58 Lee . — The Morphology of Leaf-fall. 
three vascular bundles. At the same time the hypoderm of the petiole be- 
comes converted into collenchyma, providing an additional source of strength 
for that organ. Thus again the weakest part of the petiole is its base, when 
later separation occurs (Text-fig. 4). 
As in other examples which will be described later, the Separation- 
layer (PI. IV, Fig. 3, S.L.) is well defined long before leaf-fall, its position 
closely corresponding with the furrow shown externally. It is formed very 
early by the irregular division of a layer of cells in this region which are 
distinguished by their dense living and starchy contents, but after once 
becoming well marked no further change occurs for some time. 
As is well known the stem-periderm (Text-fig. 4, St. pd) in Ribes is 
very deep seated, appearing in the pericycle or endodermis, and this fact 
exercises a great influence on the 
form and appearance of the modi- 
fications connected with leaf-fall. 
Towards the end of the summer 
the periderm of the stem attains 
an appreciable thickness, but as 
the leaf-base is approached a 
gradual diminution occurs until 
finally the periderm ceases, usu- 
ally at a distance of 2-4 cells 
below the Separation-layer. The 
cortex thus cut off by the periderm 
becomes crushed, then dies away, 
and is finally exfoliated. Its be- 
haviour in many respects is 
markedly different from that of 
the layer, 2-5 cells thick, situated 
just beneath and adjoining the 
Separation-layer, which, though it 
undergoes exfoliation along with the general cortex, exhibits modifications 
which conclusively prove its primarily protective character. 
Just before leaf-fall, the cells of the latter layer begin to undergo lignifi- 
cation. This process, which usually commences in a row about five cells 
away from the Separation-layer, is very definite and gradually spreads 
across and through the whole layer, more or less completely delimiting it 
from the ordinary cortical tissue below (PI. IV, Fig. 3, L. PI). The extent to 
which lignifi cation occurs before leaf-fail varies greatly, all stages being 
found from unaltered cells to those completely lignified, and even in very 
favourable cases partly suberized also. These facts, taken in conjunction 
with others of a like nature for other examples, point strongly to the con- 
clusion that the degree to which the leaf-fall processes are allowed to occur 
Text-fig. 4. Ribes sanguineum. Longitudinal 
section of leaf-base just previous to leaf-fall. 
