Macrophotograph of stinging hairs on the stem of a stinging nettle, Urtica dioica. The stings are long hairs with silicified tips which break off when the nettle is brushed against or stroked (not grasped). The sting penetrates the skin, injecting a mixture of acetylcholine & histamine; the former is a neurotransmitter substance normally produced at nerve terminals, the latter a chemical mediator normally produced by mast cells & responsible for many of the unpleasant symptoms of allergic reactions. Only the long hairs on the leaf are stinging hairs; the short ones are harm- less. Magnification: X 1.5 (35mm size).