THE TREE—ITS SHOOT-SYSTEM 117 
layer may be called the initial layer. This layer behaves 
essentially like the cambium of a fibro-vascular bundle, 
except that its daughter-cells become cork and phello- 
derm instead of phloém and xylem. 
The first event to notice is that each of the initial 
cells grows radially, and divides by a tangential wall 
into an inner cell nearest the axis of the branch and an 
outer cell nearer the epidermis; the outer cell becomes 
forthwith a cork-cell—z.c. its contents die and mostly 
disappear, and the cellulose cell-wall becomes suberised 
—the inner cell remains capable of repeating the pro- 
cess. But this is not the only case. After the division, 
as before, of the initial cell, it may happen that the inner 
cell becomes transformed into a collenchymatous cortical 
cell containing chlorophyll, and it is the outer of the 
daughter-cells which retains the meristem character 
and acts again as a phellogen cell, cutting off daughter- 
cells sometimes on one side and at others on the other. 
Thus, in the oak, the phellogen gives rise to permanent 
tissue on both sides of the initial layer: those cells which 
lie on the inside become phelloderm (cortical cells), those 
on the outside become transformed into phellem (cork). 
The three tissues, phelloderm, phellogen, and phellem, 
are called the periderm. 
It is obvious that the cork-cambium, by thus adding 
to the cortical parenchyma, is gradually driven radially 
outwards from the centre of the stem. This means that 
it obtains room to extend tangentially, and it does this 
by its cells occasionally dividing by walls perpendicular 
