STRUCTURE.] ARE IN ALL CASES LEAVES. 241 



fications of one common type, the leaf, have been gradually 

 discovered; so that that which, forty years ago, was considered 

 as the romance of a poet, is now universally acknowledged to 

 be an indisputable truth. It may, however, be remarked, 

 that when those who first seized upon the important but 

 neglected facts out of which this theory has been constructed, 

 asserted that all appendages of the axis of a plant are meta- 

 morphosed leaves, more was stated than the evidence at that 

 time would justify; for we cannot say that an organ is a 

 metamorphosed leaf, when, in point of fact, it has never been 

 a leaf. What was meant, and that which is supported by the 

 most conclusive evidence, is, that every appendage of the 

 axis is originally constructed of the same elements, arranged 

 upon a common plan, and varying in their manner of deve- 

 lopment, not on account of any original difference in struc- 

 ture, but on account of special, local, and predisposing causes : 

 of this plan the leaf is taken as the type, because it is the 

 organ which is most usually the result of the development of 

 those elements, is that to which the other organs generally 

 revert, when, from any accidental disturbing cause, they do 

 not sustain the appearance to which they were originally 

 predisposed, and moreover, is that in which we have the 

 most complete type of organisation. 



Proof of this will be furnished as the different modifica- 

 tions of the appendages of the axis come respectively under 

 consideration. The leaf, as the first that is formed, the most 

 perfect of them all, and that which is most constantly present, 

 is properly considered the type from which all the others are 

 deviations, and is the part with the structure of which it is 

 first necessary to become acquainted. 



1. Of the Leaf. 



The leaf is an expansion of the bark at the base of a 

 leaf-bud, prior to which it is developed. In most plants it 

 consists of cellular tissue, filling up the interstices of a net- 

 work of fibres which proceed from within the stem, and 

 ultimately separating from the bark by an articulation ; in 



VOL. I. TJ 



