Animated scanning electron micrograph (SEM) of the leaf of a stinging nettle (Urtica dioica). 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. Despite this defence, the stinging nettle has a long history of use as a medicine and as a food source.