7o 
Lee . — The Morphology of Leaf -fall. 
well into the leaf-base, and, as will be seen from the diagrams, is distinct 
from the Protective-layer formed later (Text-figs. 12 and 13). 
About the end of August or early in September, the protoplasm 
becomes very abundant in the cells at the junction of cortex and petiole, 
and the starch granules increase greatly in number. The layer which 
shows this increased activity consists of 2-3 rows of cells, and, starting at 
a point on the inner surface of the petiole just above the junction of the 
latter with the axillary bud, passes in a slanting direction across the leaf- 
base, and curving downwards joins the stem-periderm at an appreciable 
distance below its upper limit. 
Very soon, division walls appear 
in the cells of this layer, situated 
near the dorsal surface, and the 
process spreads rapidly across, the 
cells for the most part dividing 
quite irregularly. The cells of the 
lower layer, however, divide more 
regularly and give rise to a cam- 
bium which produces 4-8 rows of 
cells towards the petiole. As 
soon as they are cut off, the new 
cells undergo ligno-suberization, 
losing in turn their starch granules 
and living contents, until finally 
nothing but clear, dead cells re- 
main. The cells of the Protective- 
layer possess the form and general 
characteristics of periderm cork, 
and although the middle lamellae 
and corners of the cell-walls ex- 
hibit lignification, the main mass 
consists of suberin. During its 
activity, the Protective-cambium has become continuous with the phellogen 
of the stem-periderm, but in spite of this and the general resemblance of 
the Protective-layer to the cork in the stem, there is really little connexion 
between the two, the place of junction invariably remaining well marked. 
The activity of the cells immediately above the Protective-layer has 
already been described as having resulted in the formation of more or less 
irregular walls. These cells still retain their starch and living contents, and 
division proceeds until walls to the number of 2-5 have appeared in each 
cell. The Separation-layer thus formed is early marked off, and, except 
near the dorsal surface, it is in direct contact with the upper limit of the 
Protective-layer. Occasionally, however, near the dorsal surface a small 
Text-fig. 13. Populus balsamifera. Longitudinal 
section of two-year-old leaf-scar. 
