THE EPIDERMIS AND PERIDERM 83 



length and width. These side walls usually appear nearly white, 

 while the end wall, particularly of the outermost cork cells, 

 usually appears brown or reddish-brown, or in some cases nearly 

 black. 



Cork cells on cross-section are rectangular in form, and they 

 are arranged in superimposed rows, the number of rows being 

 gradually increased as the plant grows older. Such an increase 

 in the number of rows of cork cells is shown in the cross-section 

 of cascara sagrada (Plate 16, Fig. C). 



Cork cells fit together so closely that there is no intercellular 

 spaces between the cells. In this case two rows of cork cells 

 occupy no greater space than the solitary row of cork cells 

 immediately over .and external to them. As a rule, the outer- 

 most layers of cork cells have a narrower radial diameter than 

 the cork cells of the underlying layers. This is due to the fact 

 that these outer cells are stretched as the stem increases in 

 diameter. This view shows the height of cork cells, but not 

 always the length, which will, of course, vary according to the 

 part of the cell cut across. In a section a few millimeters in 

 diameter, however, all the variations in size may be observed. 

 The color of the walls is nearly white. 



The cavity may contain tannin or other substances. When 

 tannin is present, the cavity is of a brownish or brownish-red 

 color, or it may be nearly black. Most barks appear devoid of 

 any colored or colorless cell contents. 



The radial section (Plate 16, Fig. B) of cork cells shows the 

 height of the cells and the width of the cells at the point cut 

 across. Some cells will be cut across their longest diameter, while 

 others will be cut across their shortest diameter. Cork cells are, 

 therefore, smaller in radial section than they are in cross-section. 

 The color of the walls is white, and the color and nature of the 

 cell contents vary for the same reasons that they vary in 

 cross-sections. 



The number of layers of cork cells occurring in cross- and 

 radial-sections varies according to the age of the plant, to the type 

 of plant, and to the conditions under which the plant is growing. 



The number of layers of cork cells is not of diagnostic im- 

 portance, nor is the surface view of cork cells diagnostic except 

 in certain isolated cases. 



