MULTICELLULAR MULTISERIATE BRANCHED HAIRS 1. Apical hairs boneset (_Eupatorium perfoliatum_, L.). 2. Basal hairs boneset (_Eupatorium perfoliatum_, L.). 3. Apical hairs life-everlasting (_Gnaphalium obtusifolium_, L.). 4. Basal hairs life-everlasting (_Gnaphalium obtusifolium_, L.).] On surface view (Plate 16, Fig. A) the cork cells are angled in outline and are made up of from four to seven side walls; five- and six-sided cells are more common than the four-and seven-sided cells. Surface sections of cork cells show their 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 outermost 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 importance, nor is the surface view of cork cells diagnostic except in certain isolated cases. [Illustration: PLATE 16 PERIDERM OF CASCARA SAGRADA (_Rhamnus purshiana_, D.C.) _A._ 1, Outline of cork cells; 2, Line of contact of adjoining cork cells.