112 HISTOLOGY OF MEDICINAL PLANTS 
central cavity is very large. A black intercellular substance 
is usually adhering to portions of the outer wall. 
The color of the walls of the different stone cells is very 
variable. In Ceylon cinnamon and ruellia the walls are color- 
less; in zanthoxylium, light yellow; in rumex, deep yellow; 
in cascara sagrada, greenish yellow. 
The pores of stone cells, like the pores of bast fibres, are 
either simple or branched, and they may or may not extend 
through the entire wall. Many of the shorter pores extend for 
only a short distance from the cell cavity. 
The width of the cell cavity varies considerably in the stone 
cells of the different plants. In aconite (Plate 32, Fig. 5), in 
calumba (Plate 33, Fig. 2), and in Ceylon cinnamon (Plate 33, 
Fig. 1), the cell cavity is several times greater than the thick- 
ness of the cell wall. 
In allspice (Plate 32, Fig. 4), in bitter root (Plate 32, Fig. 
3), the diameter of the cell cavity and the thickness of the wall 
are about equal. In cubeb (Plate 33, Fig. 4), in ruellia (Plate 
32, Fig. 1), the wall is thicker than the diameter of the cell 
cavity. 
The cavity of many stone cells contains no characteristic 
cell contents. In other stone cells the cell contents are as 
characteristic as the stone cell. The stone cells of both Saigon 
and Ceylon cinnamon (Plate 33, Fig. 1) contain starch; the 
stone cells of calumba (Plate 33, Fig. 2) contain prisms of calcium 
oxalate; the stone cells of allspice and sweet-birch bark contain 
tannin. 
In cross-sections, stone cells occur singly, as in Saigon cinna- 
mon (Plate 34, Fig. 1), ruellia (Plate 34, Fig. 2); in groups, as 
in cascara sagrada (Plate 34, Fig. 3); and in continuous bands, 
as in Saigon cinnamon (Plate 34, Fig. 4). 
In powdered drugs, stone cells, like bast fibres, occur as in 
ruellia, calumba, etc.; or in groups, as in cascara sagrada, witch- 
hazel bark, etc. In most powders they occur both singly and 
in groups. 
The individual stone cells are mostly entire, as in ruellia, 
calumba, allspice, echinacea, etc. In cascara sagrada many of 
the stone cells are broken when the closely cemented groups 
are torn apart in the milling process. Many of the branched 
