108 THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 



deficient in light. Stations 3 and 4, however, are rather similar in 

 every factor, while station 7 is quite different. It is not surprising, 

 therefore, to find striking structural divergence between stations 

 3 and 4 on the one hand and station 7 on the other, and far less 

 consistent differences between stations 3 and 4. The leaf is thinnest 



Table 15. — Effect of habitat upon leaf structure. 



in station 7, and the same is true of the lower cuticle, the differences 

 in the latter case being especially noticeable. In the case of the 

 upper cuticle the differences in Arctostaphylos and Quercus are less, 

 and in Adenostoma there is reversal, the mesophytic station 7 showing 

 a cuticle slightly thicker than the others. There are also differences 

 apparent in the mesophyll, especially in Arctostaphylos. In this 

 species the sections from stations 3 and 4 are barely distinguishable, 

 but these two differ strikingly from station 7. Such differences 

 can not be expressed numerically, but are very evident in drawings 

 (figs. 40 and 41). In the leaf from station 4, the dense palisade extends 

 from epiderm to epiderm, being slightly looser below, with here and 

 there a tendency toward sponge character. In that from station 7 

 the palisade is but two layers deep, and there is a distinct region of 

 sponge tissue. Quercus and Adenostoma from the three stations show 

 far less striking differences. 



We conclude, therefore, that increased thickness of leaf and of 

 cuticle, increased development of palisade tissue, and decrease of 

 sponge coincide with decrease of moisture and increase of light. 

 Further than this we can not certainly go. The natural assumption 

 is that moisture is the controlling factor rather than light, which 

 may have an indirect effect. 



