OF PLANTS. 19 



the great extent of surface that they offer to 

 the action of the air, perforin a most important 

 part in the vegetable kingdom; and it will 

 be seen from the functions now about to be de- 

 scribed, that their importance is not overrated. 



Leaves perform, in the vegetable kingdom, 

 the same offices as the lungs in the animal 

 kingdom. Through them, from the pores co- 

 vering their surface, the respiration of the plant 

 is carried on, and more than this, for at the 

 same time that the respiration is going on 

 through their pores, a constant assimilation of 

 one of the gases of the atmosphere (hereafter 

 to be described) is also taking place — and from 

 this source the plant derives a considerable 

 portion of its nourishment; to this also we 

 must add that a constant chemical action is 

 always in operation in the leaves, in the forma- 

 tion of the resinous, and oleaginous, and acid 

 matters they contain. These processes of the 

 leaves are constantly in operation, from the 

 first formation of the leaf until the seed is per- 

 fected, and they only cease when, from the 

 ripening of the fruit, their assistance is no 

 longer required. 



It is important to remark that light is indis- 

 pensably necessary to this function of plants ; 

 in its presence, both the mechanical action of 

 evaporation of the watery parts and the secre- 

 tion of the various gases is carried on vigor- 

 ously ; but in its absence the plant loses this 

 power, and becomes subject in its turn to the 

 action of the oxygen of the atmosphere. 



In speaking of the root of plants it has been 



