104 DR. K. LEDEGANCK ON THE FALL OF LEAVES IN AUTUMN. 



Link, begins the series of phytotomical observers of the modem 

 school, which attaches itself above all, to close observation of facts. 

 According to this clever observer, the cause of the fall is inherent 

 in the leaf-structure, and pre-existsin every chemical and mechanical 

 alteration of the tissue. At the point of union of the petiole with 

 the pulvinus, a layer of cells, placed in a different direction to those 

 of neighbouring layers, determines the weak point, where, later on, 

 separation must take place. As to the proximate cause of the 

 separation, our author states nothing positively. 



Mettenius, in the course of his researches on the ferns, is led to 

 admit of the formation of a new tissue, a thing difficult to imagine oc- 

 curring in an organ in the decline of its vitality, such as a leaf ready 

 to fall. This tissue, " delicate parenchyma," is produced between 

 the base of the petiole and the pulvinus, but dying soon after, it 

 occasions the disarticulation of the parts it formerly united. The 

 imposing authority of Hugo v. Mohl is brought in to support these 

 observations, which, nevertheless, seem to us insufficiently estab- 

 lished, and which, for our part, setting aside the ferns, we have 

 never been able to confirm by microscopical research. Still, if the 

 existence of this special " separating layer," the cells of which 

 have the faculty of speedy self disorganization, were proved for all 

 articulate leaves, what would happen to the vascular bundles ? If these 

 latter remain intact, as Hugo v. Mohl admits, to be afterwards broken 

 mechanically, how could the separation of the leaf happen so com- 

 pletely and suddenly, leaving so clean a scar for all leaves alike ? 

 Besides, the production of this young tissue in the midst of other 

 tissue in process of drying up, appears to us difficult, if not im- 

 possible to admit, and that it is not established by direct ob- 

 servation. 



To Schacht (1852) is due the merit of having, by his observa- 

 tions upon Draccena reflexa, placed the first indicator in the 

 path so vaguely traced out before his time. It was whilst reading 

 his remarkable work, Die PJianzenzelle^ that we were induced to 

 commence our work ; and it is to reply to the appeal which he there 

 makes to botanical micrographers, which induced us to undertake 

 a long series of investigations on this interesting and debated sub- 

 ject. We may, perhaps, be permitted to cite here the words of 

 the passage : — 



" This fact, certain as to Draccena reflexa, may probably throw 

 light upon the full of the leaf generally. The leaf, as we know, 



