NUTRITION 



75 



taining the green substance, chlorophyll (leaf-green), the 

 chief organs for this work are the leaves. This explains 

 many facts about leaves — e.g., why they are green, why 

 they are thin and usually broad, why they are often 

 much larger in young, rapidly growing plants that need 

 much nourishment, than in mature plants (Fig. 55), 



Fig. 56. — A tree (hornbeam), seen from "outside," showing the dense 

 foliage at or near the tips of the branches. The same tree as in Fig. 57. 

 (Photo by the author.) 



why they occur at or very near the tips of the branches, 

 where they are well exposed to light (Figs. 56 and 57). 

 There is no more important fact in botany, nor indeed 

 in all natural science, than that all the food of the world 

 is manufactured in the chlorophyll-containing cells of 

 plants. 



74. Importance of Sunlight.- — Plants and plant parts 

 wrdwn in the dark are, with rare exceptions, never green. 



