_B._ Radial longitudinal section of cascara sagrada. 1, Cork cells; 2, Phellogen; 3, Forming parenchyma cells; 4, Cortical parenchyma cells. _C._ Cross-section of cascara sagrada. 1, Cork cells; 2, Phellogen; 3, Forming parenchyma cells; 4, Cortical parenchyma cells.] The presence or absence of cork or epidermal tissue in powders must always be noted. The presence of cork enables one to distinguish Spanish from Russian licorice. In like manner, the presence of epidermis enables one to distinguish the pharmacopœial from the unofficial peeled calamus. The absence of epidermis in Jamaica ginger is one of the means by which this variety is distinguished from the other varieties of ginger, etc. In canella alba the periderm is replaced by stone cell-cork. That is, the cells forming the periderm are of a typical cork shape, but the walls are lignified, unequally thickened, and the inner or thicker walls are strongly porous, and the walls are of a yellowish color. Stone cell-cork forms the periderm of clove bark also, but the cells are narrower and longer, and the inner wall is not so thick or porous as is the case in canella alba bark. STONE CELL PERIDERM In canella alba (Plate 17, Fig. B) cork periderm is frequently replaced by stone cells, particularly in the older barks. These stone cells form the periderm because they replace the cork periderm, which fissures and scales off as the root increases in diameter. The side and end walls of cork cells are of nearly uniform diameter. Exceptions occur, but they are not common. In buchu stem (Plate 101, Fig. 3), the cork cells have thick outer walls, but thin sides and inner walls. The cell cavity contains reddish-brown deposits of tannin. PARENCHYMA AND STONE CELL PERIDERM As the trees and shrubs increase in diameter, cracks or fissures occur in the periderm, or corky layer. In such cases the phellogen cells divide and redivide in such manner as to cut off a portion of the parenchyma cells, stone cells, and fibres of the cortex which is inside of and below the fissure. All the parenchyma cells, etc., exterior to the newly formed cork cells soon lose their living-cell contents, since their food-supply is cut off by the impervious walls of the cork cells. In time they are forced outward by the developing cork cells until they partially or completely fill the break in the periderm. In white oak bark (Plate 18), as in other barks, a large part of the periderm is composed of dead and discolored cortical cells. [Illustration: PLATE 17 _A._ Cross-section of Mandrake Rhizome (_Podophyllum peltatum_, L.). 1. Epidermis. 2. Phellogen. 3. Cortical parenchyma. _B._ Stone cell periderm of white cinnamon (_Canella alba_, Murr.).] [Illustration: PLATE 18 PERIDERM OF WHITE OAK (_Quercus alba_, L.)