THE MOST IMPORTANT SPECIES. 25 



ewan. It is a species especially of fields and waste places, commonly 

 coming in where the soil has been distm-bed. Railroad rights-of-way 

 are therefore often lined with it. The climate of southern Michigan 

 is apparently well suited to the growth of the plants, since they are 

 reported as very abundant in that region. Specific localities whence 

 samples may be obtained include London, Ontario; Stevensville, 

 Michigan; and Lincoln and Madison, Nebraska. The plant has been 

 successfully grown near Akron, Ohio, where it was used experimentally 

 in the preparation of rubber by Fox (1910). 



Although Asclepias syriaca is usually reported as growing in waste 

 places, there is no direct evidence available that it will grow on soils 

 unsuited to any other crop. It will make a better growth than most 

 agricultural plants on shallow soil and in worn-out pastures and will 

 flourish in places so stony that cultivation is impossible. But since 

 such conditions seldom prevail over large areas, the competition with 

 other crops, such as beans and corn, will need to be taken into account. 

 This species belongs to the more humid eastern climate and is prob- 

 ably quite unsuited to the arid districts of the West and Southwest. 

 Its ability to grow and form large plants in the climate of New Bruns- 

 wick and Saskatchewan indicates that it is the most hardy of the 

 American milkweeds and one of the few to be considered for Canada 

 or the States along the Canadian border. Other characteristics 

 favorable to this species include its large size, robust habit, abundance 

 of foliage, and especially its ability to spread rapidly by underground 

 parts, thus coming to cover large areas with a solid stand of erect, 

 leafy stems. 



Eubber-content. — The first American milkweed to be tested for its 

 rubber was Asclepias syriaca, and up to the time of the present study 

 this was the species to receive the most attention (WilUam Saunders, 

 1875; Fox, 1911). These earlier examinations demonstrated the pres- 

 ence of rubber, but in small amounts, this being due either to the 

 chance gathering of poor strains or to harvesting at the wrong season 

 or from plants grown under conditions unfavorable to the formation 

 of rubber. The percentage content was not determined and expressed 

 in a sufficiently definite manner to render comparison with the tables 

 possible. The analyses here reported give further evidence of great 

 variability in rubber-content between different plants. Whether this 

 is due to hereditary qualities or to environmental causes is still a 

 subject for experimentation. As a beginning on this problem an 

 analysis was made of green leaves for comparison with fully matured 

 and yellow leaves of plants growing in the same locality and under 

 conditions that seemed to be identical. In each case leaves were 

 taken from 5 plants, both lower and upper leaves being selected; these 

 were dried, pulverized, and thoroughly mixed. In this manner it was 



