1388 ELEMENTS OF PLANT ANATOMY. 
etally are named phelloderm; these generally contain chloro- 
phy. The phellem cells, which form what is known as the 
corky layer, are alike in form and origin; they differ only in the 
nature of their walls, some being entirely without suberin and 
therefore either soft and easily torn asunder, or if their contents 
are lost, brittle and easily broken. ‘The separation or removal 
of the sections of bark from the stem depends largely on this 
lack of cohesion in the phelloid cells. Something also depends 
on the nature of the tissues without. If these are tough and 
of such a nature as to shrink forcibly with loss of water, their 
contraction causes the tearing apart of the phelloid cells and 
results in the loosening and separation of the bark sections which 
they limit. The large scaly pieces of bark from Platanus stems, 
and the long thin strips from Clematis and Vitis are separated 
in this way, the former being called scale bark, the latter, ring 
bark. 
On the other hand, certain stems develop their periderm in 
such a way that the breaking of the weaker walls takes place so 
as to cause deep fissures or clefts in the bark without detaching 
any large pieces. Examples of this are found in several species 
of oak (Quercus). Still others form their periderm first only 
on certain places, angled stems along the corners, as in certain 
species of Euonymus, round stems in bands parallel with the 
long axis, as in some species of Quercus. In all these instances 
the phellogen produces only phellem and the masses so formed 
are named wings. One peculiarity of this formation may have 
a bearing on the question of the cause of difference between 
spring and fall wood. The number of years’ growth may be 
detected in these cork ridges quite as readily as in the woody 
portion, the regular summer’s growth being terminated always 
by several layers of cells with exceedingly short radial diameter. 
This wing-like growth is mostly confined to the younger stems 
and branches. Later on the entire surface of the stem is 
covered by the gradual extension of the phellogen layer and 
