Nettle stinging hair. Coloured Scanning Electron Micrograph (SEM) of the undersurface of a nettle leaf Urtica dioica, showing a long stinging hair. The stinging hair (at centre) is straight and sharp, joined to the leaf surface by a cellular base. The tapered region of the hair is silicified and it breaks off when the hair is brushed against or stroked. The sting easily penetrates human skin injecting acetylcholine (a neurotransmitter produced in nerves) and histamine (a chemical responsible for symptoms of allergic reaction). Only the long hairs on nettle leaves are capable of stinging; the shorter hairs (also seen here) are harmless. Magnification x24 at at 85x35mm. x70 at 8x10 inch size.