IMPORTANCE OF AGRICULTURE. 107 



gas, whilst the carbonic acid disappears. Now in these experi- 

 ments no nitrogen \s supplied at the same time with the carbonic 

 acid ; hence no other conclusion can be drawn from them than 

 that a simultaneous introduction of nitrogen is not necessary for 

 the decomposition of carbonic acid, — for the exercise, therefore, 

 of one of the functions of plants. And yet the presence of a 

 substance containing this element appears to be indispensable for 

 the assimilation of the products newly formed by the decomposi- 

 tion of the carbonic acid, and their consequent adaptation for en- 

 tering 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 consti- 

 tuting several new products; these are named sugar when they 

 possess a sweet taste, gum or mucilage when tasteless, and ex- 

 crementitious matters when expelled by the roots or other parts. 



Hence it is evident that the quantity and quality of the sub- 

 stances 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 na- 

 ture 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 number of 

 its twigs, branches, leaves, blossoms, and fruit. Whilst the indi- 

 vidual organs of a plant increase 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 containing nitrogen, and those destitute of it, 

 varies with the amount of nitrogenous matter in the food of plants. 



The development of the stem, leaves, blossoms, and fruit of 

 plants, is dependent on certain conditions, the knowledge of which 

 enables us to exercise some influence on certain of their internal 

 constituents as well as on their size. It is the duty of the natural 

 philosopher to discover what these conditions are ; for the funda- 

 mental principles of agriculture must be based on a knowledge 

 of them. There is no profession which can be compared in 



