OEGANIC CHEMISTRY. 293 



ing material of the plant, is completed, so as to be fit for 

 use in constructing all the various parts the wood, the 

 bark, the flowers, the fruit, etc. Animals, also, receive their 

 elements in part from the earth, but not in a direct man- 

 ner. They receive them from the plants which they eat. 

 The plant, then, gathers up, as we may say, the elements 

 from the earth for the use of the animal. They are com- 

 bined together in the blood, which is to the animal what 

 the sap is to the plant the common building material of 

 the body. But as in the case of the plant, so with the an- 

 imal, all is not derived from the earth. A part of the ox- 

 ygen needed comes from the air, being admitted by the 

 pores of the lungs, as part of the carbon of the plant goes 

 into it by the pores of the leaves. It is believed that the 

 leaves of plants decompose the carbonic acid that comes to 

 them from the lungs of animals, separating it into its ele- 

 ments, carbon and oxygen, and that the carbon is absorbed 

 to make a part of the plant, while the oxygen thus set free 

 again returns to the lungs of animals. Every leaf, there- 

 fore, is a laboratory to purify the air, and maintain its prop- 

 er supply of oxygen for the use of the animal kingdom. 



409. Subservience of Plants to Animals. You see, then, 

 that the subservience of plants to animals is twofold. 

 First, they supply to animals the elements of their growth 

 by gathering them from earth, air, and water into their 

 own substance. This subservience is direct in the case of 

 herbivorous animals. It is no less real, though indirect, in 

 the case of the carnivorous, for they eat the flesh of the 

 herbivorous. Secondly, plants, by their chemical action 

 upon the air, keep up that supply of oxygen which is need- 

 ed by animals. 



410. Difference of Vegetable and Animal Structures in 

 Composition. In this subservience of plants to animals 

 there is one very interesting fact to be noted in regard to 



