42 EUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



the acetone-benzene method as ah-eady described, so that the per- 

 centages are taken to represent the proportion of pure rubber in the 

 samples. This perhaps accounts for the low percentages as compared 

 with those reported by others. 



VII. SPECIES WITH VERY LOW RUBBER-CONTENT. 



In the course of the investigation it was found that, while perhaps 

 all latex-bearing plants contained rubber, it occurs in most of the 

 species in such small quantities as to be negligible. However, it is 

 quite probable that further studies will demonstrate considerably 

 higher percentages in the individual plants of some of the species here 

 reported as containing but small amounts. This assumption is based 

 on the fact that rubber-content varies with ecologic conditions and 

 with the hereditary strain or race. It is also probable that the per- 

 centage content of these low-grade species could be increased by 

 bringing them under cultivation, where they could be subjected to a 

 considerable variety of cultiu-al methods, such, for example, as changes 

 in the water relation, and where they could be improved through 

 selection and breeding. However this may be, it seems unnecessary 

 to do more than to give the results of our analyses at the present time, 

 and so in table 12, which refers for the most part to very poor species, 

 will be found a few in which the rubber-content is sufficiently high to 

 be of more than passing interest. 



All of the species listed in table 12 are latex plants, with the excep- 

 tion of Aster spinosus, Hymenoxys odorata, Pyrrhopappus muUicauUs, 

 and Silphium laciniatum. The discovery of rubber in these species is 

 especially noteworthy in that its presence has not been heretofore 

 known in any of the genera to which they belong, with the exception 

 of Hymenoxys. This is of some biologic although not of much com- 

 mercial interest. There is, of course, a possibility that the benzene 

 extract reported for these as "rubber" may, in some instances, be some 

 other substance, something that comes out with the benzene extract 

 through errors in methods or manipulation. In each case, however, 

 the results have been checked by one or more duplicate analyses and 

 the percentages are sufficiently high and sufficiently in agreement to 

 justify a fair degree of confidence in the report. 



Table 12 includes some latex plants in which the benzene extract is 

 so low, 1 per cent or less, that it can not be said with certainty that 

 rubber is present. Yet since every species with a latex is of interest 

 as a possible carrier of rubber, all of these are here listed for comparison, 

 regardless of the percentage found on analysis. 



In cases where the part of the plant analyzed is reported as "whole" 

 this is to be taken as exclusive of root and flowers; in other words, the 

 stems and foliage were ground up and analyzed together. All per- 

 centages are computed upon the basis of absolutely dry weight of the 

 material analyzed. 



