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. 



