682 NATURAL SCIENCE. ^.„,, 



process of bark, so that the tree does not suffer from the effects of an 

 open wound. The change in the cellular tissue prior to its disruption 

 he describes as a sort of self-disintegration ; " the cells contract and 

 become rounder, and separate their walls from each other, so as to 

 destroy their cohesion." He hys great stress on the point that this 

 separating process is a vital one, and there is no doubt that he gives 

 the true explanation, but it is vaguely expressed, and though fore- 

 shadowing some of the results subsequently arrived at by Mohl, 

 his work will not compare with the careful account of the 

 German botanist to whom belongs the honour of the satisfactory 

 elucidation of the problem of leaf- fall. 



In 1859, Hugo V. Mohl (15) chanced, as he tells us, for the first 

 time for many years to spend the autumn at home, and took the 

 opportunity of investigating, in a number of plants, the organic 

 changes which cause the fall of leaves. The result of his investiga- 

 tions will be found in the first two numbers of the Botanische Zeitmig 

 for i860, of which paper he was then one of the editors. Taking 

 Gymnodadns canadensis as an example, he describes the appearances 

 noticed in the tissues of the leaf-joint between the loth and 20th 

 October, during which time many of the leaves lose their leaflets, 

 and the common leaf-stalks on the lower nodes of the branches 

 gradually drop, while those on the upper nodes are only beginning 

 to prepare for their fall and cannot be removed without tearing 

 the joint. 



Separating the tissue of the twig from that of the leaf-stalk is a 

 layer of cork. The tissue of the lower part of the leaf-stalk consists 

 of shorter cells than those of the rest of the stalk or the cortex of the 

 twig, while the lowest layer immediately above the cork also has its 

 cell-walls coloured brown. Somewhat parallel with this brown layer, 

 but separated from it by colourless polyhedral cells exactly similar 

 to those forming the bulk of the leaf-joint, a thin cell-layer runs 

 across the joint. If we examine a leaf near the time for falling, but 

 which yet cannot be separated without irregular tearing, this layer 

 is seen, in longitudinal section, to consist of a few cell-rows, some- 

 what more opaque than the rest of the tissue, in which treatment with 

 iodine tincture reveals a fair amount of very small starch grains, 

 absent from the rest of the tissue of the leaf-joint, a brown-staining, 

 slimy content of a proteid nature, and a conspicuous primordial 

 utricle. Wa are e\'idently dealing with a tissue which, in contrast 

 with the rest of the leaf-joint, has all the characters of a young tissue 

 — is, in fact, meristematic. Closer observation shows, in many of 

 the cells, thin dividing walls parallel to the layer ; a process of cell 

 multiplication is evidently going on. If we examine a leaf on the 

 point of falling, where the tear is more or less complete, we find that 

 it proceeds from the separation of the walls of the cells of the starch- 

 containing layer, the free rounded surfaces of which are seen lining 

 the sides of the tear. This layer Mohl terms the " separating layer." 



