FOODOFPLANTS, 19 



the growth of the plant is accelerated, not only by the amount of 

 absorption by the roots, but also by the increased absorption by 

 the leaves. 



On this depends the importance of furnishing to young plants 

 in the spring of the year manures capable of furnishing these nits 

 with carbonic acid. For the same reason the earth is broken up 

 to admit air to vegetable matter of the soil, and thus favoring its 

 decomposition, during which it gives off carbonic acid. When 

 the roots are thus freely supplied with carbonic acid, the plant 

 sends out leaves which themselves are organs for introducing nu- 

 tritive matter from the atmosphere. We thus gain time, so impor- 

 tant for a crop which is to be produced in a single season, but 

 which is of little moment to forest trees which are to continue 

 during a long succession of seasons. 



Ammonia or JYitric acid. — This is the third article of food for 

 plants. It furnishes the nitrogen they contain. 



Carbonic acid and water contain the elements of all the non- 

 nitrogenized principles of plants. The nitrogenized principles, 

 fibrin, albumen, and casein, require the concurrence of nitrogen, 

 which can only be introduced in the form of nitric acid or some 

 salts of ammonia. 



Although plants are surrounded by an atmosphere containing 

 seventy-nine per cent of nitrogen, yet they are incapable of intro- 

 ducing and assimilating it in that form. Some experiments of 

 Boussaingault, which seem to him to prove that plants abstract 

 nitrogen from the atmosphere, admit of being differently explained. 

 It is more probable that the nitrogen was obtained from the mi- 

 nute quantity of ammonia in the atmosphere. 



Wild plants contain less of the nitrogenized principles than 

 those which are cultivated for food. Indeed, the value of vegeta- 

 ble products as food for animals, depends mainly on the quantity 

 of nitrogenized principles they contain. 



One of the great problems in agriculture, is to furnish plants 

 with a supply of nitrogen at a cheap rate. Water and carbon are 

 in general provided by nature in sufficient abundance; and for wild 

 plants, ammonia is furnished sufficiently from the sources I will 

 presently point out. But for plants which are cultivated with re- 

 ference to the amount of nitrogenized principles they contain, it 



