54 Lee . — The Morphology of Leaf-fall. 
cell-division has occurred, and the activity assumed by the cells in this 
region cannot possibly have any connexion with the presence of the stem- 
periderm, which tapers out and stops at some distance below the area 
indicated (Text-fig. i, St.pd.). The separation itself usually commences 
near the dorsal surface of the petiole, and extending rapidly across, soon 
leaves the leaf adhering solely by the vascular elements. The living cells 
of the vascular bundles split apart as described, and as soon as the non- 
living elements — vessels, &c. — become ruptured by mechanical means, 
i. e. the weight of the leaf, wind, &c., the leaf falls to the ground. 
Beyond the massing of the 
protoplasm in the cells at the 
base of the petiole, there has, 
at this time, been no formation 
which could be described as 
protecting the exposed surface. 
Directly after leaf-fall, however, 
the activity of the cells below 
the surface of separation is at 
once shown by the change in 
the chemical composition of 
the cell-walls. Very gradually 
these become more or less com- 
pletely lignified. This process 
commences in regions which 
vary with the individual speci- 
men, being generally situated 
near the epidermis, and spreads 
rapidly until the whole of the 
Protective-layer has under- 
gone lignification (PI. IV, Fig. 2). At the same time there is deposited 
on the inner surface of each cell-wall of the Protective-layer a fine 
film of suberin, the completion of which is marked by the disappearance 
of the protoplasm from the cells of the area. 
Between the area which undergoes ligno-suberization and the exposed 
surface, there is a layer consisting of 2-3 rows of cells which for a time 
remains entirely unaffected by the changes going on around it. For a long 
time these cells retain their protoplasm and their cellulose character, 
but gradually the protoplasm in the outer cells disappears, and the latter 
collapse and form a protective membrane. The change thus initiated spreads 
to the other cells of this layer until finally all the protoplasm disappears and 
the walls of the collapsed cells, while still remaining more or less cellulosic, 
become adpressed to the surface and form a very efficient method of protec- 
tion in addition to the Protective- layer proper (Text- fig. 2, P.S.). 
Text-fig. i. Castanea sativa. Longitudinal section 
of leaf-base at time of leaf-fall. 
