146 GENERAL FUNCTIONS AND STRUCTURE OF LEAVES. 



them to great risk of injury, would have impeded their rapid 

 movements, and would have been attended with many inconve- 

 niences. 



215. Now in Vegetables, the same object is to be attained ; 

 but under different conditions. The nutritious fluid of the Plant, 

 like the blood of animals, needs to be exposed to the influence of 

 the air, to preserve its power of maintaining life ; and this cannot 

 be effected, either by the underground roots, or by the hard 

 woody stems and branches, which expose so small an amount of 

 surface to the atmosphere. Nor can this be effected, by the intro- 

 duction of air into internal cavities in these parts ; since this would 

 require a continued series of movements, as in air-breathing- 

 animals, which the Plant has no means of performing. Again, as the 

 Plant is rooted in the earth, and is not adapted to move through 

 the atmosphere, there is no reason why its surface should not be 

 spread out to any extent, for the purpose of exposing the sap to 

 the influence of the air; just as the blood is exposed in the gills 

 of fishes and other aquatic animals, to the small quantity of it 

 contained in the water they inhabit. Further, a very essential 

 condition of the changes, which the sap undergoes by coming 

 into contact with air, is the influence of light ; without which 

 they would be very imperfectly performed. 



216. This general view of what is required from the Leaves, 

 will suffice to show, how beautifully their structure and situation 

 are adapted to the offices they have to perform. The leaf may 

 be said to consist essentially, of an extension of the skin or cuticle 

 of the Plant, into a flat expanded surface ; which is supported by 

 a skeleton, prolonged from the wood of the stem or branch. If 

 any leaf be but cursorily examined, it will be seen that from each 

 surface a sort of skin may be torn, which may sometimes be 

 stripped off very cleanly from the tissue beneath : the space 

 between these surfaces being occupied by soft green tissue, which 

 the naked eye can often perceive to consist of separate particles 

 loosely united, and which is seen with the magnifying-glass to 

 be composed of distinct cells, usually more closely packed toge- 

 ther near the upper surface than near the lower, where there are 

 many cavities or interspaces among them. 



