Calcium oxalate crystals form one of the most important inorganic cell contents found in plants, because of the permanency of the crystals, and because the forms common to a given species are invariable. By means of calcium oxalate crystals it is possible to distinguish between different species. In butternut root bark, for instance, only rosette crystals are found, while in black walnut root bark--a common substitute for butternut bark--both prisms and rosettes occur. This is only one of the many examples which could be cited. These crystals, for purposes of study, will be grouped into four principal classes, depending upon form and not upon crystal system. These classes are micro-crystals, raphides, rosettes, and solitary crystals. MICRO-CRYSTALS =Micro-crystals= are the smallest of all the crystals. Under the high power of the microscope they appear as a V, a Y, an X, and as a T. They are, therefore, three- or four-angled (Plate 78). The thicker portions of these crystals are the parts usually seen, but when a close observation of the crystals is made the thin portions of the crystal connecting the thicker parts may also be observed. Micro-crystals should be studied with the diaphragm of the microscope nearly closed and with the high-power objective in position. While observing the micro-crystals, raise and lower the objective by the fine adjustment in order to bring out the structure of the crystal more clearly. Micro-crystals occur in parenchyma cells of belladonna, scopola, stramonium, and bittersweet leaves; in belladonna, in horse-nettle root, in scopola rhizome, in bittersweet stems, and in yellow and red cinchona bark, etc. The crystals in each of the above parts of the plant are similar in form, the only observed variation being that of size. Their presence or absence should always be noted when studying powders. RAPHIDES =Raphides=, which are usually seen in longitudinal view, resemble double-pointed needles. They are circular in cross-section, and the largest diameter is at the centre, from which they taper gradually toward either end to a sharp point. [Illustration: PLATE 78 MICRO-CRYSTALS 1. Horse-nettle root (_Solanum carolinense_, L.). 2. Scopola rhizome (_Scopolia carniolica_, Jacq.). 3. Belladonna root (_Atropa belladonna_, L.). 4. Bittersweet stem (_Solanum dulcamara_, L.). 5. Scopola leaf (_Scopola carniolica_, Jacq.). 6. Tobacco leaf (_Nicotiana tabacum_, L.). 7. Belladonna leaf (_Atropa belladonna_, L.).] Raphides occur in bundles, as in false unicorn root (Plate 79, Figs. 6, A, B, and C), rarely as solitary crystals. In ipecac root the crystals are usually solitary. In sarsaparilla root, squill, etc., the raphides occur both in clusters, part of bundle, or in bundles, and as solitary crystals.