98 
Lee . — The Morphology of Leaf-fall. 
Before the Protective-layer is completely formed, the cells of a layer 
above, which is separated from the Protective-layer by 1-3 cells, become 
active, and display abundant protoplasm and numerous starch granules. 
Numerous division walls appear in this Separation-layer, and the cells 
increase greatly in size; next their walls begin to swell and become mucila- 
ginous, and finally the complete solution of the middle lamellae paves the 
way for the full separation of the leaf. 
There is no Lignified-layer in this species, but a ‘ parenchyme sacrifie’, 
consisting of part of the Separation-layer in addition to the cells that 
intervene between the latter and the Protective-layer, is invariably present. 
The cells gradually lose their contents, and their walls slowly undergo ligno- 
suberization. 
It is not until late winter that a cork cambium is formed beneath the 
Protective-layer, and even then it does not become very active, only 1-2 
layers resulting previous to the second season. Later, however, a thick 
layer of cork cells is produced. 
Fraxinus excelsior, Linn. 
The stout, roughly cylindrical petiole is delimited from the stem by 
a well-marked furrow, and encloses within its axil a conspicuous spherical 
bud. The lignified stereome which accompanies the leaf-trace bundles 
becomes greatly reduced at the leaf-base, and appearances point to the 
conclusion that there is also a reduction in the vascular elements in that 
region. A conspicuous periderm is present in the stem previous to the 
commencement of the leaf-fail processes. 
Tyloses and gummy lignin appear in the vessels near the leaf-base at 
a very early stage, the latter being deposited in considerable quantity. In 
a layer eight or more cells in thickness situated near the leaf-base, the 
protoplasm increases in density, and new walls (1-2) appear in each cell. 
Ligno-suberization, which immediately follows, commences near the epi- 
dermis or the vascular bundles, and quickly spreads in all directions, 
a measure of its advance being gained by noting the diminishing density of 
the protoplasm, which finally disappears when the chemical change is 
complete. 
Divisions (1-4 in each cell) in the cells of the petiole above the 
Protective-layer result in the formation of the Separation-layer, which is 
early distinguished in the usual way, i. e. presence of abundant protoplasm 
and starch. The swelling of the primary cell- wall and the mucilaginization 
and disappearance of the middle lamellae are well shown in this example, 
which altogether furnishes a very easy and diagrammatic type. 
After leaf-fall the vascular elements, which have only partly become 
ligno-suberized, proceed rapidly to complete that process, which is no doubt 
