HOW TREES GROW 15 



moves to other parts of the plant and is the basis for the for- 

 mation of all other foods. The oxygen escapes to the sur- 

 rounding air. 



But not all of the sun 's energy absorbed by the leaf is put to 

 work making food. Sometimes a leaf takes up fifty times as 

 much sunshine and energy as it can use for food making. The 

 rest is used to evaporate the water in the leaf, a process known 

 as transpiration. The amount of water given off by a tree 

 through transpiration is enormous. A white oak, of average 

 size, during a single summer day will give off 150 gallons of 

 water. A forest of such trees must give out as much moisture 

 as a river and quite probably a wind blowing over the forest 

 will absorb even more moisture than if it had blown over a 

 lake of equal size. 



Plants require the same kinds of food as do animals, with 

 the important difference that the plant manufactures its own 

 food and animals obtain theirs by browsing on plants or de- 

 vouring flesh of other animals. In one sense, and a very true 

 sense, the tree is a factory where starches and sugars are manu- 

 factured out of substances the tree finds in the earth beneath 

 it, in the air above, and in the sunlight. There are just three 

 principal groups of food for both plants and animals, carbo- 

 hydrates, fats and proteins. These are all formed in the living 

 plant, primarily from the simple sugars. 



Just as animals can get food, only by taking it in from the 

 outside, so the plant, once its stored-up food is exhausted, can 

 make its own food only by taking-in the necessary substances 

 from its surroundings. It takes its carbon dioxide from the air. 

 The supply of water comes from the soil, entering into the 

 roots and moving upward to the leaf. The soil holds its water 



