounds. It is noted, e. g., that the 

 leaves of No. 919 were very old and dry, that those of No. 360 were also 

 past maturity, although not yet hardened, while those of No. 350, which 

 gave the highest yield, were about at the stage of full maturity. 



The analysis of the dried latex is interesting, since it is one of the 

 few analyses made of this substance as it occurs in Asclepias. The 

 results are considerably higher than those given by Fox (1911) for 

 Asclepias syriaca, and also higher than those found in tropical latex 

 plants, such as Hevea brasiliensis. 



