LECT. X.] ANATOMY OF LEAVES. 547 



classes present, and the difficulty of pointing 

 out the general features which should be re- 

 garded as the fixed characteristics of each class, 

 present too many difficulties to permit the adop- 

 tion of this arrangement. Equal difficulties occur 

 to render objectionable any classification founded 

 on the form, or the substance, or the arrangement 

 of the parts of leaves. The only plan, therefore, 

 that we can adopt to render our investigation me- 

 thodical, is to discover in what circumstances all 

 leaves agree; and to examine the nature of these, 

 with their modifications in the three great divisions 

 of plants which we have just noticed. 



In the most cursory examination of the ma- 

 jority of leaves, we perceive that these organs are 

 composed of three distinct parts : one part, firm 

 and apparently ligneous, constitutes the framework 

 or skeleton of the leaf; another, succulent and 

 pulpy, fills up the intermediate spaces of this 

 framework; and a third, thin and expanded, 

 incloses the other two, or forms the covering for 

 both surfaces of the leaf. On a closer examina- 

 tion we find that the first of these parts is vascular, 

 the second cellular, and the third a transparent 

 cuticular 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 



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