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.
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Plants in the family Urticaceae, also known as Stinging Nettles, also form leaf cystoliths, but only during their later flowering and seed setting stages.