The stone cells of cubeb (Plate 33, Fig. 4) are very small, mostly rounded in outline, with a great number of very fine simple pores which extend from the outer wall to the central cavity. The wall is yellow and very thick. The stone cells of echinacea root (Plate 33, Fig. 5) are very irregular in form; the walls are yellowish and porous, and the 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 colorless; 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 thickness 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 cinnamon (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 singly, 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 stone cells of witch-hazel bark and leaf, wild cherry, etc., also occur broken in the powder. [Illustration: PLATE 32 POROUS AND STRIATED STONE CELLS 1. Ruellia root (_Ruellia ciliosa_, Pursh.). 2. Winter’s-bark (_Drimys winteri_, Forst.). 3. Bitterroot (_Apocynum androsæmifolium_, L.). 4. Allspice (_Pimenta officinalis_, Lindl.). 5. Aconite (_Aconitum napellus_, L.).] [Illustration: PLATE 33