The Resin-Canals in the Guayule. 175 



RESIN-CONTENT OF GUAYULE BY ANALYSIS. 



The percentage of resin in the branches and twigs of field plants, 

 according to figures obtained by Whittelsey (in manuscript) , is between 

 about 10 per cent for the smaller and about 17 per cent for the larger 

 branches. The amount probably varies according to the structure, and 

 this with the rate of growth of the parts. For irrigated plants the follow- 

 ing figures were obtained. The material was the same as that referred to 

 in table 53. 



TABLE 51. 



Percentage 

 Parts. of resin. 



I. Stump 2.46 



Ila. Wood of 1907 growth i .36 



116. Cortex of this , 4 . 06 



III. Growths of 1908 intact 7 . 56 



IV. New growth of 1909 with leaves 2 . 70 



V. Roots 10.80 



Aside from possible errors, it seems that, bulk for bulk, the irrigated 

 plant contains less resin than the field plant. This is due in part to the 

 larger relative volume of the wood cylinder. The reduction of the amount 

 in older growths is due also in part to the radial compression of the resin- 

 canals in irrigated plants, whereby their capacity is much reduced. The 

 force of this explanation of the figures appears when we compare the per- 

 centage of resin in III above. When we introduce the rate of growth as 

 a factor we must conclude that the total secretive activity is not reduced 

 under irrigation, nor is the secretive activity of the resin-secreting cells 

 lowered. The result, however, is had that in a given volume of cortex 

 there is less resin in irrigated plants. In the pith, however, this does not 

 hold, since the relative volume of the resin-canals under irrigation is as 

 great or greater than in field plants. The reduced amount of resin of the 

 cortex , volume for volume , appears , therefore , to be a secondary matter only , 

 and bears, so far as we can see, no explanation in terms of adaptation. 



