85 
Lee. — The Morphology of Leaf-fall. 
in each cell. Little growth in size occurs in this Protective-layer, but the 
cells near the dorsal surface (where the layer is thicker) begin to undergo 
a chemical change, the process of lignification commencing in this region 
and extending across the leaf-base to the vascular bundle. Little or no 
suberization occurs previous to defoliation. 
During the processes just described a varying number of tyloses appear 
in the vessels of the leaf-trace near the active area, and are accompanied by 
an abundant supply of gummy lignin, together very efficiently closing the 
lignifaed conducting elements. 
The Separation-layer also arises about the same time. It traverses 
the petiole in the plane of the external furrow and is produced by the 
repeated division by (2-6) parallel walls of 2-3 rows of cells above and 
adjacent to the Protective-layer. There is an abundance of living and 
starchy contents in these cells ; their walls begin to swell, and the middle 
lamellae between the outer cells degenerate into pectic mucilage and 
finally disappear, leaving the neighbouring cells quite free. 
Before the final separation occurs, the cells immediately above the 
Separation-layer, after undergoing one or two divisions, become more 
or less completely lignified. This layer, which always retains a certain 
amount of protoplasm, is thickest at the sides and gradually decreases 
as the centre is approached. 
As separation takes place between the upper rows of the Separation- 
layer there is invariably a mass of unaltered cells left attached to the 
Protective-layer, which for the most part, until properly exposed, retain their 
contents and cellulose walls. After complete separation, however, these 
cells soon lose their turgidity, and dying away, they collapse and become 
firmly pressed to the surface of the Protective-layer. 
In P. Cerasus little or no suberization occurs in the cells of the Protec- 
tive-layer before leaf-fail, but subsequently the deposition of suberin on the 
inner face of each cell goes on rapidly, the living elements of the vascular 
bundle also undergoing this process. The crystal cells in the Protective- 
layer become lignified, but no suberin could be detected in their walls. 
During the first winter the cork cambium is formed by the regular 
division of the cells below the Protective-layer, and by its activity gives rise 
to a few layers of cork. By the end of the second year a thick layer of cork, 
traversing the vascular bundle and continuous with the stem periderm, has 
been preserved. 
P. VIRGINIANA, Linn. 
P. COMMUNIS-DULCIS. 
The description of the course of events in P. Cerasus applies generally 
for P. virginiana and P. communis-dalcis. There is, however, one point of 
difference. So far as the examination goes, there is never any ligno- 
