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 

 101, 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. 



