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"Stinging nettle (Urtica doica) leaf. The stinging nettle is a herbaceous perennial flowering plant. It has hollow stinging hairs, called trichomes on its leaves and stems. They act like needles that inject chemicals into animal and human skin, creating a stinging sensation. The chemicals injected are acetylcholine, histamine, serotonin, moroidin, leukotrienes and sometimes formic acid. As the hairs grow upward along the stalk and outwards on the leaves, it is possible to touch them in the opposite direction without getting stung."