83 
Lee . — The Morphology of Leaf fall. 
Invariable. It commences near the ventral surface of the petiole just above 
a small ridge on the inner surface of the petiole, and runs across obliquely 
to a point very near the upper limit of periderm formation in the stem. 
The separation which soon follows commences near the ventral surface of 
the petiole. As the division of the cells (and consequently, their separation) 
is more and more backward the nearer we approach the dorsal surface, it 
usually happens that the separation in the latter region is more or less 
mechanical, and is due to the whole weight of the leaf being thrown on this 
small area. Although it seems highly probable that the middle lamellae 
between the separating cells become changed in some way or other to lessen 
the resistance to separation, no such change could be traced, the tests for 
mucilage especially giving no positive results. From a comparison of the 
cell-walls of the Separation-layer before and after separation it seems 
possible that the disunion is accomplished by differential growth of the 
respective cell-walls. 
At the time of separation there is usually no ligno-suberization of the 
cells beneath the exposed surface, although a varying number of new cell- 
walls have appeared. Soon after the cells immediately below begin to 
undergo change, the process spreading in all directions until a fairly thick 
layer, including the living tissue within the vascular bundle at that level, 
has become ligno-suberized. Tyloses appear in some of the larger vessels, 
and the gummy lignin produced is fairly abundant. Towards the end of 
the first winter a cambium arises beneath the Protective-layer which in the 
first year produces a considerable quantity of cork, and which continues its 
activity in the second year. 
Carpinus Betulus, Linn. 
The ordinary characters of stem and petiole (both external and 
internal) are present in this species. In the processes which take place 
before and after leaf-fall there is little to distinguish it from any of the 
other examples of this type. Previous to leaf-fail the cells at the base of 
the petiole undergo irregular division, but no ligno-suberization occurs 
until after the leaf has fallen. The Separation-layer, which is usually a fair 
distance above the junction of the stem and petiole, is formed by more or 
less irregular divisions in a layer of cells 2-3 rows in thickness, and separa- 
tion occurs between the outer cells of this layer. A well-marked Lignified- 
layer is almost invariably present above the Separation-layer. After leaf- 
fall, ligno-suberization commences in the cells of the Protective-layer, but is 
very diffuse, although apparently aided by the distribution of gummy lignin 
from the vessels. In late winter divisions in the cells beneath the Protective- 
layer result in the formation of a cambium, which, however, is not very active. 
It is well known that in this species the leaves often remain on the 
tree during the winter, and it is interesting to see what happens in such 
