2S2 ANATOMY OF LEAVES'. 



CHAPTER V. 



ANATOMY OF LEAVES. 



The Leaf, physiologically speaking, may be defined 

 to be a temporary organ of plants, which performs 

 nearly the same function in the economy of vegetable 

 life, as the lungs perform in that of the animal ; or, in 

 other words, leaves are the respiratory organs of plants. 

 When these organs are absent as in the Dodder, and 

 in the Cactus tribe, where their appearance, and that 

 very inconspicuously, is only temporary, the green 

 surface of the stem evidently performs the function of 

 the leaves. 



In the most cursory examination of the majority of 

 leaves, we perceive that those organs are composed 

 of three distinct parts : one part, firm and apparently 

 ligneous, constitutes the frame work or skeleton of 

 the leaf; another, succulent and pulpy, fills up the in- 

 termediate spaces of this frame work ; and a third, 

 thin and expanded, encloses the other two, or forms 

 the covering for both surfaces of the leaf. On a closer 

 examination we find that the first of these parts is vas- 

 cular, the second cellular, and the third a transparent 

 articular pellicle. Admitting, therefore, that these 

 parts are present in every leaf, although we may not 

 be able to discover all of them distinctly, owing to the 

 imperfection of our instruments ; we may conduct 

 our inquiries into the structure of leaves in reference 

 to their vascular, their cellular, and their cuticular 

 systems. 



I. OF the vascular system of leaves. 



Among fallen leaves, which have been exposed to 

 the action of the atmosphere in a damp place, or 

 which have dropped into a pond, we generally find 



