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 
