PLANTS. 61 



effect the chang'c. For this reason a continuation of 

 dark, rainy weather is injurious to the growth of most 

 phmts. In sucli weather they turn i»ale from the absence 

 of coloring material, which requires the aid of sunlight 

 for its formation. 



The air is admitted to the interior of a leaf through 

 minute openings, or mouths, which generally exist in 

 gi'cat numbers. Upon an ordinary apple leaf there may 

 be found as many as 100,000 of these openings. 



Plants Purify the Air. — This absorption of carbonic 

 acid and liberation of oxygen are of the greatest impor- 

 tance to animal life. 



In the lungs of men and other animals, oxygen from 

 the air is continually absorbed into the blood, and in 

 place of it carbonic acid gas passes from the lungs with 

 the breath. An accunudation of this gas in the atmos- 

 phere would soon l)ecome jtoisonous, but the danger is 

 avoided, and the balance maintained, by the fact that all 

 growing vegetation is constantly withdrawing carbon and 

 setting oxygen free. It is estimated that an acre of 

 forest trees will consume the carbonic acid produced 

 by the breathing of fifteen men. 



In this way vegetable and animal life mutually benefit 

 each other, each requii'ing and making use of that ele- 

 ment which is rejected by the other. 



Do Plants Breathe? — While plants are taking in car- 

 l)on and thr(^\ving out oxygen, they are at the same time, 

 though only to a slight extent, doing exactly the reverse, — 

 taking in oxygen and throwing out carl)onic acid. This 

 is very similar to the act of breathing in animals. 



It would seem unnecessary for the plant to take these 

 two seemingly oj)j)osite courses, but they are for entirely 

 different purposes, 



