170 



POMOLOGY 



Table XLVII 



average annual twig growth to a tree in inches 



141. Leaf area. — In all these experiments the foliage is 

 a notable index to the vigorous condition of the trees or 

 the reverse, both in the abundance of leaves and in their 

 size and color. As an illustration, the following figures may 

 be cited: 



Table XLVIII 

 effect of cultural methods on leaf size 



Average area of leaves 

 in square inches 



Trees in sod 4 . 24 



Clean tillage 5 .07 



Tillage-cover-crops 5 . 28 



Not only are the area and abundance of the leaves reduced 

 under sod culture, but the foliage often turns yellow and 

 drops prematurely. A cross-section of such leaves shows 

 that there is one layer of palisade cells and another layer 

 about half the length of the surface or first layer. In con- 

 trast with this structure, the leaves on trees in a high state 

 of vigor show three and sometimes a partial fourth layer of 

 palisade cells and a denser structure of the mesophyll. 



