332 Prof. Vaucher on the Fall of Leaves. 



from the stem. There exists, however, one case where the 

 pressure of the bud, if not the principal cause, is at least the 

 secondary one of the fall of the leaf ; and this is when the leaf- 

 stalk, instead of being placed under the bud, according to the 

 common law, envelopes it like a bonnet ; but this arrangement 

 is not common. The only trees in which it has been observed 

 are the Plane, the arborescent Sumach, the Ailantha glandu- 

 losa, and Acacia. 



Disease or plethora of the leaves cannot occasion the rup- 

 ture of the leaf-stalk ; for it happens sometimes, and particu- 

 larly after white frosts, that they fall whole and green. Be- 

 sides, in dry autumns, when the juicies are less abundant, the 

 leaves fall as quickly, and even sooner than in moist seasons. 

 In short, this hypothesis does not explain why, in the case of 

 disease, the leaf separates by the base of the leaf-stalk rather 

 than at the leaf ; how it always takes place in the same man- 

 ner and at the same point ; and how, above all, the fracture 

 appears smooth and well-defined, in place of presenting an ir- 

 regular and lacerated surface. 



The third supposition, which attributes the fall of leaves to 

 the increase of the diameter of the twig, although more con- 

 formable to the course of nature than the preceding, does 

 not explain all the appearances which accompany the rupture. 

 For example, it is easily conceivable that the increasing thick- 

 ness of the twig must favour the separation of the leaf-stalk ; 

 but it is not known how this separation, in place of presenting 

 all the irregulai'ities of ordinary fracture, is found so well 

 marked, and so similar to itself in all plants. Farther, al- 

 though this explanation may suffice for simple leaves, that is 

 to say, for those of which the leaf-stalk is not divided, it can- 

 not apply to compound leaves, for the leaflets of these separate 

 from the common leaf-stalk, without its receiving any more 

 growth than the smaller petioles which it supports. 



If the point of adherence of a leaf-stalk, says Professor 

 Vaucher, be examined at the moment of separation, it will be 

 remarked, that it forms a clean and perfectly defined section. 

 This species of cicatrix, of which the impression is also seen 

 upon the twig, is differently figured, according to the confor- 

 mation of the leaves. In some it presents the appearance of 



