32 



RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



between the different samples tested, with the highest one yielding 4.8 

 per cent from the leaves. It seems impossible to correlate this varia- 

 tion either with geographic distribution or ecologic conditions. The 

 inference, therefore, is that the species comprises a considerable num- 

 ber of genetic strains, that further exploration may discover better 

 ones than any thus far found, and that breeding experiments with so 

 large a number to select from might yield surprising results. On the 

 other hand, the foliage is more sparse than in most other milkweeds 

 and constitutes only a minor portion of the total weight of the plant. 

 A form in which the stems as well as the leaves would carry a high 

 rubber-content is therefore desirable. The possibility of producing 

 such a strain is indicated by sample No. 918, from alkaline soil of the 

 San Joaquin Valley, in which the stems yielded 2.3 per cent at the 

 same time that the leaves carried 4.4 per cent. If it is found that these 

 figures can be increased by scientific breeding and the plants grown on 

 alkaline soil to the size and density of that shown on plate 3, Asclepias 

 mexicana will take its place as one of the most promising species. 



TABLE 6. Chemical analyses of Asclepias mexicana. 



1 Single plant. 



2 Soils trongly alkaline. 



3 Second growth; mixture of leaves from 80 plants. 



4 Mixture of leaves of 40 plants. 

 6 Second growth. 



8 Computed. 



7 Second growth; mixture from 40 plants. 



8 Second growth from areas adjacent to 



941 and 1038. 



9 Third growth from same roots as 941. 



