Cystoliths occur, then, in special cavities, in parenchyma cells (rubber-plant leaf, fig, pellionea, and mulberry), and in non-glandular hairs (cannabis indica). In powdered ruellia root the cystoliths occur in or are separated from the parenchyma cells. TESTS FOR CYSTOLITHS When dilute hydrochloric acid or acetic acid is added to cystoliths a brisk effervescence takes place with the evolution of carbon dioxide gas. Part III HISTOLOGY OF ROOTS, RHIZOMES, STEMS, BARKS, WOODS, FLOWERS, FRUITS, AND SEEDS In Part II the different types of cells and cell contents found in plants have been studied. In Part III it will be shown how these different cells are associated and the nature of the cell contents in the different parts of the plant. These parts are the root, the rhizome, the stem of herbs, bark and wood of woody stems, the leaf, the flower, the fruit, and the seed. CHAPTER I ROOTS AND RHIZOMES Some fifty-five roots, rhizomes, and rhizomes and roots are official in the pharmacopœia and national formulary. About 5 of these are obtained from monocotyledonous plants, and 50 from dicotyledonous plants. In studying the structure of roots and rhizomes, then, it must first be determined whether the root in question is monocotyledonous or dicotyledonous. This fact is ascertained by determining the type of the fibro-vascular bundle. The bundle is of the open collateral type in all rhizomes and roots obtained from monocotyledonous plants, but it is closed, radial, or concentric in the monocotyledonous type. In both of these groups the cellular plan of structure is similar, the chief variation being the absence of one or more types of cells, the variation in the amount, in arrangement, in the anatomical structure, in the color, and in the cell contents of the individual cells. These facts will be impressed on the mind while studying the rhizomes and the roots. CROSS-SECTION PINK ROOT The cross-section of pink root (Plate 89) has the following structure: =Epidermis.= The epidermal cells are small, nearly as long as broad, and the outer wall is thicker and darker in color than the side and inner walls. The cells usually contain air. =Cortex.= The cortical parenchyma cells are very large and somewhat rounded in outline, and the walls are white. There are about twelve rows of these cells, and each cell contains numerous small, rounded starch grains. =Endodermis.= The endodermal cells are tangentially elongated, and the walls are very thin and white. There are two or three layers of endodermal cells; the cells’ outer layers are larger than the cells of the inner layers. [Illustration: PLATE 89 CROSS-SECTION OF ROOT OF SPIGELIA MARYLANDICA, L.