=Solitary crystals= are the most variable of all the forms of calcium oxalate. They usually occur in crystal cells associated with bast fibres and stone cells, less frequently in stone cells (Plate 33, Fig. 2). There are many different and characteristic forms of prisms. The more common are: 1. Rectangular: A. Parallelepipeds. B. Cubes. 2. Polyhedrons: A. Irregular polyhedrons. I. Flat bases. (_a_) Non-notched. (_b_) Notched. II. Tapering bases. B. Octohedrons. The crystals occurring in Batavia cinnamon and henbane leaves are parallelopipeds (Plate 82, Figs. 1 and 2). The crystals occurring in cactus grandiflorus, hemlock bark, krameria root, and soap bark are irregular polyhedrons (Plate 83). They are longer than broad, and the ends are tapering. The crystal of cactus grandiflorus has the narrowest diameter of these four, while the crystals of soap bark have the widest diameter. In coca leaf, xanthoxylum bark, elm bark, Spanish licorice, and in white oak (Plate 84), and in cocillina bark (Plate 82, Fig. 4) the crystals are all irregular polyhedrons with flat bases. They are mostly longer than broad and they are all widest in the centre; in each a few crystals are notched, but most of the crystals are not notched. The crystals in quassia wood, uva-ursi leaf, and most of those of quebracho and wild cherry bark (Plate 86, Figs. 1, 2, 3, and 4) are irregular polyhedrons with flat ends. They are longer than broad, widest at the centre, and non-notched. [Illustration: PLATE 81 INCLOSED ROSETTE CRYSTALS 1. Hops (_Humulus lupulus_, L.). 2. Bracts of cannabis indica (_Cannabis sativa_, variety _Indica_, Lamarck). 3. Medullary rays of canella alba. 4. Parenchyma cells of mandrake (_Podophyllum peltatum_, L.).] [Illustration: PLATE 82 SOLITARY CRYSTAL 1. Batavia cinnamon (_Cinnamomum burmanni_, Nees). 2. Henbane leaves (_Hyoscyamus niger_, L.). 3. Morea nutgalls. 4. Cocillana bark (_Guarea rusbyi_ [Britton], Rusby).] [Illustration: PLATE 83 SOLITARY CRYSTALS 1. Cactus grandiflorus (_Cereus grandiflorus_ [L.], Britton and Rose). 2. Hemlock bark (_Tsuga canadensis_ [L.], Carr.). 3. Krameria root (_Krameria triandra_, Ruiz and Pav.). 4. Soapbark (_Quillaja saponaria_, Molina).] [Illustration: PLATE 84 SOLITARY CRYSTALS 1. Coca leaf (_Erythroxylon coca_, Lamarck). 2. Xanthoxylum bark (_Zanthoxylum americanum_, Miller). 3. Elm bark (_Ulmus fulva_, Michaux). 4. Spanish licorice root (_Glycyrrhiza glabra_, L.). 5. White oak bark (_Quercus alba_, L.).] Cubes occur in senna, cascara sagrada, frangula, white pine, tamarac (Plate 85), quassia, uva-ursi, quebracho, and in wild cherry (Plate 86). The crystals of morea nutgalls (Plate 82, Fig. 3) are octahedrons, and they resemble the crystals of calcium oxalate found in urinary sediments.