Scanning electron micrograph of the leaf of a stinging nettle, Urtica dioica. The picture shows parts of three stinging hairs (yellow/brown). In contrast to normal leaf hairs ( background, green), the stinging hairs have an enlarged base (bottom, to left), and taper to a narrow end, which terminates in a small bulbous structure ( here red). The covering of the bulb is a glass-like wall containing silica. A defence mechanism, the bulb is easily broken open by a passing animal. The broken surface is sharp, and penetrates soft skin tissue, particularly the mouth parts of herbivores. The bulb injects a mixture of acetylcholine and histamine, chemicals that produce irritation and inflammation in mammals. The sting is effective at the slightest touch; grasping the nettle can sometimes evade the sting, because the hair breaks or bends without the tip piercing the skin. Mag x 190 at 10x8