WHY THE LEAVES FALL. 
375 
or periderm cells, which, passing through the leaf-stalk at nearly 
a right angle to the other tissues, prevented the passage of the 
sap from the stem to the leaf, and ultimately caused the detach- 
ment of the latter, pretty much in the same way as that by 
which the removal of the large slabs of bark from the plane- 
tree is effected. Yon Mohl, however, shows that this statement 
of Schacht’s is too general ; that the periderm layer is as often 
absent as present ; and so far as we have ourselves noticed, the 
periderm cells are often not formed till after the fall of the leaf, 
when they gradually extend over the wound, and close it as by 
a plaister. Yon Mohl’s account of what happens is as follows : — 
Shortly before the fall of the leaf, there begins to be formed a 
very delicate layer of cells, the growth of which is from above 
downwards, so that, beginning from the axillary side of the leaf, 
and gradually extending downwards and outwards nearly at 
right angles to the long diameter of the cells of the leaf-stalk, 
at any rate at right angles to the plane of the leaf, it effects a 
gradual separation between the stem and the leaf as effectually 
as a knife would do. The layer in question does not sepa- 
rate, as might be supposed, the comparatively dry distal 
portions of the leaf, from the still active cells of the stem, which 
are filled with juices, but it is formed in the midst of soft tissue, 
the cells of which are turgescent with fluids. It is, therefore, 
not so much a separation of dead from living tissue, as a passage 
of an active “ line of demarcation ” through living, still active 
tissue. This “ separating layer,” unlike the periderm cells of 
which Schacht speaks, is composed of young tissue endowed with 
a large share of vitality ; its cells are usually filled with starch 
and with albuminoid matters, indicative of a young and active 
tissue. The vessels of the petiole, according to Yon Mohl, are not 
affected at all by these changes ; they simply become broken 
through as the leaf falls. We allude to the spiral and pitted 
vessels especially, and not to those “ vasa propria ” which have 
of late been so minutely studied by M. Trecul. The latter ob- 
server, in the Gomjptes Rendus , July 1, 1867, p. 25, mentions 
the formation, in the vasa propria, of a layer of cells, just prior to 
the fall of the leaf, in several plants. The effect of this forma- 
tion of new cells is to fill up and completely obstruct the reser- 
voirs of secretions. These cells are said by M. Trecul to be 
formed from the subdivision and growth of those forming the 
boundaries of the channels in question, and thus are wholly dif- 
ferent from those constituting the separating layer of Yon Mohl, 
though it is obvious that they must have a similar effect in 
promoting the detachment of the leaf from its support. It may 
be as well to point out that the circular constriction so often 
seen at the base of a leaf-stalk, forming the joint, or articulation, 
as it is called, exists from the beginning, and while it indicates 
