A method of isolation of crystalline inclusions of plant cells is described. The crystals consist mainly of calcium oxalate in plants grown under normal conditions, but when calcium is replaced by magnesium, barium, or strontium in the culture solution these elements substitute for calcium in the crystals; even under normal conditions magnesium occurs in the crystals to the extent of about 2 per cent. The crystal morphology vanes in the species examined from raphides to complex conglomerates and X-ray diffraction demonstrates an association of raphides with calcium oxalate monohydrate whilst other solitary forms and conglomerates are associated with calcium oxalate 2.25H2O. On this basis the species examined can be divided mto three groups.