154 ON THE TORPIDITY OF PLANTS. 



cause or aiiolher evaporate but little, often last for several years. We 

 may, therefore, in general, say that the duration of life in leaves is in 

 inverse proportion to the force of their evaporation. When this time 

 has arrived, the leaf gradually dries up, and finishes by dying ; but 

 the death of the leaf ought not to be confounded with its fall ; for 

 these two phenomena, although frequently confounded, are in reality 

 TGiy different. All leaves die sometime or other, but some are 

 gradually destroyed by exterior accidents without falling, while others 

 fall, separating from the stem at their base, and fall at once, either 

 already dead, or dying, or simply unhealthy." 



It is probable that both these explanations are required to under- 

 stand the phenomena of the fall of the leaf; but that it is not altogether 

 owing to the rupture of the spiral vessels by the acquisition of new 

 wood, is proved by the fact, that in some trees which increase in wood 

 the fastest, the leaves remain upon the branches the longest. Nor 

 can it invariably be caused by the choking up of other kinds of tissue, 

 from the fact, that in plants which are known to evaporate their moisture 

 very rapidly, the process of defoliation may be greatly retarded by plac- 

 ing them in the green-house, and by that means sheltering them from 

 the influence ofthe cold and wet weather of autumn. If we examine 

 a leaf as it is fixed upon the branch of a tree in autumn, we shall find 

 at the axil of its footstalk, a bud already formed and ready for expan- 

 sion into leaves and flowers in the ensuing spring. I do not profess 

 myself to be in position to substantiate the point, but I must confess 

 that I am strongly impressed with the opinion that the formation and 

 development of the bud causes pressure to be made upon the vessels 

 communicating with the leaf of the present year and the medullary 

 sheath, and that it is here the obstruction of the flow of sap takes place, 

 thus cutting off the principal supply of nourishment which is neces- 

 sary for the health and well being of the leaf ; and that it is not as, De 

 Candolle supposes, in the cellular and other tissues of the blade ofthe 

 leaf that the obstruction takes place. I say that 1 am not in a posi- 

 tion to substantiate the truth of such an opinion, but there cannot be 

 any doubt that the presence of the bud must create some degiee of 

 pressure upon the spiral vessels of the leaf immediately preceding it, 

 and thus causing the leaf to fall off at this point ; for where healthy 

 and natural defoliation takes place, we shall invariably find a scar 

 immediately under the leaf-bud, which is not the case if a leaf withers 

 and dies from accidental causes. 



