50 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



in another 3.2 per cent, and in the third 4.1 per cent of rubber. In 

 no species have approximately equal amounts been found in more than 

 2 plants from a single small area. 



The evidence thus far available seems, therefore, to point to the 

 presence of innumerable strains within at least some species of Asclepias, 

 the rubber-content varying with these strains, as well as with changes 

 in the environment. 



The influence of soil, moisture, and other external conditions has 

 not been studied in detail, but there is some indication that the presence 

 of alkali in the soil increases the rubber-content of Asclepias mexicana. 

 On the other hand, there is no evidence that poor or dry soil or defi- 

 ciency in rainfall increases the amount. In fact, some of the best 

 rubber-producing species, such as Asclepias sulUvanti, grow only on 

 rich and moist bottom-lands. Within single species the plants from 

 poor soil average about the same as those from good soil, but with 

 some indication that especially robust plants are likely to carry rather 

 high percentages of rubber, this being associated, perhaps, with their 

 increased photosjoithetic activity. 



Seasonal variation. — Detailed studies were carried out on several 

 species in order to determine the presence or absence of a correlation 

 between the age of the plant and its rubber-content. It was thought 

 that such correlation, if it existed, would throw some light upon the 

 perplexing question of the r61e that rubber plays in the economy of 

 the plant. The effect of other factors, however, is too great to permit 

 the drawing of any definite conclusions. Aside from such obvious 

 modifying influences as changes in temperature and moisture, the 

 translocation of rubber withia the plant must be allowed for, and more 

 important than all, the wide extent of individual variation. It is 

 obvious that analyses made from leaves of a single plant but taken 

 on different dates would yield data of very doubtful value, since there 

 is no evidence that leaves in different positions carry equal percentages 

 even at the same time. Moreover, the removal of a sufficient number 

 of leaves to permit of analysis would almost certainly modify the 

 process of rubber formation in the rest of the plant. With these 

 difficulties in view, seasonal studies on several species were inaugurated. 

 There is a lack of concordance in the results, but the indications are 

 that both the total amount and the percentage increases as the herbage 

 matures, but that they again fall off as the parts approach senility, 

 or, in the case of stems, as lignification sets in. 



An experiment, the results of which seem to controvert the theory 

 that the percentage increases with maturity, will be first described. 

 Several plots of Asclepias sulUvanti were selected near Lincoln, 

 Nebraska, and samples from them were taken at intervals of approxi- 

 mately 8 days each. Each lot was chosen because of the apparent 



