64 EUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



Per cent. Per cent. 



Asclepias sullivanti 1.2 to 8.1 Asclepias galioides 0.6 to 5.2 



syriaca 1.1 to 4.4 brachystephana 2.1 to 2.9 



califoTnica 2.6 to 4.3 speciosa 1 to 3 



latifolia 2 to 3.8 Apocynum cannabinum 0.7 to 5.1 



mexicana 1.4 to 4.8 



(8) The product has been examined for only a few species, and these 

 examinations have not been thorough. They indicate, however, that 

 the rubber is probably of low grade, not to be compared with fine 

 imported Para, with plantation rubber, nor with chrysU. It would 

 doubtless find use for mixing with other rubbers and in the manufac- 

 tures where great elasticity is not demanded. 



(9) A marked variability in the wild plants, and the number of 

 species involved, indicate that strains with a higher rubber-content 

 than any thus far discovered could be developed by breeding and then 

 perpetuated either by using pedigreed seed or by vegetative repro- 

 duction. It is possible that improvement ia the quality of the rubber 

 can also be effected by such methods. 



(10) By-products in the form of fiber and paper-pulp can be obtained 

 from the residue after the extraction of the rubber. The pulp of 

 Asclepias subulata has been worked up into paper of good quality, 

 the yield of dry fiber being 43 per cent and of bleached paper 28.5 

 per cent of the air-dry weight of the stems. The previous extraction 

 of rubber reduces the cost of paper-making. It seems not unlikely 

 that for this species the profits from fiber would exceed those from 

 rubber. 



(11) No opinion is expressed regarding the financial results that 

 might follow from an attempt to grow the plants on a commercial 

 scale. It is certain, however, that considerable scientific experiment 

 should precede any such attempt. Sufficient data have been accumu- 

 lated to justify the recommendation that these experiments be now 

 undertaken. In this connection there should be considered those 

 native shrubs in which the rubber occurs as soUd particles and also 

 various exotic shrubs and herbs, as well as the native species of Asclepias 

 and Apocynum. 



