Cystolith

"cavity" and "stone") is a botanical term for outgrowths of the epidermal cell wall, usually of calcium carbonate but sometimes of silicon dioxide also, formed in a cellulose matrix in special cells called lithocysts, generally in the leaf of plants.

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

The cystolith is a spindle-shaped body composed of concentric layers of longitudinally oriented cellulose microfibrils associated with pectins and other cell wall polysaccharides.

Cystolith formation occurs at the tip of a peg that grows in from the lithocyst wall.

The peg is composed of heavily staining amorphous material like that of an apoplastically sealed cell wall.