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EUBBEE-CONTENT OF NOETH AMEEICAN PLANTS. 



I. PURPOSE OF THE INVESTIGATION. 



Rubber is now considered by every civilized community as one of 

 its prime necessities. Since this important commodity is almost 

 wholly a product of the tropics, those countries which lie entirely or 

 mostly in the temperate zones are dependent upon importations, mostly 

 from overseas, for their supply. This places the importing nation 

 at a disadvantage, since it is subject at all times to labor and other 

 economic disturbances in the rubber-producing countries, while in 

 time of war it is dependent upon the good-will both of the producers 

 and of whatever power happens at the moment to be in control of those 

 seas over which the importations must travel. The United States is 

 in a particularly hazardous position in this respect. The normal 

 annual importation into this country is approximately 300,000,000 

 pounds. War-time conditions would make such increased and impera- 

 tive demands that, unless this amount could be very materially 

 augmented, it would be necessary greatly to restrict the use of rubber for 

 ordinary commercial purposes. This inconvenience, however, would 

 be trivial as compared with the perilous condition of the country in 

 case importations from overseas should at the same time be seriously 

 curtailed or stopped entirely by the enemy. It therefore behooves 

 the United States and other countries so situated to discover some other 

 method of supplying their needs, and it is possible that one such method 

 lies in the improvement and cultivation of rubber plants suited to 

 extra-tropical conditions. The responsibility for the initiation of such 

 studies lies primarily with the botanist, since he is the one who should 

 be the best qualified to recognize the kinds of plants most likely to 

 yield results. It was with these considerations in mind that the 

 present investigation was undertaken, although the general scientific 

 interest which attaches to the formation and presence of rubber in 

 plants was also an impelling motive. 



The work described in this paper is to some extent a continuation 

 of a survey of western North America for rubber-producing plants 

 begun in 1917 as a war-emergency project under the State Council of 

 Defense of California and of which there is more specific reference on 

 p. 8. That project, however, centered around a study of the genus 

 Chrysothamnus and related shrubs, plants in which the rubber occurs 

 in the form of solid particles in the plant cell, whereas the present 

 studies are concerned chiefly with the genus Asclepias (milkweeds) 

 and similar herbaceous plants in which the rubber occurs somewhat 

 as an emulsion in the latex, or milky sap, which in turn occupies the 

 vessels of the highly specialized laticiferous tissues. Furthermore, 



