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Plant hairs

Stinging Nettle (Urtica dioica)
Stinging trichome x84

Robert Hooke in 1665 was fascinated by the common Stinging Nettle and though unable to explain how the pain of a sting was "so suddenly created....and at length quite extinguished", described in detail, from personal microscopical observations, how the stinging mechanism worked.

When brushed against, the bulb at the tip of the hollow shaft snaps off producing a sharp, needle-like point that easily punctures skin. Downward pressure on the shaft causes the cell at its base to rupture forcing a cocktail of chemicals, including formic acid, along the shaft into the wound.

Secretory cells