False-colour scanning electron micrograph (SEM) of the tip (foreground) & base (background) of two stinging hairs on a leaf of the common stinging nettle, Urtica dioica. The long stinging hairs have silicified tips which break off when the nettle is brushed against or stroked, but not when it is grasped. The sting penetrates the skin, injecting a mixture of acetylcholine & histamine. The former is a neurotransmitter substance normally produced at nerve terminals, the latter a chemical mediator responsible for many of the unpleasant symptoms of allergic reactions. Magnification: x45 at 35mm size, x320 at 8x10- inch size. Reference: MICROCOSMOS, figure 4.21, page 77.