While studying the prisms, focus first on the upper surface and then down to the under surface in order to observe the forms accurately. There are several plants in which more than one form of crystal occur. Rosette crystals and prisms are associated, for instance, in cascara sagrada, frangula, condurango, dogwood, and pleurisy root (Plate 87, Figs. 1, 2, 3, 4, and 5). An important factor to be kept in mind in studying crystals is the number--whether abundant, as in rhubarb, or sparingly present, as in mandrake, etc. Variation in the number of crystals is not uncommon, even in different parts of the same plants. In wahoo stem bark, for instance, there are several times as many rosette crystals as there are in the root bark. Crystals of calcium oxalate are freely soluble in dilute hydrochloric acid without effervescence; but they are insoluble in acetic acid and in sodium and potassium hydroxide solutions. With sulphuric acid they form crystals of calcium sulphate. CYSTOLITHS =Cystoliths= consist of calcium carbonate deposited over and around a framework of cellulose. FORMS OF CYSTOLITHS The =forms of cystoliths= differ greatly in the different plants in which they occur. In the rubber-plant leaf, the cystolith resembles a bunch of grapes and is stalked; in ruellia root (Plate 87, Fig. 1) the cystoliths vary from nearly circular to narrowly cylindrical, and no stalk is present; also the cystolith nearly fills the cell in which it occurs. In the hair of cannabis indica (Plate 88, Fig. 3), the cystolith varies in form according to the size and shape of the hair, but in all the hairs the cystolith appears to be attached to the upper curved part of the inner wall of the hair. [Illustration: PLATE 85 SOLITARY CRYSTALS 1. India senna (_Cassia angustifolia_, Vahl.). 2. Cascara sagrada bark (_Rhamnus purshiana_, D. C.). 3. Frangula bark (_Rhamnus frangula_, L.). 4. White pine bark (_Pinus strobus_, L.). 5. Tamarac bark (_Larix laricina_ [Du Roi], Koch).] [Illustration: PLATE 86 SOLITARY CRYSTALS 1. Quassia (_Picræna excelsa_ [Swartz.], Lindl.). 2. Uva-ursi leaf (_Arctostaphylos uva-ursi_ [L.], Spring.). 3. Quebracho bark (_Aspidosperma quebracho-blanco_, Schlechtendal). 4. Wild-cherry bark (_Prunus serotina_, Ehrh.).] [Illustration: PLATE 87 ROSETTE CRYSTALS AND SOLITARY CRYSTALS OCCURRING IN 1. Cascara sagrada bark (_Rhamnus purshiana_, D.C.). 2. Frangula bark (_Rhamnus frangula_, L.). 3. Cundurango bark (_Marsdenia cundurango_, [Triana] Nichols). 4. Dogwood root bark (_Cornus florida_, L.). 5. Pleurisy root (_Asclepias tuberosa_, L.).] [Illustration: PLATE 88 CYSTOLITHS 1. Ruellia root (_Ruellia ciliosa_, Pursh.). 2. Pellionia leaf. 3. Cannabis indica (_Cannabis sativa_, variety _Indica_, Lam.).]