Stinging nettle (Urtica dioica) leaf, coloured scanning electron micrograph (SEM). This plant has many hollow stinging hairs called trichomes on its leaves and stems. They act like hypodermic needles, injecting histamine and other chemicals that produce a stinging sensation when contacted by humans and other animals. The stinging nettle has a long history of use as a medicine and as a food source. Magnification: x5 when printed at 10 centimetres wide.