Lee . — ■ The Morphology of Leaf -fall. 
59 
before defoliation is largely if not solely dependent upon the external con- 
ditions which usually determine the moment of leaf-fall. When external 
conditions have not interfered a more or less ligno-suberized layer is present 
before the separation of the leaf from the stem. The process of lignification 
takes place as above described, and later a thin film of suberin is added to 
the inner face of each cell-wall. The only other changes which occur 
before leaf-fall in the elements at this level are the appearance of a few 
tyloses and the production of a great amount of gummy lignin in the 
vessels of the vascular bundles. 
Defoliation is accomplished by the splitting apart of the cells of the 
Separation-layer. The middle lamellae between the cells of the lower two 
rows begin to swell and are gradually converted 
into pectic mucilage (which is also impregnated 
with tannin) and later disappear, leaving the 
neighbouring cells quite free from each other. 
About the time of separation there is a great 
increase in size of the walls of the separating 
cells, and it seems probable that a differential 
growth of the separating walls occurs which, 
if true, will greatly facilitate the process of 
separation. 
In those cases where external conditions 
have resulted in defoliation before ligno-sube- 
rization has been completed, this is rapidly ac- 
complished in the newly-formed scar. By the 
end of the first year, therefore, the external 
portion of the scar including the vascular bundle 
is completely ligno-suberized, but no periderm 
has been produced which can be said properly 
to belong to the scar. During the second year, 
however, divisions occur below the Protective- 
layer which result in the production of a definite periderm. A fairly 
thick layer of cork is produced at the expense of the living cells of 
both vascular bundle and cortex in this region, and this, starting from 
the outer portion of the periderm of the axillary branch, traverses the 
vascular bundle in the form of a basin-shaped curve, which finally ends in 
the stem-periderm at some distance below its upper extremity (Text-fig. 5, 
P. pd.). Subsequently the vascular tissue above undergoes complete sube- 
rization in places where lignification had not previously occurred, the change 
being very striking even in unstained preparations. 
L.P 
St pd 
Text-fig. 5. Ribes sangui- 
neum. Longitudinal section of leaf 
scar at end of second year, x = 
crushed cortex. 
