LEAF EXPOSURE TO SUN AND AIR 



141 



FIG. 107. Leaf Arrangement of Horse-Chestnut on 

 Vertical Shoots (top view). 



The rows are produced, as it is easy to see on examining 

 such a leafy twig, by a twisting about of the petioles. 



The adjustment 

 A 



<mjj, in many opposite- 



leaved trees and 

 shrubs consists in 

 having each pair 

 of leaves cover 

 the spaces be- 

 tween the pair 

 below it, and 

 sometimes in the 

 lengthening of 

 the lower petioles 

 so as to bring 

 the blades of 

 the lower leaves outside those of the upper leaves. Ex- 

 amination of Figs. 107 and 108 will make the matter 

 clear. 



The student 

 should not fail to 

 study the leafage 

 of several trees of 

 different kinds on 

 the growing tree 

 itself, and in 

 climbers on walls 

 (Plate III), and to 

 notice how circum- 

 stances modify the position of the leaves. Maple leaves, for 

 example, on the ends of the branches are arranged much 



FIG. 108. Leaf Arrangement of Horse-Chestnut 

 on Vertical Shoots (side view). 



