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 will 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. 
