Crystals very often appear in plant cells, being widely distributed. Generally such crystals are comprised of hydrated calcium oxalate, which develops in order to protect the cells from the toxicity of the oxalic acid produced in plant metabolism, or act as a calcium reservoir. Calcium carbonate crystals also appear frequently in vegetal tissue, probably having a regulatory role regarding calcium concentration. Other substances, such as silica, calcium phosphate, organic substances, etc., could also appear as crystalline material in plants.
Eduardo L. Varetti and Carola R. Volponi, "Characterization of Crystals in Plant Cells Using FT-IR Microspectroscopy," Appl. Spectrosc. 49, 537-539 (1995)