False-colour scanning electron micrograph (SEM) of the stinging hairs along the main vein on the undersurface of a leaf of the stinging nettle, Urtica dioica. The stings are long hairs (short ones are harmless) with silicified tips (intact here) which break off when the nettle is brushed against or stroked. The sting penetrates the skin, injecting a mixture of acetylcholine & histamine. The former is a neurotransmitter substance normally produced at nerve terminals; the latter is a chemical mediator responsible for many of the unpleasant symptoms of allergic reactions. The result is a burning sensation, itching & redness. Magnification: x25 at 6x4.5cm size.