10 INTRODUCTION 
that it is often nearly impossible to find the lumina or to 
distinguish one cell from another; the layer may be two 
cells in thickness, or it may be only half a cell in thickness, 
only one side of the cell having taken on a thick wall. The 
new phellogen. then forms a few layers of cork cells and 
a fresh phelloderm. 
This process is repeated year by year, so that a large 
number of cork layers are formed, all in the same radial 
rows with the rest, except where phellogen cells have 
divided tangentially, and 
without encroaching on the 
inner cortex, which remains 
intact. 
In special cases, how- 
ever, e. g. where the cortex 
has become injured, a fresh 
phellogen may be formed 
through the cortex or outer 
Fic. 5.—Diagram showing method 
of formation of periderm: ¢., cortex ; phloem, as In OF dinary scale 
¢,, first-year cork; c,, second-year bark trees. 
habe fy penis ag ;oeliees A longitudinal section 
shows at the top of each leaf 
cushion a leaf scar where the abscission layer has cut off 
the leaf. A poorly developed cork layer is formed under the 
abscission layer, and the cells beneath this become suberized. 
The cork layer is joined on to the primary cork layer of the 
stem on all sides. These leaf scars would appear to be weak 
points in the defensive armour of the stem. At the same 
time they may serve for the interchange of gases, since no 
Jenticels are present in the cork until the second year. 
Inner cortex. The parenchyma inside the cork layer 
remains alive, and the cells contain chloroplasts. The cells 
are at first nearly circular in all sections, but later become 
extended tangentially to keep pace with the increasing 
girth of the stem. These cells usually divide by radial 
walls, and each elongated cell is replaced by two or three 
circular ones. This continued vitality of the cortex is 
rendered necessary by the peculiar form of periderm 
