DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY. DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY.
Macrophotograph of stinging hairs on the stem & leaves 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. 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 harmless. Magnification: X 1 (35mm size).
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