ANATOMY OF LEAVES. 2Q5 



dicotyledonous leaves, the radiated arrangement of 

 the spiral vessels is found in all : the petiole in this 

 respect, as well as in the other parts of its structure, 

 closely resembling the stem or the branch from which 

 it springs. In simple leaves, with a few exceptions, 

 although the vascular part forms at first several fasci- 

 culi, at the base of the petiole, yet these soon coa- 

 lesce into one compound fasciculus ; but in compound 

 leaves they remain distinct. Thus, in the common 

 kidney-bean, as an example of a composite leaf, in 

 which the petiole is channelled, with an articulation 

 at the base of the common petiole, and, also, at that 

 of each partial petiole, we find that the vascular fas- 

 ciculi are distinct, and form a circle situated immedi- 

 ately under the bark in the channelled parts of the 

 petiole ; with a considerable portion of lax cellular 

 substance or medulla, inclosed within the circle which 

 they form : whereas, in the articulated parts, there is 

 one central fasciculus only, surrounded by a large 

 mass of very firm cellular matter. The advantage 

 of this change of disposition of the vascular bundles, 

 in the articulations, is very obvious ; for, had the fas- 

 cicles remained distinct, and surrounding the pith, as in 

 the articulations, those on the outside of the flexure, 

 in every considerable motion of the joint, must have 

 described so large a circle, as would have endanger- 

 ed the organization of the vessels by the extension ; 

 while those on the inner side would have suffered, 

 equally, by the compression to which they must ne- 

 cessarily have been subjected. But, by the whole 

 of the vessels being situated in the centre of the 

 petiole, the extension and compression produced 

 by the flexure in every part of the fascicle, is not 

 more than can be borne by any individual vessel, 

 whether spiral or entire ; and, thence, the freest and 

 most varied motion of the joint can be exercised with 



