Chapter 3. 
STEMS AND LEAVES. 
Mechanical requirements of stems and branches. Twining and 
climbing plants. The food conducting function of stems. The 
work of leaves and their structure. The wilting and recovery of 
leaves. Protection of leaves against excessive drought. Direct 
effect of surroundings on leaf development. Hardening off 
plants. ; 
The stem is not as important an organ of plants as are 
the roots and leaves. The latter are essential for the 
nutrition of plants, and their particular work can only be 
carried on if the leaves are fully exposed to the sunlight. 
There is consequently considerable competition among 
plants for “a place in the sun,’ and in the course of 
evolution there has been a development of plants of 
increasing size, which by overlapping the smaller ones 
have been able to reach the light. This has led gradually 
to the production of trees and shrubs, but in herbaceous 
plants too, the stem and branches have the function of 
displaying the leaves to the best advantage. This and 
certain mechanical principles which are involved account 
for the peculiarities of branching typical of different 
plants. In addition to the stem and branches the leaf 
stalks are also concerned in the ultimate positions in which 
the leaves are expanded. By varying growth in length 
and direction the leaves -become so placed that there is 
little or no overlapping and the dovetailing of the various 
leaves causes the formation of a regular “leaf mosaic.” 
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