86 
Lee . — The Morphology of Leaf fall. 
suberization of the Protective-layer previous to leaf-fail. In P. Cerasus , 
as was stated above, lignification of the cells of the Protective-layer almost 
invariably occurs before the leaf is finally amputated, and in some cases 
there is a strong tendency towards suberization. But in P. virginiana and 
P. communis-dulcis this has not been observed. 
CLASS II (b). 
Platanus orientalis, Linn. 
In this well-known plant, the slender cylindrical petiole passes 
gradually into the swollen base, which externally displays 7-8 prominent 
ribs corresponding to the leaf-trace bundles beneath. Within the inflated 
leaf-base there exists a spacious cavity which, while opening to the exterior 
by a transverse slit on the ventral surface, effectively protects the large axillary 
bud. Internally, there is the usual distribution of starch granules and crystals 
(simple and compound) of calcium oxalate, as well as the usual reduction at 
the leaf-base of the sclerenchyma accompanying the leaf- trace bundles. 
The formation of tyloses in the vessels of the leaf-trace usually gives 
the first indication of approaching leaf-fall. These appear in small numbers 
in the vessels of the primary wood near the junction of petiole and stem, 
and are generally accompanied by a small amount of gummy lignin. 
As the tyloses increase in number and the gummy lignin in quantity, 
there is also a slight massing of the protoplasm in the petiolar cells near the 
leaf-base, which may at the same time acquire a small amount of starch in 
the form of granules. Slight division then follows, thin walls appearing 
singly in a varying number of these cells ; rarely do all the cells divide, and 
it is not uncommon to find that no division has occurred at the time of 
defoliation. Little or no growth takes place in the divided cells, which now 
begin to undergo ligno-suberization. The cells which first become lignified 
are usually situated near the epidermis of the basal part of the Protective- 
layer, and the process spreads rapidly in the cells of the outer part of the 
leaf-base and to some extent in the cells of the inner portion. In the same 
way the cell-walls acquire an inner film of suberin, but though the double 
process is very apparent in ordinary cases before leaf-fall, it is not until long 
after that the process is complete, the final stage being reached when the 
whole of the cells at this level become ligno-suberized and lose their living 
content 
In consequence of the conical shape of the leaf-base, and the added 
strength given by the regular deposition of the numerous leaf-trace bundles 
(each of which has its own special patch of sclerenchyma), one might expect 
to find the provision for separation rather exaggerated. As a matter of fact 
the Separation-layer in the numerous cases examined was found to be a very 
transitory affair, consisting of cells in which apparently no preparations had 
