6 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



while the prime object of that earUer search was to locate an emergency 

 supply of rubber in native shrubs, the object of the present one is to 

 discover, if possible, some latex plant sufficiently promising to justify 

 experiments looking towards its cultivation as a rubber-producing 

 crop. A second object is to extend the knowledge of the occurrence 

 of rubber in plants, quite irrespective of any immediate practical 

 results. During both of these investigations attention has been also 

 given to many other plants than those of the two genera just men- 

 tioned, so that definite information is now available as to the presence 

 or absence of rubber in about 225 species and varieties of west American 

 plants. 



II. EARLIER INVESTIGATIONS. 



The most important studies thus far made on North American 

 rubber plants have been those on Castilla and guayule, the former a 

 tree of tropical America, the other a low shrub of northern Mexico 

 and southwestern Texas. Investigations of latex plants native to the 

 United States have been few and, at least so far as indicated by 

 published reports, they resulted in no promising discoveries. This 

 was due in some cases to chemical and other difficulties encountered 

 in the analyses of the samples; sometimes to a lack of persistence in 

 assembling samples from a large number of species and from a con- 

 siderable variety of habitats; sometimes, and perhaps always in part, 

 to the chance which resulted in the wrong botanical species as the 

 object of study or to its examination at an unfavorable season. 

 One of the outstanding results of the present inquiry is the discovery 

 that not only do closely related species vary among themselves in 

 their rubber-content to a surprising degree, but also that a remarkable 

 range of variation may be expected within a single species. Even the 

 different parts of the plant are usually found to carry very unequal 

 percentages, and these proportions vary somewhat with seasonal and 

 other ecologic conditions. It is therefore evident that, if one sets 

 out to discover the best native rubber-producing species, he should not 

 rest content until a very large series of examinations has been made. 

 It will be seen from the following summaries of the results obtained by 

 earlier workers that with the exception of Castilla and guayule the 

 investigations were not carried far enough to give an adequate basis 

 for judgment, and that in a number of cases the plants deemed as 

 unworthy of further study have since been shown to possess consider- 

 able promise as rubber plants. 



Central American Rubber Tree {Castilla elastica). — Since this is a trop- 

 ical plant not suitable for cultivation in the United States, it will here 

 receive but the briefest mention, and that only for the sake of making 

 the list as nearly complete as possible. The most authoritative 

 account of the plant is one by Cook (1903). The conclusions of this 



