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high, and collection can commence when it is from five to seven 

 years of age. Although a number of trees can often be found 

 in proximity, the species can by no means be classed as gre- 

 garious. The milk, after collection, must be coagulated arti- 

 ficially. This is mostly accompHshed by boiling in water, which 

 causes the rubber to separate as a superficial crust ; it is then dried 

 and hardened by rolling. The same result is sometimes obtained 

 by merely mixing the milk with water and allowing it to stand. 

 Sea water acts much better than fresh water. Sometimes the 

 coagulation is accomplished by means of adding citric or sulphuric 

 acid. The yield of rubber is nearly one half of the weight of the 

 latex, and the rubber is of only medium quality. 



The second variety of rubber to be considered is produced in a 

 region where all the conditions are opposed to those of the well- 

 watered Castilla region, namely, the high and dry table-land of 

 the northwestern district. Owing to the high degree of radiation, 

 this region differs also in being subject to a great variation of tem- 

 perature by day and night, respectively, yet it can be regarded as 

 a hot district. During midday the heat is often extreme. It is 

 excessively dry, the amount of rainfall, even in the short rainy 

 season, being but moderate. Except for some large yuccas, and 

 a few leafless species, trees are almost wanting, and the shrubs 

 are mostly low and stunted. Among these shrubs occurs one 

 which has been described before in Torreya, namely, Parthenmm 

 argentatuni ; it is an important rubber-yielder, and therefore 

 called " guayule," the Indian equivalent for "wild rubber." It 

 is a low shrub of some two or three feet in height, of robust and 

 densely branching habit, and somewhat gregarious. The stem is 

 rarely so thick as the wrist and branches from the base, the 

 branches being rather short and stout. This shrub is of very 

 slow growth, requiring probably forty or fifty years to reach its 

 full size. It is as yet too little known to enable us to say how 

 many years it must grow before it will yield sufficient rubber to 

 be worth harvesting, but this is believed to require fifteen years 

 or more. Little is known about its natural methods of reproduc- 

 tion, but it appears to propagate sparingly, in the desert, from 

 seeds. The prospects for a new crop of rubber within a human 



