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 cues 

 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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