SUMMARY AND CONCLUSIONS. 63 



XIII. SUMMARY AND CONCLUSIONS. 



(1) The purpose of the investigation was to make a preliminary 

 study of native North American plants in regard to their rubber- 

 content. While this is a matter of general scientific interest, it was 

 hoped that some species would be found that carried enough rubber 

 to justify their further study with a view to establishing a rubber- 

 growing industry within the United States. This is desirable in that it 

 would utilize land now lying idle, and, furthermore, would render the 

 nation to some extent independent of overseas importations. 



(2) The species examined were mostly latex plants, especially 

 species of Asclepias (milkweeds) and Apocynum (Indian hemp). The 

 work is a continuation of earlier studies on native shrubs. A total 

 of about 225 North American species have now been examined and a 

 record made as to the presence or absence of rubber in their tissues. 

 About 6 species of shrubs and 16 species of latex-bearing herbs are 

 considered worthy of further study and experiment. Nothing is 

 known concerning the rubber-content of about 80 other native species 

 of Asclepias, all of which presumably contain at least a small amount 

 of rubber. 



(3) The researches of earlier workers on Asclepias and Apocynum 

 revealed but very small amounts of rubber. By carrying the examina- 

 tions to other species, and to other ecologic and genetic forms of the 

 same species, much larger percentages in the plant have been found. 



(4) The acetone-benzene method of analysis was employed. The per- 

 centages reported are therefore for rubber, practically free from resin or 

 other admixtures. They are based upon the dry weight of the material 

 analyzed. 



(5) The species of Asclepias and Apocynum reported as of special 

 interest are all perennials. Some make a good growth on land so 

 poor or dry that it is not at present utilized. They grow rapidly 

 from seed and from portions of the root. It is possible that several 

 crops could be obtained in one year without replanting. The crop 

 could be handled almost entirely by machinery. 



(6) The amount of rubber in the plant appears to vary with the 

 species, with small races, or strains within each species, and also with 

 the ecologic conditions obtaining during the period of growth. High 

 percentages are often associated with vigor of growth. Mature herb- 

 age often carries higher percentages than young herbage. 



(7) The latex plants examined usually carry much higher per- 

 centages in the leaves than in the stems or roots. The one exception 

 is a nearly leafless desert milkweed (Asclepias subulata), the stems of 

 which contain from 2 to 6.4 per cent. This is a species of outstanding 

 promise. Others considered as of special interest, with the rubber- 

 content of their mature leaves, are: 



