76 TALKS AFIELD. . 



gas which is exhaled by animals is necessary 

 to the life of plants. Assimilation can take 

 place only in the sunlight, but growth — the 

 formation of new cells — takes place more 

 rapidly at night. During this growth and 

 the metastasis which is necessary to it, — 

 the changing of one organic compound into 

 another, — the plant is breathing ; air is 

 taken in through the stomata, or the air of 

 the air-spaces is used if the stomata are 

 closed. This breathing is strictly compara- 

 ble to that of animals, as the oxygen is used 

 and carbonic acid gas is given off. The sto- 

 mata act as valves ; they regulate largely 

 the amount of water given off and the 

 amount of air taken in. They are open in 

 sunlight, but are nearly closed in darkness. 

 During a severe drouth, when the roots can- 

 not find sufficient water, they close and allow 

 no evaporation to take place. When the 

 atmosphere is moist they are wide open. If 

 the leaves are the lungs of the plant be- 

 cause they breathe, they are more emphat- 

 ically the stomachs of the plant because they 

 assimilate and digest. 



