NOURISHMENT OF PLANTS. 29 



an extremely pungent odor. This latter gas is al- 

 ways evolved in the putrescence of animal and vege- 

 table matter. 



The proximate constituents of plants may be 

 grouped, according to the elementary substances of 

 which they are composed, into two principal divis- 

 ions. The first includes those vegetable substances 

 which are composed of but three elements, — car- 

 bon, hydrogen, and oxygen ; the second comprises 

 those which consist of four elements, — carbon, 

 hydrogen, oxygen, and nitrogen. This classification 

 possesses not merely a theoretical, but in an emi- 

 nent degree also a practical interest, since very im- 

 portant conclusions are deducible therefrom in rela- 

 tion to the food of plants and the refuse matter 

 engendered from them. The distinction upon which 

 it rests will be seen at once to depend upon the 

 presence or the absence of nitrogen. Hence the 

 first group may be said to comprise non-azotized 

 vegetable substances, or such as are destitute of 

 nitrogen ; and the second, azotized vegetable sub- 

 stances, or those which contain nitrogen. The latter 

 contain, besides nitrogen, small quantities of phos- 

 phorus and sulphur. 



The proximate constituents of plants already 

 known may accordingly be thus arranged : — 



