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 
