140 



THE VEGETATIVE FUNCTIONS OF PLANTS 



favorable exposure to light of a large amount of chloro- 

 phyll, upon which the plant is absolutely dependent for 

 the manufacture of its food. Leaves that develop in 

 reduced light-intensity (shade), ordinarily dispose of their 



FIG. 99.* Geotropic correlation among the branches of a young spruce 

 tree. After the terminal bud of the sapling was destroyed one of the lat- 

 eral branches (normally transversely geotropic) became negatively geo- 

 tropic; ultimately it assumed a vertical position and became the "leader" 

 of the tree. 



tissue in such a way as to become thinner and of larger 

 area than when developed in more intense light. When 

 the light-intensity is increased, the palisade layer often 

 becomes double (Fig. 27). If a leafy stem is bent, so 



