88 HISTOLOGY OF MEDICINAL PLANTS 
partially or completely fill the break in the periderm. In white 
oak bark (Plate 18), as in other barks, a large. part of the peri- 
derm is composed of dead and discolored cortical cells. 
ORIGIN OF CORK CELLS 
The cork cells are formed by the meristimatic phellogen 
cells, which originate from cortical parenchyma. These cells 
divide into two cells, the outer changing into a cork cell, while 
the inner cell remains meristimatic. In other instances the 
outer cell remains meristimatic, while the inner cell changes 
into a cortical parenchyma cell. The development of a cortical 
parenchyma cell from a divided phellogen cell is shown in Plate 
ror, Fig.6. Both the primary and secondary cork cells originate 
from the phellogen or cork cambrium layer. Cork cells do not 
contain living-cell contents; in fact, in the majority of medicinal 
barks the cork cells contain only air. 
The walls of typical cork cells are composed, at least in part, 
of suberin, a substance which is impervious to water and gases. 
In certain cases layers of cellulose, lignin, and suberin have been 
identified. Suberin, however, is present in all cork cells, and 
in some cases all of the walls of cork cells are composed of suberin. 
Suberized cork cells are colored yellow with strong sodium 
hydroxide solutions and by chlorzinciodide. 
