130 THE WHOLE ART OF RUBBER-GROWING 



CHAPTER XIII. 

 Assimilative and Secondary Rubbers. 



T N the foregoing essays I have naturally con- 

 fined myself to those rubber-producing trees 

 which more or less readily lend themselves to 

 profitable exploitation in that particular zone 

 which has become the recognised home of plan- 

 tation rubber ; but there are, of course, several 

 other plants which contribute — many of them 

 heavily — to the world's yearly output of marketable 

 caoutchouc, and for the guidance and assistance of 

 those interested in the problem of the future supply 

 of natural rubber I give a list of the chief secondary 

 rubbers, whose properties may be said to be essen- 

 tially assimilative, inasmuch as they owe all the 

 value they possess to the aids of costly, secret, and 

 highly scientific machinery to convert their latices 

 into what is at most a cheap and useful adulterant 

 to the pure article of the genuine rubber tree. 



Guayule (pronounced Gwy-u-lie). — This is the 

 well-known Parthenium argentatum, a desert shrub, 

 three to four feet high, which kindly nature permits 

 to thrive only in the rainless territories throughout 

 the northern part of Mexico and the neighbouring 

 areas of Texas. 



