8 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



Guayule (Parthenium argentatum). — The most extensive use thus far 

 made of North American rubber plants has been that of guayule, a 

 Mexican and Texan shrub that came into prominence about 1902. 

 This plant was formerly very abundant in some parts of Mexico and 

 the value of the exported product exceeded $10,000,000 in 1911. In 

 recent years, however, the production has fallen off very markedly. 

 As the native supply began to wane, attempts were made to bring the 

 species under cultivation, at first with but little success because of 

 difficulties encountered in the germination of the seed. It was also 

 found that whereas the native shrub yielded an average of 10 per cent 

 of guayule gum, or ''rubber," the content of the cultivated plants fell 

 in some cases to less than 2 per cent. After considerable experimenta- 

 tion a high rate of germination was finally obtained, and through the 

 segregation and selection of superior strains the content of field-grown 

 plants is said to have been increased to 18 per cent. In fact, a few 

 strains carrying a considerably higher percentage have been developed, 

 but it is understood that these have certain characteristics which 

 render them unsuitable for commercial planting. Whether the 18 

 per cent just mentioned is for pure rubber or whether it includes a 

 considerable amount of resin and other impurities, as in commercial 

 guayule, is not known to the writers. It is now well established that 

 the wild plants belong to a series of innumerable races, that these 

 vary to a considerable extent, and that reproduction is partheno- 

 genetic, at least in part. The method of improvement, therefore, 

 has been to select the best strains as determined by analysis and 

 cultural characters and to propagate from these plants without the 

 introduction of cross-breeding. The rubber in guayule occurs as 

 minute bodies in the cells, there being no latex as in Hevea, Ficus, and 

 most other rubber plants. The planting of guayule on a large scale 

 has been started in southern Arizona under the scientific direction of 

 Maccallum, but the success of the industry is still a matter for the 

 future to decide. The only extensive account of rubber as it occurs 

 in guayule has been presented by Lloyd (1911). This author describes 

 field conditions and factory methods, in addition to the results of his 

 histological investigations on the formation and distribution of rubber 

 in the plant. 



Chrysil and related rubbers. — Immediately upon the entrance of the 

 United States into the great war in 1917, the State of California under- 

 took a survey of the Great Basin and Pacific Coast areas for rubber- 

 producing plants, the work being carried out under the direction of the 

 State Council of Defense and the University of California. The 

 results have been published by Hall and Goodspeed (1919). Accord- 

 ing to these authors, rubber of good quality has been found in 12 

 varieties of ChrysothammLS nauseosus or rabbit-brush, the best shrubs 



