52 RUBBEB-CONTENT OF NORTH AMERICAN PLANTS. 



the lack of concomitance in the various lots and thus to warn against 

 the danger of explaining seasonal variation on the basis of a limited 

 number of analyses and without taking all possible factors into account. 



With due regard to the warning just given, the results of an experi- 

 ment on Asclepias mexicana may now be considered. A uniform 

 growth of this plant on an area a few meters square was located near 

 Mount Diablo, California. A swath was cut across the middle of this 

 plot on July 9, 1920. The leaves at that time carried 3.7 per cent. 

 The growth along both sides of this strip was cut on October 12 of the 

 same year, when it was found that the percentage had increased to 4.6. 

 The content of the stems had dropped during this interval from 1 to 0.8 

 per cent, but in view of the small amount of rubber in the stems this 

 slight difference is not considered significant. 



Further evidence may be deduced from certain of the tables of 

 analyses. For example, in table 3 several instances may be noted 

 where the analysis of young leaves yielded lower percentages than 

 older leaves taken from farther down on the same stem. But most 

 convincing of all are comparisons between Nos. 1034, 1032, and 1035, 

 made in this order from table 3. Here the increase with age is per- 

 fectly regular. Also, in the case of A. subulata (table 1), the green 

 stems are found to carry smaller percentages in the young growth 

 than in the more mature but not woody portions. On the other hand, 

 the percentage of rubber in very old leaves and stems is always much 

 reduced (see No. 919 of table 4, No. 342 of table 5, and various numbers 

 of table 1). This lends support to the theory that caoutchouc may 

 undergo considerable modification, or even entirely disappear, after 

 it is once formed in the plant. 



In case it is found that the amount of rubber increases as the season 

 advances, as seems to be indicated by the above considerations, it 

 win be interesting to know whether this is due to an increase in the 

 volume of the latex or to an increase in the percentage carried by the 

 latex itself. The only evidence at hand indicates that the latter may 

 be the case. A number of plants of Asclepias hdlli growing in Ute 

 Pass, Colorado, were "bled" on July 17, 1919, and 7 c. c. of latex 

 obtained and analyzed. The results showed but 4.7 per cent of 

 rubber in the water-free latex. A similar sample was obtained from 

 another colony of plants on July 28, 1919. This was found to carry 

 21.0 per cent of rubber on the same basis. This seems to indicate 

 a considerable increase in the richness of the latex for A. halli as the 

 season advances, although the difference may be due to individual 

 variation. 



Variation in successive crops. — When the stems of some and perhaps 

 all species of Asclepias are removed by mowing or cutting in any 

 manner near the base, a new growth of shoots will spring up. Thus 



