PLANT FOOD AND MANUREH. 117 



CHAPTER VI.-PLANT FOOD AND MANURES. 



Reference has already been made to 

 water and carbon dioxide as two of the con- 

 stituents of plant food. These two substances 

 are obtained from the atmosphere, the former 

 falling as rain and usually entering plants 

 through their roots, the latter being absorbed 

 and assimilated by the green leaves. In addition, 

 certain constituents of the food of plants are 

 derived from the soil. We will divide these 

 into two classes : nitrogenous and mineral mat- 

 ters. This division is convenient, for the absorp- 

 tion of nitrogen is sufficiently interesting to 

 make it desirable to devote separate attention 

 to it. Moreover, when a plant, or any other 

 vegetable substance, is burned, the nitrogen 

 disappears in the gases or vapours, together 

 with the carbon, hydrogen and oxygen, while 

 the mineral matter remains behind in the 

 form of ash. 



NITROGENOUS MATTER. 



1. As the air by which we are surrounded 

 consists of four parts of nitrogen and one 

 part of oxygen, it would seem reasonable to 

 suppose that its nitrogen would amply pro- 



