62 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



It may be found on further examination that other species than 

 Asclepias subulata are suited to the purposes under consideration. 

 A. sullivanti, for example, carries a higher percentage of rubber in 

 the leaves alone than is found in the stems of the leafless A. subulata, 

 and the plants are very tall, thus yielding a large tonnage. But 

 usually its stems are very poor yielders and as far as known the plants 

 grow only in fairly good soil. The fiber would be of value, but is 

 probably of lower grade than that of the desert species. If j A . sullivanti 

 can be induced to grow on cheap land, or if the rubber-content can 

 be very materially increased, particularly in the stems, it would have 

 a chance as a possible rubber plant, occupying its own geographic 

 area. Other species to be considered are the ones given special con- 

 sideration in this report. Their qualifications, as far as known, are 

 briefly discussed in each case under the subheadings of "Distribution 

 and ecology" and "Rubber-content." It also should be remembered 

 that only a few of the native latex plants have been studied. Possibly 

 there are others which on examination would be found to hold out 

 much more promise than any of these. 



It is readily seen from the above statements that a much more 

 extended and detailed investigation will be necessary before anything 

 definite can be said regarding the possibilities of these plants as 

 agricultural crops. The present study was undertaken chiefly to 

 determine whether or not such investigations could be carried forward 

 with some prospect of ultimate success. It seems to the authors that 

 the facts thus far established amply demonstrate the desirability and 

 wisdom of instituting further studies and experiments. In case these 

 are carried out, they should be extended to include Chrysothamnus 

 and other plants not treated in detail in this paper and should aim to 

 cover the following points: 



1. Further field studies of wild plants to discover better species and 



strains than those already found. 



2. Experiments in breeding and selection. 



3. Modifications in environment, especially the water and alkali relations, 



and their effect upon growth and yield. 



4. Manipulation of the plant itself, with the object of increasing the 



amount of rubber-bearing tissue. This is already under way for 

 chrysil. 



5. Field experiments to determine the tonnage per acre to be expected, 



and the best methods of culture. 



6. Studies in the seasonal formation of rubber in the plant. This includes 



a combination of the most refined chemical methods, with micro- 

 scopic studies of cell-contents and their changes precedent to and 

 during the deposition of rubber. 



7. Quality and value of the product. Investigations along this line are 



already in progress. Chrysil and several species of Asclepias 

 rubber will be studied. 



8. By-products, including fiber, pulp for paper, acetone, alcohol, dyes, 



potash, essential oils, waxes, and resins. 



