VEGETABLE PHYSIOLOGY. 87 



exists any example whatever of vegetable sloughing, which the 

 same tree will also furnish in the annual or rather continual 

 exfoliation of its bark, but that the fall of the leaf does not seem 

 to afford that example. 



We can foresee an objection that may be urged against the 

 above argument from the fact of the sloughing of the entire skin 

 of the snake, and other species of serpents, which may be 

 regarded as a distinct organ. But although the skin of the snake 

 or any other animal may be regarded as a distinct organ, yet it 

 must be in a light very different from that of an organ attached 

 to the body of a plant or animal by a natural joint or articulation 

 that comes asunder of its own accord ; for the skin of the animal 

 in question is forced off in the manner of a slough merely by 

 means of the formation of a new skin beneath it, which has 

 already taken the place of the old skin in the living system, and 

 to which it has just been shown that there exists nothing whatever 

 analogous in the fall of the leaf. So that, after all, the best 

 reason we can give is, perhaps, that the leaves fall in consequence 

 of their being worn out, and no longer necessary \o the immediate 

 process of vegetation ; which is evidently divisible into animal 

 stages commencing with the approach of spring, and terminating 

 with the return of winter, which is to the vital principle, 

 apparently, a period of rest. If it is necessary, however, to 

 attempt an exemplication of the process by which the leaf is made 

 ultimately to detach iisclf from the plant, it may be observed 

 that it consists wholly in the change that is effected in the 

 articulation uniting the foot stalk to the branch ; for in the case 

 in which the injury extends suddenly beyond the leaf, the leaf 

 may wither and decay, but will not fall off, because the articulation 

 has not been duly prepared, and because the vital principle can 

 now no longer act upon it from the intervention of the dead or 

 diseased portion of the plant beyond which it has withdrawn 

 itself. But in the natural process of vegetation the necessary 

 change is effected by the leaf on the one hand, in its yielding 

 to the influence of physical or chemical agencies, and withering 

 and shrinking into narrow compass, when the usual supply of sap 

 is no longer transmitted to it ; and by the vital principle on the 



