578 CONSERVATIVE ORGANS. [LECT. X. 



disposition of the vascular bundles, in the articu- 

 lations, is very obvious: for, had the fascicles 

 remained distinct, and surrounding the pith, 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 endangered 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 necessarily have been subjected. 

 But> by the whole of the vessels being situated in 

 the centre of the petiole, the extension and com- 

 pression produced by the flexure, on 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 impunity. The neces- 

 sity of such a modification of structure, in the 

 petioles of compound leaves, susceptible of mo- 

 tion, may indeed be inferred from the fact, that 

 articulations are present in all those which per- 

 form certain movements; as, for example, those 

 which fold together their leaflets at night ; those 

 which are endowed with the power of spon- 

 taneously moving their leaflets, as Hedysarum 

 gyrans; and those which fold their leaflets together 

 when touched, as Mimosa sensitiva and pudica. 



Some simple leaves, those for instance of 

 the Holly Hock, of the Geranium tribe, &c. 

 which have several principal costse diverging from 



