Stinging nettle leaf. Coloured scanning electron micrograph (SEM) of a stinging nettle leaf (Urtica dioica). Its long poisonous hairs protect the plant from herbivores. When touched, they cause pain and inflammation to the skin by the release of a mixture of acetylcholine (a neurotransmitter that activates nerve cells) and histamine (a substance produced in allergic reactions). The shorter hairs are harmless. The nettle plant is common on grassland and wasteland throughout Europe. Magnification: x30 at 6x7cm size.