xviii 



INTRODUCTIOX. 



and the latter^ while in their growing state and 

 exposed to the action of lights are engaged in se- 

 parating from the air all that is noxious to animals, 

 and restoring to it in its pure state that which 

 supports their ^-itality. Thus we are permitted 

 to see, that, were not the earth occupied by 

 the proportion of animals and vegetables which 

 He has ordained, it would be fit for the support 

 of neither. The leaves, too (to borrow a simile 

 from machinery), serve as a safety-valve to the 

 whole complex structure of which they form a 

 part : if the roots absorb a greater C[uantity of 



CUTICLE OF A LEAF, SHOWING THE STOMATA. 



moisture than is consistent \\dth the well-being 

 of the tree, the leaves are furnished ^^ith an 

 apparatus for transmitting it in the form of va- 

 pour to the air; and if, on the other hand, the 



