112 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



For comparison with Arbutus it is necessary to use the shoots of all 

 species that were cut in station 7 and left there (table 17). 



Table 17. — Loss per square decimeter, station 7. 



e. e. 



Adenostoma (average of 5 shoots) 2 . 77 



Arctostaphylos (average of 5 shoots) 3 96 



Arctostaphylos (both surfaces considered) 1 . 98 



Arbutus (average of 5 shoots) 3 . 69 



Arbutus (both surfaces considered) 1-85 



The two shrubs show the same relations here as in station 10. 

 Arbutus seems to be slightly more effective in reducing water-loss 

 than Arctostaphylos, and therefore more effective than Adenostoma 

 per unit total surface. However, the average total leaf-surface 

 per shoot was 38 per cent greater than Arctostaphylos and thus far 

 greater than Adenostoma, indicating that even for a plant of equal 

 size the total leaf-surface would be greater; and there is the further 

 fact of the tree stature of Arbutus. The conclusion seems to be 

 that it is not so much differences in cuticular or stomatal effectiveness 

 that make the difference in drought resistance as it is the total leaf- 

 area exposed in plants of approximately equal size, the separate 

 elements in this character being number and size of leaves. 



One other point may be brought out. Table 18 exhibits the 

 difference in evaporation-rate which appears in the same species 

 growing in the two habitats, due fundamentally to differences in 

 the environmental conditions. 



Table 18. — Loss per square decimeter, station 7. 



c. c 



Adenostoma (station 7) 2 . 77 



Adenostoma (station 10) 5 . 32 



Arctostaphylos (station 7) 1-98 



Arctostaphylos (station 10) 3 .34 



In Arctostaphylos the rate in the xerophytic station is 70 per cent 

 greater than in the other, while in Adenostoma the difference is 

 nearly 100 per cent. The study of the transpiration of these plants 

 is one of the most promising fields for further investigation. 



