96 Lee . — 7 /k? Morphology of Leaf-fall. 
highly lignified. Quite early in the formation of the Protective-layer 
tyloses appear in the vessels at that level, and in such numbers as to 
completely close the latter. 
The Separation-layer, which appears just before leaf-fail, is formed by 
the division of 2-3 rows of conspicuous cells situated just above the Pro- 
tective-layer. These cells possess the usual characteristics and proceed 
to divide by 1-3 walls. The outer cells increase considerably in length, 
their walls become quite mucilagi- 
nous, and finally the leaf is freed by the 
solution of the middle lamellae and 
the rupture of the vascular bundles. 
Some time previous to defoliation, 
the cells below the Protective-layer 
divide up and form a cambium, which 
produces 8-10 layers of cork in the 
first year. Almost immediately it 
traverses the vascular bundle, and 
the elements above it soon undergo 
their final change. In the second 
year a new periderm is produced 
below the first one, and this, after 
traversing the vascular bundle, curves 
gradually to join the stem periderm, 
enclosing between it and the first 
Protective-periderm a small area of 
unaltered cells. 
In the scar tissue the cells of the 
Separation-layer which remain when 
Text-fig. 18. Celastrus articulatus. . . c , 
Longitudinal section of leaf-base just previous the leat has been removed are the 
to separation. last to l ose their contents, and for 
a time they form a conspicuous layer 
of unaltered cells. No lignified zone is present above the Separation-layer. 
Celastrus articulatus, Thunb. 
In the stem the ring of bundles is supplied with an internal layer of 
sclerenchyma, none being present outside the vascular cylinder. A single 
vascular bundle leaves the ring and enters the petiole, and during its whole 
course sclerenchyma is entirely absent. In the cortex, the cells of which 
are generally smaller than those of the petiole, starch granules are invari- 
ably present, while crystals of calcium oxalate are only rarely found 
throughout the plant. 
The formation of the Protective-layer (Text-fig. 18, L.P.) takes place 
in the usual manner of this type. There is a slight increase in the proto- 
