LEAVES KEEP THE AIR MOIST 



443 



power to make food. Some of us wonder why the plants do 

 not constantly grow new leaves to take the place of those eaten 

 by insects. This is because the tree whose leaves have been de- 

 stroyed by insects has no means of making food, and loses its 

 vigor and its ability to produce new growth. 



If we wish our trees and shrubs to remain active and healthy, 

 we must protect them against leaf-destroying insects. Cater- 

 pillars should be watched particularly in the early spring and 

 should be removed from plants. A good way to protect the 

 trees is to destroy the insect eggs before they are hatched, or to 

 spray the trees with a substance injurious to the young cater- 

 pillars. 



Leaves keep the air moist. Plants require soil food and 

 get it by absorbing water in which minerals are dissolved. 

 The amount of minerals dissolved 

 in ground water is very small, 

 and the roots must absorb enor- 

 mous quantities of water in order 

 that the plant may get the re- 

 quired amount of mineral matter. 

 The large quantity of water taken 

 in by the roots passes upward 

 through the stems and finally 

 reaches the leaves. Some of this 

 water is used in starch making 

 and the rest is given off through 

 pores in the leaves. This giving 

 off of water through leaf pores, or 

 stomata, is known as transpira- 

 tion (Fig. 309). Most of us have no correct idea of the 

 quantity of water absorbed by the roots and released by the 

 leaves. It has been calculated that a large, healthy oak tree 

 has about 700,000 leaves and that it throws off to the air 



FIG. 309. Through transpiration 

 water collects in the upper glass. 



