Scanning electron micrograph of part of the stem from a stinging nettle (Urtica dioica). The large stinging hairs are hollow tubes which act like tiny glass needles. The bulb at the base of each hair contains the stinging liquid (including formic acid, histamine and acetylcholine). The tips of the glassy hairs are easily broken when brushed, which easily pierces the skin to deliver the sting. The plant and roots have been used historically in alternative medicine to treat a variety of conditions, possibly by decreasing inflammation and increasing urine output. Width of image is 2 mm. Supplied by Kevin Mackenzie, courtesy of Wellcome Images.