6 4 
Lee . — The Morphology of Leaf -fall. 
lignin is produced in great quantity, and fills up the interspaces between 
the tyloses as well as the vessels from which tyloses are absent. 
With the first change in the chemical composition of the Protective- 
layer, the adjacent cells above begin to organize into a Separation-layer. 
This layer (PI. IV, Fig. 5, S.L.), which is 2-6 cells thick, is at once 
distinguished by the abundance of its living and starchy contents, and by 
the fact that the chemical changes going on in the walls induce the latter to 
give a reaction different from that of the surrounding cells. A few irregular 
division walls may appear, but these are never very extensive. The primary 
walls begin to swell ; the middle lamellae between the outer cells become 
mucilaginous, and later disappear, and the neighbouring cells separate. 
Above the Separation-layer the adjacent cells become completely 
lignified, and finally lose most of their living contents before the leaf 
separates. The Lignified-layer thus formed is generally 2-3 cells thick, 
and no doubt aids in the separation of the leaf (Text-fig. 9, and PI. IV, 
Fig- 5, Lig .). 
The cells of the Separation-layer which remain on the scar retain for 
a time their living protoplasm, as well as the cellulose nature of their walls. 
Later, however, the protoplasm disappears, and the cell-walls collapse and 
form a thick membrane over the surface of the Protective-layer. 
Although most of the changes just described take place quite early 
the leaves of the Birch are among the last to fall. Probably this fact is 
associated with the comparative lightness of the leaf, and the slight resis- 
tance it offers to the wind ; and this may also explain the degree of com- 
pleteness attained by the leaf-fall processes in this species before the leaf is 
finally cast off. Before the latter event occurs, a layer of cells beneath the 
Protective-layer divides up by regular walls running at right angles to the 
direction of the petiole (PI. IV, Fig. 5, cal). A regular cambium is thus 
produced, which by division gives rise to a layer of cells towards the 
Protective-layer. As these are cut off suberization immediately takes place, 
so that a slight layer of cork is produced before leaf-fall. Subsequently 
the activity of the cambium increases, and before the end of the first season 
4-6 rows of cork cells have been produced, which, however, are interrupted 
by the vascular bundles and accompanying stereome (Text-fig. 9 ,P.pd). 
In the second year a new cork cambium is produced below the first, 
and is at once continuous with the periderm of the stem. It curves down- 
wards and traverses the vascular bundles, and by the end of the second 
year it gives rise to a thick layer of cork. 
The examples which have already been described have only one 
constant difference, i.e. the presence or absence of irregular division walls 
in the Protective-layer. With this exception the final result is substantially 
the same. In all these cases the Protective-layer is formed by the meta- 
morphosis of existing cells, and although there is invariably formed beneath 
