VARIATIONS IN RUBBER-CONTENT OF ASCLEPIAS AND APOCYNUM. 51 



uniformity of the plants, and 5 specimens were taken from each lot 

 at the close of each period. It was thought that by thoroughly mixing 

 the dried and ground material of 5 plants a reliable average for the 

 whole lot would be obtained. The results are shown in table 14. 



TABLE 14. Seasonal variation in the rubber-content of Asclepias sullivanti at Lincoln, 



Nebraska. 



In considering the results set down in table 14, only the rubber-con- 

 tent of the leaves need be taken into account, since the amount in 

 the stems is so slight that the differences are not significant. For 

 lot 2 it will be noticed that, with the exception of the first period, the 

 rubber-content gradually increased as the season advanced. This is 

 in agreement with results often but not always obtained in other 

 species. The analyses of lots 1 and 3, however, entirely negative 

 this conclusion and indicate an irregular fluctuation difficult to account 

 for, except on the basis of some unknown factor or on that of individual 

 variation, which, as shown in table 13, is so great that it is not wise to 

 draw conclusions unless a very large number of plants are included in 

 each analysis. Therefore the principal value of table 14 is to indicate 



