WHALEY: RUBBER 21 



of rubber but because of the part which it plays as a wild rubber source. It 

 was at one time as important as Hevca in the production of natural rubber, 

 and in the late nineteenth century and early twentieth century large numbers 

 of Castilla trees were planted in Latin America. 



Costilla, also called, though erroneously, Castilloa, is a member of the 

 mulberry family and extends from Mexico southward into Bolivia, Brazil and 

 Peru (Loomis 1942). Rubber is gathered from Castilla by tapping, but it 

 differs from Hevea by giving a relatively large amount of latex at each tapping, 

 but can be tapped only a few times a year without serious injury. It has been 

 a common procedure to cut large Castilla trees for their rubber yield which 

 may be as much as 50 pounds from one felled tree. Castilla has played a large 

 part in the supplying of so-called "wild" rubber during the present emergency. 



Guayule, Parthenimn argentatum, was "discovered" by Dr. J. W. Bige- 

 low of the Mexican Boundary Survey in 1852 "near Escondido Creek, Texas." 

 It was described and named by Asa Gray in 1859. Guayule is native to the 

 North Central plateau region of Mexico extending into the Big Bend region 

 of Texas. Within the area it is confined rather strictly to limestone soils and 

 is generally restricted to altitudes between 3000 and 7000 feet, where rainfall 

 averages from 10-15 inches a year. 



A detailed description of Guayule and its characteristics has been published 

 by Lloyd (1911, 1932). Guayule is a member of the Compositae, a low-grow- 

 ing, much branched, woody shrub with small silver-gray leaves. Wild plants 

 are generally about two feet in height, and have a dry weight of one or two 

 pounds after several years growth. Hardy perennials in habit, undisturbed 

 plants probably live some 30 to 40 years. There is occasionally some vegetative 

 reproduction but most of the reproduction is by seed. Under favorable con- 

 ditions large numbers of seeds are produced, but in very dry periods partic- 

 ularly, seed production is limited. In semi-arid regions to which the plant is 

 native it has great capacity to withstand long continued droughts. However, 

 under drought conditions very little growth takes place. 



The rubber in guayule occurs in latex, but in contrast to the situation in 

 Hevea and the other so-called latex-bearing plants the latex is not found in 

 vessels or tubes but is a component of the individual parenchymatous cells. 

 Rubber is stored in all parts of the plant except perhaps the leaves. In wild 

 plants the rubber content usually averages around 7% of dry weight for mature 

 plants. Cultivated selected strains run as high as 22% rubber at maturity. 



The Germans began experimenting with the extraction of guayule rubber 

 before 1900 but the product remained unimportant until in 1904, a factory, 

 using a pebble-mill extraction process, was set up in Torreon, Mexico. Other 

 factories followed and small-scale production of rubber from guayule has been 

 almost continuous since, except during one period of very low rubber prices. 



