found in comparative abundance, and they consist, in the great majority of cases, of calcium oxalate. In the stem and root of the rhubarb, for instance, in the leaf-stalk of Begonia, and in countless other cases, sometimes within the cell, sometimes in the substance of the cell-wall, we find large and well-formed crystals of this salt; their varieties of form, which are extremely numerous, are simply the crystalline forms proper to the salt itself, and belong to the two systems, cubic and monoclinic, in one or other of which, according to the amount of water of cryôÙstalôÙliôÙsaôÙtion, this salt is known to crystallise. When calcium oxalate crystallises according to the latter system (as it does when its molecule is combined with two molecules of water of cryôÙstalôÙliôÙsaôÙtion), the microscopic crystals have the form of fine needles, or ãraphides,ã such as are very common in plants; and it has been found that these are artificially produced when the salt is crystallised out in presence of glucose or of dextrin2.
Calcium carbonate, on the other hand, when it occurs in plant-cells (as it does abundantly, for instance in the ãcystolithsã of the Urticaceae and Acanthaceae, and in great quantities in Melobesia {413} and the other calcareous or ãstonyã algae), appears in