368 PLANTS AND MAN 



of North America, but these are mostly herbaceous plants, and 

 exploiting them for their rubber necessitates destruction of the 

 plant. However, studies on a number of the most promising of 

 these have been under way for a number of years, and since most 

 of these species are plants of arid or semi-desert climates, they 

 may constitute a profitable use for now unproductive regions. 

 They may be of no great commercial importance in the near 

 future, with plantation rubber developed to such a high degree, 

 but in the event of removal of plantation rubber from the reach 

 of the American rubber industry by war embargoes, such a 

 domestic source would be of utmost importance. 



GuAYULE is the leading source of rubber in temperate regions 

 at the present time, Mexico exporting several million pounds of 

 guayule rubber yearly. Guayule is a low semi-shrubby plant 

 which grows in arid parts of the southern United States and 

 Mexico, and is treated as a crop plant, maturing in about four 

 years to yield about 1 5 % of its weight in rubber. The rubber 

 occurs as tiny granules scattered through the tissues; they are 

 removed by a milling process which consists of reducing the 

 plants to pulp in a water medium and then skimming off the 

 rubber particles which rise to the surface. The crude rubber 

 obtained in this way is cleaned, rolled into sheets, and dried. 

 Rubber produced from guayule is dark gray in color and inferior 

 to Brazilian rubber for most purposes, although it is said to excel 

 the latter for certain uses in the manufacture of automobile tires. 

 Most of the experimental work on guayule has been done in the 

 United States, where attempts are being made to develop 

 varieties of increased rubber content. Studies on the growth, 

 care, and harvesting of the crops are likewise being carried on. 



Desert milkw^eed, an herbaceous plant of the arid southwest, 

 is under experimental cultivation at the present time as a possi- 

 ble commercial rubber source. The stems of this plant yield 

 2% to 6% of their weight in a low-grade rubber; after extraction 

 of the rubber, the pulpy residue has been manufactured into 

 paper. Studies at the present time are for the purpose of determin- 

 ing the proper age at which to harvest, methods of curing to favor 

 higher rubber content, most efficient methods of extraction, and 



