THE OBGANIZATION OF THE PLANT 



75 



but they make up in number what they lack in size. There 

 may be several million in the epidermis on the underside of a 

 single ordinary leaf. The stomata have been estimated to oc- 

 cupy nearly one twentieth of the area of the leaf. It is a curi- 

 ous fact that gases can enter the leaf through these minute 

 openings more rapidly than they can pass through a single 

 opening equal in area to all the stomata. 



Formation of plant food. Food is formed only in the green 

 cells of the plant. This is because the energy necessary for 

 combining the food materials is derived from the sunlight by 



the green coloring matter called 

 chlorophyll. In the cell this color 

 is found in small bodies known 

 as chloroplasts. The chloroplasts 

 really form the food, though they 

 are helpless without chlorophyll. 

 The first food product formed is 

 usually grape sugar, represented 

 by the formula C 6 H 12 O 6 , but this 

 is soon turned to starch, a more stable form of plant food, with 

 the formula C 6 H 1Q O 5 . Plants of the iris, lily, and amaryllis 

 families rarely form starch. In such 

 plants oil formed from the same three 

 chemical elements may be the first visi- 

 ble product of photosynthesis. Starch, 

 wood, and several other substances 

 contain the same proportion of carbon, 

 hydrogen, and oxygen, and the differ- 

 ence between them is accounted for by 

 assuming a different multiple of the 

 formula for each. The hydrogen and 

 oxygen in the combination are derived from the soil water, 

 and the carbon comes from the carbon dioxide in the air. The 

 latter goes into the leaf through the stomata, and, spreading 



FIG. 62. Cells of a moss with 

 chloroplasts 



FIG. 63. Starch grains in 

 the cells of a potato 



