202 TROPICAL AGRICULTURE 



rasping and boiling the roots. The composition of the Landol- 

 phia heudelotii rubber is, ordinarily, caoutchouc 91.3 per cent., 

 resin 5.9 per cent., protein 1.4 per cent., and ash 1.4 per cent. 



Guayule (Parthenium argentatum) is a barely shrubby plant 

 belonging to the Composite family and native of Texas, New 

 Mexico, and Mexico. The plant attains a height of i to 3 

 feet and is characterized by silvery gray bark and leaves, and 

 yellow flowers which appear in September. The plants are 

 cut, dried, and shredded and the rubber is then extracted by 

 patented processes. Commercial attention was first called to 

 the plant in 1876, although it must still remain doubtful 

 whether the Indians were not familiar with the guayule plant 

 as a source of rubber. The latex cells are in the bark and 

 pith. The crude rubber obtained by the patented processes 

 now in use equals 8 or 10 per cent, of the dry weight of the 

 stems. Guayule rubber contains 20 to 30 per cent, of resin. 

 The rubber is of fair grade and is used in the manufacture 

 of rubber boots and for similar purposes. 



The rubber-yielding plants of commercial importance be- 

 long chiefly to the botanical families of Euphorbiacese, Urtica- 

 ceae, and Apocynaceae, and to a smaller extent to Asclepiada- 

 ceae and Compositae. More than 80 species of trees and vines 

 have been used as a source of rubber, but only those which 

 have been mentioned in the previous discussion have attained 

 any real commercial importance. All commercial rubber-bear- 

 ing plants, with the exception of the guayule, are confined to 

 the Tropics. It is a well known fact that the development 

 of latex in plants seems to occur much more commonly in the 

 Tropics than in the cold climates. Much discussion has been 

 had as to the function of latex in plants, but no agreement 

 has been reached as to what this function is. The latex has 

 been supposed to be a storage material for the nutrition of 

 plants. It has also been suggested that it has an important 

 function in protecting trees against attacks of insects and 

 against injuries. The fact that the latex runs out so promptly 



