Cystolith

Cystolith (Gr. "cavity" and "stone") is a botanical term for the inorganic concretions, usually of calcium carbonate, formed in a cellulose matrix in special cells called lithocysts, generally in the leaf of plants of certain families, e.g. Ficus elastica, the Indian rubber plant of the family Moraceae. Plants in the family Urticaceae, also known as stinging nettles, also form leaf cystoliths, but only during their later flowering and seed setting stages. Cannabis and other plants in the family Cannabaceae also produce leaf and flower cystoliths.

... The cystolith is a spindle-shaped body composed of concentric layers of longitudinally oriented cellulose microfibrils associated with pectins and other cell wall polysaccharides. At maturity it is heavily impregnated with calcium carbonate. Some cystoliths also contain silicon and are covered in a sheath of siliceous material. Cystolith formation occurs at the tip of a peg that grows in from the lithocyst wall. Evidence from ultrastructure suggests that the lithocyst cytoplasm transports carbohydrates to the cystolith via Golgi vesicles, and organizes the deposition of cystolith cellulose microfibrils via a system of microtubules lying beneath the plasma membrane that envelopes the growing cystolith. The peg is composed of heavily staining amorphous material like that of an apoplastically sealed cell wall. It is incapable of supporting the migration of lanthanum ions into the cystolith.

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