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 layers increase. 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- 

 lying 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 



