PAPERS OX BIOLOGY AND AGRICULTURE 211 



different enviroumeuts. lu the ease of Rhus Toxicoden- 

 dron there is not this contrast. In almost no case is 

 there more than one layer of palisade cells. Xor is there 

 any marked dift'erence in the thickness of the one layer 

 of cells. In the spongy tissue there is variation, not in 

 the thickness, bnt in the restriction of the amount of 

 air spaces. This, of course, is what causes the variation 

 in the coefficient of compactness. 



In conclusion. The variations in the structure of the 

 leaves may be summed up in the following : 



1. The leaves are greater in area where the situation 

 is mesophytic and smaller in the xerophytic locations. 



2. There is no apparent agreement in the relation 

 between leaf area and leaf thickness. This is contrary 

 to the usual condition. 



3. There is agreement between the compactness of 

 the leaf and its environment. 



