82 
Lee . — The Morphology of Leaf -fall . 
Class II (a). 
CORYLUS COLURNA, Linn. 
Externally the stem and petiole display no special characteristics. 
Of the internal arrangements it may be noted that the cells of the cortex 
are smaller than those of the petiole, and are separated by a layer of still 
smaller cells ; starch granules are abundant in cortex and in the transition 
layer, but are absent, with the exception of the bundle sheath, from the 
petiole; cluster crystals of calcium oxalate are numerous in both regions, 
and a superficial periderm is present in the stem before leaf-fall, while at the 
leaf-base there is an almost complete reduction in the sclerenchyma accom- 
panying the three vascular bundles which supply the leaf. 
The irregular division of the cells at the junction of petiole and cortex 
by walls which appear singly in each cell and which run in all directions, is 
preceded by a marked increase in the living contents of these cells, so that 
although no ligno-suberization occurs previous to leaf-fall, the Protective- 
layer is easily recognized. 
The Separation-layer, which now appears, is formed by the division of 
1-3 layers of cells situated above the Protective-layer, and is at once dis- 
tinguished by abundant living and starchy contents, and by its reactions 
with stains. Separation occurs in the usual way by mucilaginization and 
subsequent disappearance of the middle lamella. Throughout the whole 
process a few tyloses are present, and later gummy lignin is very abundant 
in the vessels at the leaf-base. A slight Lignified-layer is present above the 
plane of separation. 
Ligno-suberization of the Protective-layer occurs after leaf-fall. The 
double process takes place in the upper portion, the cells below remaining 
for a time almost wholly lignified. The chemical alteration of the included 
crystals and vascular elements takes place in the usual way. 
Scar periderm arises about the end of the second season,. It traverses 
the vascular bundles (in which it has a curved course) and becomes con- 
tinuous with the stem periderm on both sides. In later seasons new peri- 
derms arise beneath the first one. 
Cercis Siliquastrum, Linn. 
The petiole is slender, and a slight groove at its junction with the stem 
indicates the position of the Separation-layer. Three bundles supply each 
leaf, and the usual reduction in the accompanying sclerenchyma occurs at 
the leaf-base. The first indication of approaching leaf-fall is the regular 
division of a layer, 2-4 cells in thickness, which runs in an oblique direction 
just above the leaf-base. New walls appear in these cells quite suddenly, 
the only previous indication being the slight massing of the protoplasm in 
this layer. The plane along which the divisions occur is quite definite and 
