55 
Lee . — The Morphology of Leaf-fall. 
The layer just described is distinguished by Tison as the ‘ parenchyme 
sacrifie ’, though it would be better perhaps to look upon it as the remains 
of the Separation-layer. Its premier characteristic — that of retaining for 
a considerable time its protoplasm and unaltered cell-walls — is possibly due 
to the production within the separating cells of a small amount of some 
substance, the effect of which is to overcome the tendency to become ligno- 
suberized ; while the second characteristic of collapsing is probably a result 
of the exposed position of the layer. The thickness of this layer varies 
greatly ; in some cases it consists simply of the remnants of the cells which 
have separated, while in other 
cases it is much thicker and 
consists of several layers of 
cells. At a later'stage there is 
no obvious distinction between 
this layer and the Protective- 
Jayer, the two having become 
completely ligno-suberized. 
In late autumn or winter 
when the ligno-suberization of 
the Protective-layer is nearing 
completion, and when the same 
change is rapidly occurring in 
the living elements of the vas- 
cular bundle at that level, 
another process commences 
which results in the formation 
of a periderm beneath the Pro- 
tective-layer. The living cells 
adjacent to the latter become 
active, and commence to divide 
by walls approximately parallel 
to the surface of the scar 
(PL IV, Fig. 2 , c ). In this way a regular cambium is produced, which, 
starting from the outer epidermis of the leaf-base at some distance above 
the upper extremity of the stem-periderm, runs across the leaf-base and 
traversing the vascular bundles joins the inner epidermis just above the 
junction of the latter with the axillary bud (Text-fig. 2, P.pd.). The new 
cambium becomes continuous across the vascular bundle by division of the 
living parenchymatous cells of the latter, and when tyloses are present they 
also divide in the same manner (PI. IV, Fig. 2, ty. 1 ). The generative layer 
thus produced differs in no particular from the cork cambium of the stem, 
and like the latter it cuts off the usual regular cells towards the exterior, and 
occasionally one or two layers of phelloderm cells towards the cortex. 
Text-fig. 2. Castanea saliva. Longitudinal 
section of scar in first winter. 
