92 
Lee , — The Morphology of Leaf-fall. 
layer (PI. V and VI, Figs. 15,16, 17). These walls are invariably very definite ; 
they give the usual cellulose reactions, and are often curved so that the 
convex side is towards the Protective-layer. This arrangement, although 
not invariable, is noteworthy, for it often appears to be connected with the 
presence of abundant latex on the concave side of the walls and with the 
degeneration (and therefore lower pressure (?)) of the substance enclosed 
by the walls. 
With regard to the formation of these walls nothing very definite has 
been observed. As Tison has said, they probably arise very rapidly, and 
the first stages are therefore difficult to obtain. The same author holds 
that the coagulation of the latex near the Protective-layer often provides 
an obstruction, and may possibly aid in the formation of the transverse 
wall. In the present case a coagulated mass of latex, such as Tison 
describes as being present on the convex side of the transverse wall, has 
never been found, though masses of various size have been observed dis- 
persed throughout the tubes; and while it is highly probable that there is 
some relation between the formation of the transverse wall and the nuclei 
within the tube in that region, it has as yet been impossible to establish 
any such connexion. 
The condition of the Protective-layer at the time of leaf-fall has 
already been described. The amputation of the leaf is followed by the 
extension into the leaf-trace of the process of ligno-suberization and by 
the formation beneath the Protective-layer of a cambium, which gives rise 
to a few regular rows of cork cells during the first part of the winter. 
Without traversing the vascular bundle, this cambium soon becomes con- 
tinuous with the phellogen of the stem ; it is not very vigorous during the 
first year after leaf-fall, rarely more than 4-6 rows of cells being produced. 
In the second year a new cork cambium arises just beneath the old one, 
and in turn gives rise to a layer of cork which completely traverses the 
vascular bundle. 
JUGLANS NIGRA, Linn. 
The course of events is very much as in Morns and other examples of 
this type. The usual reduction occurs in the vascular and strengthening 
tissue at the leaf-base. A superficial periderm is present in the stem and 
often extends a little way up the petiole. Aggregation of the protoplasm in 
the cells of the leaf-base is followed by irregular division, the new walls 
appearing, 1-3 in each cell. A fairly thick layer is thus produced which 
has a slightly curved course. Ligno-suberization takes place as usual. 
The Separation-layer is formed by the more regular division of a layer 
of 2-3 rows of cells adjacent to the Protective-layer. Separation follows as 
in other examples. A slight Lignified-layer is also present. 
Protective-periderm does not arise until about the middle of the first 
