8o 
Lee . — The Morphology of Leaf-fall. 
About this time a few tyloses are developed in the vessels near the leaf- 
base, and a quantity of gummy lignin also appears. A little later the cells 
just above the periderm begin to undergo lignification, a process which at 
first is most vigorous in the cells adjacent to the vascular bundle. Suberi- 
zation first commences near the epidermis, and, though spreading in all 
directions, it is invariably most conspicuous in the region where it first 
appears. The Protective-layer (L. P. and u. L . P.) thus formed is thin 
and indefinite above, and consists of cells which have not previously 
undergone division. The walls of the included crystal cells become 
lignified only. 
The Separation-layer (S. LI) has little or no relation to the Protective- 
layer. It is situated at some distance above the latter, and is usually com- 
posed of 2-3 rows of cells which rarely undergo slight division, and which 
possess the usual distinction of abundant protoplasmic and starchy contents. 
Their walls swell and the middle lamellae become mucilaginous and finally 
disappear, leaving the neighbouring cells quite free. The position of the 
Separation-layer is noteworthy. It is situated just below the place where 
the leaf-trace thickens and forks into two, and is so high up as to leave 
behind enough of the leaf-base to afford efficient protection to all but the 
largest of the axillary buds. 
Lignification occurs in the cells of the petiole immediately above the 
Separation-layer, and produces a well-marked Lignified-layer (. Lig .). After 
leaf-fall the cells beneath the exposed surface, which are still cellulosic, now 
undergo varying degrees of ligno-suberization, the latter process spreading 
upwards from the already ligno-suberized portion of the Protective-layer 
below. In the upper part, however, its further progress is delayed by the 
desiccation of the cells, which lose their contents and allow their walls to 
collapse and form a flattened layer on the surface of the scar. Little or no 
further change occurs in the scar tissue until the beginning of the second 
season, when a new phellogen arises in the tissue immediately below the 
old one. Near the epidermis the two phellogens are almost in contact with 
each other, but towards the centre they diverge considerably, the primary 
one rising towards the vascular bundle, the secondary cork cambium dipping 
down a little towards the leaf-trace, which it completely traverses. A thick 
layer of regular cork cells is produced before the end of the second season, 
at which time all the tissue above has undergone complete ligno-suberization. 
The scar is triangular in shape, and shows on its surface the three 
cicatrized nodules representing the three vascular bundles. The uppermost 
and largest axillary bud alone is visible. 
