20 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



reproduction can be practiced without great difficulty. This investi- 

 gator worked entirely with the stooling type, and, in addition to mak- 

 ing a number of successful transplants, also grew plants from portions 

 of the crown removed for this purpose. The division was made in 

 such a manner as to leave a few coarse, fibrous roots attached to the 

 propagules, which were grown first in a pot and then in the open ground. 

 The divisions were made January 19, 1921. By March 26 the young 

 shoots were 4 inches high. During the same period the parent plant, 

 which had been pruned back and transferred to the experiment station 

 at Bard, California, along with the propagules, had sent up new 

 shoots 6 inches long. Other portions of the same crown were started 

 without irrigation by covering with 2 feet of sandy soil, but were lost 

 when shallowly planted in the field. This was probably due to the 

 extreme exposure to the hot sun before the new roots had become 

 established. 



In order to determine whether plants would send up new shoots if 

 the tops were removed, Mr. Reed cut back some bushes on January 

 30, 1921. These were growing near Dome, Arizona. One of the 

 plants, which had 96 stems when cut back, had sent up 260 new 

 shoots by May 25 of the same year. At this time the new growth 

 was 30 inches high and "absolutely full" of latex. 



The above experiments indicate (1) that Asclepias subulata can be 

 successfully transplanted; (2) that when plants are cut back a new 

 "crop" of stems will be produced; and (3) the possibility of vegeta- 

 tive reproduction by division of the crowns. 



Rubber-content. It will be noted from table 1 that all of the analy- 

 ses are of the stems. This is due to the fact that the leaves are so 

 small and often so sparse at tune of collection as to be negligible. The 

 results show that the laticiferous tissue of the stem takes over the 

 rubber-holding properties of the leaves of other species, just as it also 

 assumes their photosynthetic activities. The green and glabrous 

 nature of the stem is doubtless correlated with this modified role. 



The tabulated analyses give evidence of a considerable fluctuation 

 in the percentage of rubber present. The possible explanations of 

 this include seasonal variation, the effect of different environments, 

 the presence of genetic strains, and a lack of uniformity in the selec- 

 tion of the material to be analyzed. Only by detailed experiments 

 can the actual causes be determined. It is noted, however, that 

 almost uniformly low yields were obtained from the samples gathered 

 at Dome, Arizona, on January 30, 1921. It is possible that the plants 

 at this station are of a poor type, but the two collected on October 18, 

 1920 (Nos. 1121 and 1122), at the .same place ran fairly high. The 

 explanation is suggested that all plants are higher in rubber-content 

 during the autumn than in the early spring. This seems not unlikely, 

 since the stems dry up and become brittle during the winter months, 

 thus rendering probable the decomposition of rubber at this time. 



