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), 

 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 primarily manufactured in the chlorophyll-containing cells 

 of plants. 



74. Importance of Sunlight. — Plants and plant parts 

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

 This means that sunlight is necessary in order to make 

 chlorophyll. But green plants cannot elaborate food in 

 the dark. This means that sunlight is necessary, not 

 alone for the formation of chlorophyll, but for food mak- 

 ing as well. Non-green tissues, even in sunlight, cannot 

 manufacture food; for this process both chlorophyll and 

 sunlight are necessary. The green cell has often been 

 likened to a factory; the chlorophyll is the machinery, 

 the sunlight is the energy, while the product of the factory 

 is the manufactured food. 



75. Details of the Process. — The manufacture of carbo- 

 hydrates involves three essential steps: 



1. Taking in the raw materials (water and carbon 

 dioxide) . 



2. Recombining these parts into carbohydrates. 



3. Giving off the waste material (chiefly oxygen). 

 Taking in the Raw Materials. — We have seen in Chapter 



IV that the air spaces between the green cells of a leaf 



