ANATOMY OF TISSUES. 135 
is cracked and worn off, leaving the outer periderm layers to 
take its place. In the second and third classes, that is also the 
fate of all the cells outside the corky layers. From this descrip- 
tion it is evident that the farther from the circumference the 
phellogen layer originates, the better its adaptation for continued 
secondary growth. Take the first case for example; here the 
phellogen layer is as near the surface as possible, and all the rind 
layers except the epidermis are left intact between the inner 
layer of periderm cells and outer layer of the new cambium 
growth. The cells of these rind layers must increase their 
circumference or their tangential diameters to keep pace with 
the increasing circumference of both zones between which they 
lie, that is, the zones of the secondary growth from the cambium 
and the phellogen rings. This is probably accomplished, in 
part, by growth in surface of their periclinal walls, and in part, 
by the formation of new radial walls increasing the number of 
cells. In the second and third classes, the number of layers to 
which this growth is necessary regularly decreases. 
Long continued growth from the cambium ring renders still 
other changes necessary, and this not only in the primary rind, 
but also in that part of the primary zone itself which lies nearest 
to the primary rind. This is supplied by the formation of the 
secondary or deep-seated periderm, which is called secondary 
because its formation is usually preceded by some form of super- 
ficial periderm. 
It is much more difficult to give even a general description 
of the development of the deep-seated periderm, as it varies not 
only in different plants but on different organs of the same 
plant, and even on different parts of these organs. The reasons 
for such variety will be seen from the fact that the deep-seated 
or internal phellogen layer always arises deep enough in the 
rind to pass through or between all the elements of the phloem 
tissues, whether these tissues belong to the original bundles or 
to the subsequent growth of the cambium ring. The phellogen 
