The walls of hairs are composed of cutin, of lignin, and of cellulose.
The =periderm= is the outer protective covering of the stems and roots of mature shrubs and trees. The periderm replaces the epidermis. The periderm may be composed of cork cells, stone cell-cork, or a mixture of cork, parenchyma, fibres, stone cells, etc.
The typical periderm is made up of =cork cells=. Cork cells vary in appearance, according to the part of the cell viewed.
MULTICELLULAR MULTISERIATE BRANCHED HAIRS
1. Apical hairs arnica (_Arnica montana_, L.). 2. Basal hairs arnica (_Arnica montana_, L.). 3. Apical hairs grindelia (_Grindelia squarrosa_, [Pursh] Dunal). 4. Basal hairs grindelia (_Grindelia squarrosa_, [Pursh] Dunal).]
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.