202 THE PHENOMENA OF MOTION 



products formed by its activity from the food are va- 

 ried. The composition of the buds, of the radical 

 fibres, of the leaf, of the flower, and of the fruit, are 

 very different one from the other ; and the chemical 

 force by which their elements are held together is very 

 different in each of these cases. 



Of the non-azotized constituents of plants we may 

 assert, that no part of the momentum of force is ex- 

 pended in maintaining their form and structure, when 

 their elements have once combined in that order in 

 which they become parts of organs endued with vitality. 



Very different is the character of the azotized vege- 

 table principles ; for, when separated from the plant, 

 they pass, as is commonly said, spontaneously, into 

 fermentation and putrefaction. The cause of this de- 

 composition or transformation of their elements is the 

 chemical action which the oxygen of the atmosphere 

 exercises on one of their constituents. Now we know, 

 that, as long as the plant exhibits the phenomena of 

 life, oxygen gas is given off from its surface ; that this 

 oxygen is altogether without action on the constituents 

 of the living plant, for which, in other circumstances, it 

 has the strongest attraction. It is obvious, therefore, 

 that a certain amount of vital force must be expended, 

 partly to retain the elements of the complex azotized 

 principles in the form, order, and structure which be- 

 long to them ; and partly as a means of resistance 

 against the incessant tendency of the oxygen of the 

 atmosphere to act on their elements, as well as against 



