THE PRODUCTION OF ENERGY IN PLANTS 



69 



In green plants during the night, when carbon dioxid is not 

 being used for starch manufacture, this gas is given off to the 

 surrounding air, which probably is not true to any great 

 extent during the daytime. The taking in of oxygen and the 

 giving off of carbon dioxid by plants corresponds to breath- 

 ing in animals. This exchange of gases is carried on 

 through the thin walls of roots, through the lenticels of 

 stems, and through the stomata of leaves. The process of 

 breathing must not be confused with that of carbohydrate 

 manufacture, and the following outline will show the funda- 

 mental difference between the two. 



