118 . THE OAK 
to the far more numerous tangential walls. It is also 
easy to see that the cork-cells must be arranged in 
radial rows, and this arrangement is very conspicuous 
(fig. 18). The earlier cork-cells have very thin walls, 
later ones have the walls thicker. 
After the development of the first layer of cork the 
stretched epidermis dies, and forms simply a dead mem- 
brane outside the thin cork. In succeeding years 
layers of phellogen are formed annually beneath the 
older ones, and thus the cork layersincrease. Moreover, 
since the successive layers cut out thin, scale-like areas 
of cortex, trapping them, as it were, between the present 
and the preceding cork, the thickening corky covering 
is stratified—consists of successive and obliquely over- 
laying thin sheets of dead cortex and cork proper (fig. 
30). Again, since the increase in thickness of the stem 
or branch is continually driving these corky and dead 
structures outwards, they at length crack, and form the 
fissured bark found on older parts. Bark is thus seen 
to be something more than cork, or even periderm, and 
it is defined to be all the dead tissues cut out by the 
phellogen. 
It is also to be noticed that the successive phellogen 
layers of different years are not concentric, but the new 
ones cut the old ones at acute angles (fig. 30), thus cut- 
ting out scale-like areas of cortex ; the consequence of 
this is the formation of the very irregular scales of bark 
thrown off from the older stems and branches of the oak. 
It follows from what has been said that in older trees the 
