75 
Lee. — The Morphology of Leaf-fall. 
In the mature leaf the junction of stem and petiole is marked internally 
by a layer of smaller cells, in which later the protoplasm increases in 
amount and starch granules appear. The layer of cells thus distinguished 
runs rather obliquely from near the upper limit of the stem periderm to the 
junction of the petiole with the axillary bud, and varies in thickness from 
6 to 12 cells. No divisions of any kind occur, but the activity of the cells 
is soon apparent in the changed character of the cell- walls, which now give 
slight reactions for lignin and suberin. The process of ligno-suberization, 
which is really very diffuse, probably commences near the upper limit of 
the Protective-layer, and from thence spreads in all directions ; the proto- 
plasm gradually decreases in amount, and the disappearance of the nucleus 
marks the complete conversion of the cell-wall. 
The amputation of the leaf is effected by the separation of the cells 
adjacent to the Protective-layer on the petiolar side. This layer (2-3 cells 
thick) consists of cells which are rich in protoplasm and starch, and which 
become divided by a single wall in each case. The new walls invariably 
remain thin and cellulosic, but the mother-cell walls begin to swell and take 
the haematoxylin and ruthenium-red with great avidity. The swelling 
continues and the middle lamellae become entirely mucilaginous and finally 
disappear, leaving the adjacent cells completely separated. 
Some time before leaf-fail a phellogen is formed by the regular division 
of the cells immediately beneath the Protective-layer, which gives rise before 
leaf-fall to a layer of 2-6 cells. As the cells are cut off they undergo various 
changes, but are chiefly distinguished by the fact that they become filled 
with a dense mixture of mucilage and tannin, just as is the case with the 
cells cut off by the stem phellogen. This stage may continue for a long 
time, but sooner or later the mucilaginous mass disappears, and the cells 
assume the regular appearance and composition of periderm cork. 
Little attention has so far been paid to the behaviour of the vascular 
elements near the affected area. Very early in the course of the changes 
just described, tyloses appear in the vessels of the primary xylem, and are 
soon followed by the introduction of a great quantity of gummy lignin. 
The tyloses are usually far from numerous and seldom appear in the later 
formed vessels of the leaf-trace. Gummy lignin, however, is invariably 
abundant, and is found in the majority of the vascular elements situated 
near the leaf-base. The changes which occur in the parenchyma of the 
Protective-layer are reproduced in the living elements of the vascular bundle, 
though the process is usually not completed until after the leaf has fallen. 
Vascular bundles and patches of sclerenchyma alike provide insuperable 
obstacles to the continuity of the Protective-periderm formed in the first 
year, but later formed periderm is interrupted only by the sclerenchyma 
outside the vascular bundles. 
