THE EPIDERMIS AND PERIDERM 85 
The presence or absence of cork or epidermal tissue in pow- 
ders 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 pharma- 
copoeial 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 ror, 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 fis- 
sures 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 
