THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. Ill 



bottles and weighed. Five of each species were left in station 10 

 and the rest were placed in station 7 (Quercus agrifolia- Arbutus associa- 

 tion, p. 38). In station 7, 10 branches each of the same species 

 were cut, and in addition 10 of Arbutus menzicsii, and half of each 

 set were left in station 7 and the others taken to station 10. The 

 experiment was allowed to run for about 48 hours, from August 24 

 to August 26. The branches were pressed between driers, and the 

 areas of the leaves of Arctostaphylos and Arbutus were later deter- 

 mined with a planimeter. In the case of Adenostoma, with its 

 terete leaves, a different and less exact method was necessary. Each 

 branch was stripped of its leaves and these counted. Then 20 were 

 selected at random, placed in boiling water to restore them approxi- 

 mately to their original size, and measured accurately as to length and 

 diameter. The area was computed as a cylindrical surface, the 

 average of the 20 taken, and this applied to the total number of 

 leaves on the shoot. The total losses recorded for each shoot were 

 reduced to the amount per square decimeter of surface. So much of 

 the results as are worth giving at present are seen in tables 16, 

 17, and 18. 



Table 16. — Loss per square decimeter, station 10. 



c. c. 



Adenostoma (average of 5 shoots) 5 32 



Arctostaphylos (average of 5 shoots) 6 . 68 



Arctostaphylos (both surfaces considered) 3.34 



It is shown that when the stoma-bearing surface alone is con- 

 sidered, the loss per unit is less in Adenostoma; but if both surfaces 

 of the leaf are considered in Arctostaphylos, as we must fairly do, 

 the relation is reversed. It would seem, therefore, that the Adeno- 

 stoma leaf is the more effective in reducing water-loss so far as the 

 actual stoma-bearing surface is concerned, but that Arctostaphylos 

 compensates for this by the entire absence of stomata (at least in 

 the plants studied; see p. 104) over half its leaf-surface. The more 

 perfect xerophytism of Adenostoma can not be explained, therefore, 

 upon the basis of cuticular and stomatal regulation; and this is 

 confirmed by the descriptions presented in the last section, in which 

 these structures are seen to be very similar in the two species. We 

 may fall back upon the hypothesis that the total leaf-area is less 

 in Adenostoma than in Arctostaphylos, plants of equal size being con- 

 sidered; individuals of the two species are apt to be so when growing 

 together. The general appearance would indicate this, especially 

 the small size of the leaves of Adenostoma, and confirmation is ob- 

 tained from the fact that the average leaf-area of the shoots used in 

 the experiment (both surfaces considered in Arctostaphylos) was 

 75 per cent greater in Arctostaphylos. Of course, this is merely a 

 rough approximation, but since care was taken to select branches 

 as nearly alike as possible, it doubtless indicates the truth. 



