336 Prof. Vaucher on the Fail of Leaves. 



ed before those of the summit, as happens indeed in most 

 trees. It is also easily understood why leaves fall in warm 

 as well as in cold countries — in stoves as well as in the open 

 air ; the heat, which favours the increase of the stem, ad- 

 vances the moment of separation, and the more southerly we 

 advance, the more defoliation ought to be accelerated. The 

 cold and snows which, in altering the organization of a petiole, 

 destroys its adhesion, hasten also the fall of- leaves, and on this 

 account they sometimes fall when green. Trees of which the 

 shoots are more tardy or more vigorous, ought, on the con- 

 trary, to preserve their leaves till the twigs acquire a woody 

 consistence ; and this is found to be the case with oaks and 

 elms which have been lopped. Branches likewise which have 

 been cut before autumn, ought not to part with their leaves 

 after drying, because these have been prematurely stopped in 

 their vegetation, before the natural period of their fall. 



The chief objection to this theory is, that there are trees 

 which preserve their leaves during autumn and even in win- 

 ter. This, according to M. Vaucher, so far from forming an 

 exception to the general law, rather tends to confirm it. If 

 the leaves of these trees be examined, they will be found dis- 

 similar in structure to the other leaves. Harder, more coria- 

 ceous or ligneous, their tissue approaches nearer to that of the 

 stem upon which they are produced. But when the stem has 

 acquired sufficient size, its adherence with the leaf-stalk is 

 broken, and the leaves follow the common law. The epoch 

 of the fall of leaves of this description, varies with the nature 

 of the tree — in spring — summer — or even after some years. 

 But even admitting these considerations, and others of a simU 

 lar nature, it is finally found, says Professor Vaucher, that the 

 defoliation of these trees depends upon the cause which he has 

 assigned, viz. a solution of organic continuity between the 

 vessels and fibres of the stems, and the vessels and fibres of the 

 leaf-stalk. 



M. Vaucher concludes his memoir with some reflections, and 

 with the observation, that the circular ring or stricture found 

 at the base of the petiole, and common to all trees, is not per- 

 ceived in annual plants, nor in those which, though peren- 

 nial, decay down to the root. When leaves are torn from 



