Lee. — The Morphology of Leaf -fall. 65 
the scar, by the activity of a cambium, a layer of regular cork cells, this is 
a formation quite distinct in character and usually bears a definite relation 
to the periderm of the stem. Its function is to reinforce the already exist- 
ing Protective-layer, and, later, by becoming continuous with the stem- 
periderm. to assist in removing the now useless scar structures. 
Class III. Salix Caprea, Linn. 
The next class, of which Salix Caprea is the first example, is charac- 
terized by the possession of a Protective-layer which is entirely secondary 
in origin. It is produced by the activity of a single layer of cells which 
become merismatic, but which for a long time have no possible connexion 
with the stem-periderm. The 
whole appearance is quite different 
from what obtains in other ex- 
amples even in later stages. But 
it is upon the morphological dis- 
tinction that the class is founded. 
Taking the Willows as a 
whole, the external morphology 
is very similar. The petiole is 
slender and cylindrical, giving 
place above to the narrow lamina, 
and below spreading out where 
it joins the stem. The sheathing 
leaf-base, which is provided with 
a pair of stipular wings, encloses 
the large dorsally-compressed 
axillary bud, and a distinct furrow 
marks its junction with the stem. 
Internally, there is no obvious 
difference in size between the cells 
of the cortex and those of the petiole. In the former starch granules are 
abundant, but are absent from the latter with the exception of the bundle 
sheath. Cortex and petiole alike display fine intercellular space systems, 
and in both tissues compound crystals of oxalate of lime are abundant. 
No periderm is present in the stem previous to leaf-fall, but a very con- 
spicuous cuticle protects the surface of both stem and petiole. 
On leaving the vascular ring to supply the next leaf, each of the three 
vascular bundles is accompanied by a patch of sclerenchyma, which, how- 
ever, disappears before the leaf-base is reached (Text-fig. 10, Sell). Scleren- 
chyma is absent from the petiole, but the thick hypoderm is collenchymatous, 
and no doubt serves to strengthen the petiole. 
The absence of periderm in the stem at the time of leaf-fail rids us of 
F 
I 
Text-fig. io. Salix Caprea. Longitudinal 
section of leaf-base at the time of leaf-fail. 
