LATEX-PRODUCING PLANTS 369 



crossing of species and varieties in attempts to get a higher yield 

 and higher grade of rubber. 



In recent years several goldenrods have been experimentally 

 grown for rubber production. Thomas Edison was working on 

 this problem when he died, and his experiments are being con- 

 tinued. As in guayule, the rubber occurs in the form of tiny 

 granules which must be extracted from the tissues containing 

 them. 



Synthetic rubbers always command popular interest; the 

 idea of man producing in a test tube, or under his own controlled 

 conditions, a material for which he has been dependent upon 

 nature, is very fascinating. Synthetic rubbers have been most 

 highly developed commercially in Germany and Russia, espe- 

 cially during the World War when Germany was cut off from 

 natural sources of rubber. After the war, this expensive com- 

 mercial production went out of use. An American chemical 

 company is now manufacturing an 2irtificial rubber which is said 

 to be more wear-resistant than natural rubber, but less elastic. 

 The principal objection to artificial rubber is its cost in com- 

 parison to that of plantation rubber. 



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