IMPORTANCE OF AGRICULTURE. 143 



from them than that nitrogen is not necessary for 

 the decomposition of carbonic acid, — for the exer- 

 cise, therefore, of one of the functions of plants. 

 And yet the presence of a substance containing this 

 element appears to be indispensable for the assimila- 

 tion of the products newly formed by the decompo- 

 sition of the carbonic acid, and their consequent 

 adaptation for entering into the composition of the 

 different organs. 



The carbon abstracted from the carbonic acid 

 acquires in the leaves a new form, in which it is 

 soluble and transferable to all parts of the plant. 

 In this new form the carbon aids in constituting 

 several new products ; these are named sugar when 

 they possess a sweet taste, gum or mucilage when 

 tasteless, and excrementitious matters when expelled 

 by the roots. 



Hence it is evident, that the quantity and quality 

 of the substances generated by the vital processes of 

 a plant will vary according to the proportion of the 

 different kinds of food with which it is supplied. 

 The development of every part of a plant in a free 

 and uncultivated state depends on the amount and 

 nature of the food afforded to it by the spot on 

 which it grows. A plant is developed on the most 

 sterile and unfruitful soil as well as on the most 

 luxuriant and fertile, the only difference which can 

 be observed being in its height and size, in the num- 

 ber of its twigs, branches, leaves, blossoms, and 

 fruit. Whilst the individual organs of a plant in- 

 crease on a fertile soil, they diminish on another 

 where those substances which are necessary for their 

 formation are not so bountifully supplied ; and the 

 proportion of the constituents which contain nitrogen 

 and of those which do not in plants varies with the 

 amount of nitrogenous matters in their food. 



The development of the stem, leaves, blossoms, 

 and fruit of plants is dependent on certain con- 

 ditions, the knowledge of which enables us to ex- 

 ercise some influence on their internal constituents 



