LEAVES. 31 



rapid emission of air, when it is necessary that 

 sucli a function should be performed. 



The action and functions of stomata being 

 of such vital importance, it is absolutely neces- 

 sary to the health of a plant that they be not 

 choked up with dust or dirt or injured by in- 

 sects, the cleaner therefore the leaves of a plant 

 are kept the more it will flourish. 



Leaves growing in air are covered with a 

 cuticle. 



Leaves growing under water have no cuticle. 



All the secretions of plants being formed in 

 the leaves, or at least the greater part, it fol- 

 lows that secretions cannot take place if leaves 

 are destroyed. 



And as this secreting property depends upon 

 specific vital powers connected with the decom- 

 position of carbonic acid, and called into action 

 only when the leaves are freely exposed to 

 light and air, it also follows that the quantity 

 of secretion will be in direct proportion to the 

 quantity of leaves, and to their free exposure 

 to light and air. 



The leaf therefore is a beautiful contrivance 

 for exposing a large surface of crude sap to the 

 influence of the external air and solar light, by 

 the operation of which it is rendered capable 



