8 4 
Lee . — - The Morphology of Leaf-fall. 
cases. It is found that the usual processes occur, leading to formation 
of Protective-layer, and that even the Separation-layer may be partially 
formed. That is to say, an incomplete Separation-layer is produced 
which possesses the usual characteristics, but not being continuous it is 
powerless to effect the separation of the leaf, which therefore remains on 
the tree. In late winter divisions which occur beneath the Protective-layer 
result in the formation of a second Separation-layer by the agency of which 
in the following spring the leaf is thrown off. Tison, who has worked out 
the formation in some detail, states that the divisions beneath the Protec- 
tive-layer lead to the formation of a cambium which gives rise to many 
layers of cork towards the scar. The first layer to be formed, however, 
retains its cellulose character, the side walls elongate and finally rupture, 
and the leaf separates from the stem. In the cases examined during the 
present research the complete course of events has not been ascertained due 
to failure of material. It appears, however, that little or no cork is formed 
previous to the throwing off of the dead leaf, and that the elongation of the 
cell-walls of the Separation-layer is not very great. 
PRUNUS CERASUS, Linn. 
The external morphology of the petiole and adjacent parts is well 
known, and calls for no special remarks. It might be noted, however, that 
a fairly deep furrow marks the transition from stem to petiole, and provides 
a line of weakness which, quite apart from the fact that the plane of the 
Separation-layer is later coincident with it, is easily recognized by the ease 
with which the leaf when submitted to pressure breaks at that place. 
We have here again to notice the reduction that occurs in the scleren- 
chyma accompanying the three leaf-trace bundles. When they first leave 
the vascular cylinder in the stem each is provided with a conspicuous mass of 
lignified stereome situated outside the phloem, but this entirely disappears 
just below the leaf-base, and does not again reappear. Higher up the petiole 
the strengthening tissue consists of collenchymatous hypoderm, and patches 
of thick-walled cellulose cells which accompany the phloem ; but little or 
none is present at the leaf-base. In a median longitudinal section the 
vascular bundle is prominent, and the furrow before mentioned is very well 
marked' on the ventral surface of the petiole. There is practically no differ- 
ence in size between the cells of the cortex and those of the petiole, but 
the transition region is marked by an area of smaller cells. Starch is present 
in the cortex, less abundant in the leaf-base, and entirely absent from the 
petiole, while compound crystals of calcium oxalate are common in all 
these regions. A hypodermal periderm is present before leaf-fall. 
The first stage in the leaf-fail processes is shown by the increased 
abundance of the protoplasm in the smaller cells of the leaf-base, which 
almost immediately begin to divide by irregular walls which appear singly 
