LEAF EXPOSURE TO SUN AND AIR *141 
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 
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 
Fig. 107. — Leaf Arrangement of Horse-Chestnut on so as to bring 
Vertical Shoots (top view). 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 
(P late IIT) ’ and to Fic. 108, —Leaf Arrangement of Horse-Chestnut 
notice how ecircum- on Vertical Shoots (side view). 
stances modity the position of the leaves. Maple leaves, for 
example, on the ends of the branches are arranged much 
