Plant Industries and Climatic Plant Formations 369 



PF. 5. Cooper 



Fig. 234. Mixed forest of oak {Quercus chrysolepis) and fir {Pseudotsuga mucronata) ; 

 Santa Cruz Mountains near Palo x\lto, California. 



of economic importance. The various species of cactus {Opuntia)j 

 especially those forms with but few spines, are valuable as forage 

 for cattle. These plants contain at all times a considerable 

 amount of water, and in a region of such little rainfall this is an 

 important factor. When they are used as cattle food, it is cus- 

 tomary to burn off the spines with torches before feeding. 



Guayule is one of the minor sources of rubber. It is a small 

 shrub which accumulates resinous material in the cortex of the 

 stem. Most of the supply has heretofore been obtained from 

 wild plants, but now its cultivation has been begun in Arizona 

 and California. The agaves of northern Yucatan and the pla- 

 teau of Mexico furnish the sisal and hennequin fibers used in 

 making binder twine. Other agaves furnish fibers of less im- 

 portance. The cultivation of agaves has spread to the West 

 Indies and other semi-arid parts of the tropics. " Pulpue," 

 the Mexican national alcoholic drink, is made by fermenting and 

 distilling the juice of an agave. 



