THE PLANT IN ACTION. 251 



As we have before stated, the various inorganic 

 substances, taken from the soil by the roots, and from 

 the air by the leaves, are the food of plants. In the 

 leaf-cells they undergo remarkable and very complex 

 changes, some of which are understood, while others 

 are not, and which it is the proper business of chemis- 

 try to explain. The most important action of the leaf 

 is the reduction of carbonic-acid gas, ammonia, and 

 water, to their elements, which are used for the forma- 

 tion of organic compounds. This may be regarded as 

 the first step in the process of organization, and it takes 

 place in the leaf only under the influence of light. 

 Light is the motive power of the vegetable kingdom, 

 and the countless myriads of expanded leaves are all 

 little machines, upon which it takes effect. Light 

 impels the actions of a leaf as falling water impels a 

 water-wheel. The light is an active force, which is ex- 

 pended upon the leaf, is absorbed, and produces chem- 

 ical decompositions. Carbon, the substance of char- 

 coal, is thus separated from carbonic acid, and is ready 

 to be used in the production of organic compounds, 

 of which it is a universal constituent. The decompo- 

 sition of water and ammonia gives also hydrogen and 

 nitrogen, and these, with oxygen, form the chief bulk 

 of all organized substances. 



Animals have no such power of creating the or- 

 ganic substances which compose them. So that the 

 whole animal world, and the entire vegetable king- 

 dom, may be said to have their origin in leaves. 

 But it is only th first step that is here taken. After 

 carbonic acid, water, and ammonia, are decomposed, 

 their elements are recombined in new groups under 

 the constructive agency of the plant, and their sub- 



