Lee. — The Morphology of Leaf-fall. 61 
also abundant. Periderm is not present in the stem until long after the leaf 
has fallen. 
As is commonly the case, the increased abundance of the protoplasm 
in the cells of the leaf-base provides the first indication of approaching leaf- 
fall. The small cells of this area become densely filled with brown proto- 
plasm, and starch granules also appear. Considerable activity is displayed, 
resulting finally in the production of a few new cell-walls, which are scattered 
(never more than one in each cell) throughout the leaf-base. Beyond the 
appearance of a few tyloses and a varying quantity of gummy lignin in the 
vessels of the leaf-trace at this level, no other change occurs in the Protective- 
layer previous to defoliation. 
Separation in this species is very simple. A layer of cells (1-3 cells 
thick) just above the Protective-layer becomes conspicuous by the great 
increase in the starchy contents. A few irregular cell-walls may appear, or 
these may be entirely absent. The primary cell-walls begin to swell and 
take the pectic stains more strongly ; the middle lamellae become obviously 
mucilaginous and finally disappear, and separation, commencing near the 
dorsal surface of the leaf-base, immediately follows. 
In Tilia europaea , a further modification is introduced which does not 
occur in the principal examples previously described. In favourable cases, 
when the lignin tests are applied to longitudinal sections of the leaf-base 
taken before the separation of the leaf from the stem, it is seen that the 
cells above, but adjoining the Separation-layer, are more or less completely 
lignified. This layer, to which Tison has appropriately applied the term 
‘ Lignified-layer ’, is usually one cell in thickness near the vascular bundles, 
but gradually increases to three cells at the periphery. The living contents 
become reduced in amount and may even disappear. The effect of such 
lignification is obvious ; it gives rigidity to the whole layer, making the ad- 
jacent and ever-weakening Separation-layer much weaker by comparison, 
and thus aiding in the final separation which soon follows (Text-fig. 6, Lig.). 
Immediately after the leaf has fallen, the cell-walls of the Protective- 
layer begin to undergo lignification followed by a slight amount of suberiza- 
tion. The whole process at this stage is very diffuse and not easily recog- 
nized until later, when the Protective-layer becomes delimited below by the 
production of a regular cork cambium. The crystal cells and secreting 
cavities which occur in the Protective-layer, but which, of course, have not 
undergone division like their neighbours, now become highly lignified, but do 
not display any suberization. 
During the first winter the unaltered cells just below the Protective- 
layer undergo division by walls approximately parallel to the surface of the 
scar, and form a cambium which produces a few layers of regular cells 
towards the Protective-layer. Almost immediately the cells thus cut off 
become suberized and lose their living contents (Text-fig. 7, P.pd.). A 
