I9ii^] Hall-Goodspeed: Chrysil 245 



h. VABIATIOX DUE TO ENVIRONMENT 



Attempts have been made to correlate the differences in the rubber 

 content of individual plants with various factors of the environment. 

 This has not led, as yet, to positive conclusions. One reason for the 

 failure to obtain definite results is the low percentage in even the best 

 plants, so that auA* slight variation in the selection, preparation, or 

 analysis of any two samples might more than offset any original differ- 

 ence in their composition. Only through extensive experimental work, 

 can conclusive results be hoped for. 



Notwithstanding the difficulties just mentioned, a few observations 

 may be permitted. The water content of the soil is often considered 

 to exert an influence on rubber deposition in plants. Specimens of a 

 single variet}' of Chrysothamnus }wuseosus selected to determine this 

 point did not var}" widely in the amount of Chrj'sil present. There 

 was, if anything, a slight balance in favor of those growing on moist 

 banks but tliis difference was so slight that it is probably of no sig- 

 nificance. However, when different varieties are considered, and only 

 the average of a large number of samples taken into account, it is 

 found that those varieties which inhabit the moist and poorly drained 

 vallev bottoms contain rubber in larger amounts than do the varieties 

 of the \ery arid and well drained surrounding slopes. 



This is indicated by the table on page 244. The varieties there 

 numbered 1, 2, and 6 are lowland forms and these run higher in rubber 

 content than do numbers 3, 4, 8, 9, 10, 11, and 12, which are upland 

 forms. The single apparent exception to this rule is number 7, but 

 this may be due to the small number of samples analyzed for tliat 

 variety. 



But. while the formation of rubber thus seems to be favored by 

 the more abmidant moisture of the lowlands, another factor may be 

 the real cause of the increa.se in percentage, namely, the presence of 

 alkali. It is not impossible that the alkali acts indirectly through the 

 water relation. It is well known that its presence reduces the amount 

 of water available to plants, and it is conceivable that by thus lowering 

 the chresard it causes a deficiency in physiological moisture which is 

 even greater than the deficiency on the surrounding slopes. However 

 this may be, it is certain that the best rubber producing varieties of 

 Chrtjsothamnus nausposus are those of the alkaline valley bottoms. 



As to temperature, it seems unlikely that this factor exerts an 

 influence on rubber formation except as it acts through other agencies. 



