56 Lee . — The Morphology of Leaf-fall. 
While the latter retain their protoplasm and undergo no change, the 
former become suberized and slightly lignified, and finally entirely lose 
their contents. Only 3-4 layers of cork cells are produced in the first year. 
In the second year, another cambium is produced beneath the leaf-scar 
and is situated at some distance below the first, and unlike the latter it 
immediately becomes continuous with the periderm of the stem (Text- 
fig. 3, P.pd}). It is approximately parallel with that produced in the first 
year, but on nearing the vascular bundle it dips down and pursues a curved 
course within the latter. Four to eight layers are produced before the end 
of the second year, and 
at this time all the cells 
above the periderm have 
become completely ligno- 
suberized. 
Before going on to 
describe other examples 
the behaviour of the more 
peculiar elements present 
within the Protective-layer 
may be noticed. It is ob- 
vious that the lignified 
elements of the vascular 
bundle, such as vessels, 
strengthening tissues, &c., 
from which the proto- 
plasm has finally disap- 
peared long before the 
complete maturation of 
the leaf, cannot participate 
in the further chemical 
changes which occur in 
the formation of the Pro- 
tective-layer. The sieve tubes, companion cells, and other elements which 
have retained their living protoplasm undergo the process of ligno-suberiza- 
tion just as do the ordinary parenchymatous cells outside the vascular 
bundle, and that process is usually not completed until after the formation 
of the Protective-periderm. Finally there is the behaviour of the crystal 
cells. When these are included in the Protective-layer the course of events 
varies with the presence or absence of protoplasm. Sometimes an appre- 
ciable amount of protoplasm is present, but more often it is impossible 
to demonstrate the slightest trace of that substance, and in these cases 
it appears that the last act of the expiring protoplasm was the secretion 
of the crystal which now fills its place. It follows then that if protoplasm 
