Learning about stinging nettles the hard way is something a lot of children learn the hard way. Stinging nettles use hollow hairs, called trichomes. Are like thin brittle hypodermic needles, filled with a chemical cocktail or irritants to dissuade you from annoying the plant any further. However, it’s not just Urtica dioica that can sting.
Adeel Mustafa and colleagues have examined plants from five different families, put them under the microscope and examined the chemistry of the hairs themselves. What the team found is that the needles are made from different materials, but whatever the plant uses it ends up with a similar looking hair.
The needles of a stinging nettle use silica, but Loasaceae mainly use calcium carbonate and Cnidoscolus doesn’t use any minerals at all. All the physical shapes of the hairs are similar. In AmJBot, the authors say: “The stinging cells are essentially hollow from the base to the bulbous tip and break off with the slightest touch. Breakage creates a sharp edge connected to a large liquid reservoir, similar to a hypodermic needle. Pressure applied to the trichome will compress the bladder‐like base and eject the irritant fluid from the tip in an action analogous to the plunger of a hypodermic syringe.”
Mustafa and colleagues conclude: “Stinging hairs—even as mechanical structures—are not simple cells with mineralized walls, but stunning examples of unique plant microengineering.” They suggest that comparative biomechanical studies would help uncover some of the details of how plants use different methods to make the same weapon. But you probably wouldn’t want it to be a hands-on study.