10 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



several days, drying, then adding carbon bisulphide, he was able to 

 obtain nearly 5 per cent of this substance, which evidently was a 

 mixture of rubber and various other ingredients. No serious attempt 

 was made to commercialize this discovery. A. T. Saunders (1910) 

 reports the results of examinations made in 1900 of a milkweed the 

 species of which is not given. Only a very small amount of rubber was 

 prepared, but the physical properties are described as indicated 

 later (p. 54). 



Apparently the most extensive examination of the milkweeds 

 hitherto made was that of Fox (1911), who carried on his experiments 

 at Akron, Ohio. This worker obtained 2 to 3 per cent of rubber from 

 Asclepias syriaca "on the basis of the latex." His conclusions were 

 that 'Vhile rubber is a product of the plant, the amount is so small, 

 its quality is so inferior, and its cost of production is so high, that a 

 profitable industry is out of the question." A year later Fox (1912) 

 reported more favorably concerning his work on Apocynum cannahinum 

 or Indian hemp. The latex of this sj)ecies was found to contain only 

 1.12 to 2.36 per cent of rubber, but the quality of the product was much 

 superior to that obtained from Asclepias. Other characters of the 

 rubber are given on p. 55) It is to the credit of this investigator that 

 he carried his examination so far, although working with plants 

 which gave such discouragingly low yields. 



Widtsoe and Hirst, working at the Utah Agricultural College, made 

 percentage determinations for several species and prepared a small 

 amount of rubber for examination. The method consisted in extrac- 

 tion with benzene in a Soxhlet apparatus and precipitation with 

 alcohol. The precipitate was in some cases treated with acetone to 

 remove fats, etc. The best result was obtained with Asclepias speciosa, 

 the leaves of which yielded about 2.25 per cent by this process. A. 

 incarnata gave 3.15 per cent, but this includes at least a portion of the 

 fats and resins. Asclepiodora decumhens yielded about 1 per cent, 

 while no rubber was obtained from Apocynum cannahinum and Lactuca 

 scariola. These results are here given from a manuscript report with 

 the consent of the investigators. 



Miscellaneous reports. — It is reported by Pearson (1916) that during 

 the recent war the Germans, being unable to import freely, resorted 

 to the manufacture of rubber from certain weeds, one of which, 

 Sonchus oleraceus, or common sow-thistle, grows wild as an introduced 

 species in the United States. Apparently the most important weed 

 used was a sort of wild lettuce, or Lactuca. 



No other accounts of studies on latex rubber in North American 

 plants have been found in the literature, although it is possible that 

 such exist. At any rate, the results did not indicate a commercial value 

 noi' did they lead to extensive experiments in the improvement of 

 the plants. Not infrequently, however, erroneous reports of the dis- 



