Nettle leaf. Coloured scanning electron micrograph (SEM) of long, sharp trichomes on a leaf from a stinging nettle plant (Urtica sp.). Stinging nettles have a variety of trichomes that range from long to short. The shorter trichomes are harmless, but the longer trichomes that have a silicified tip are the poisonous ones that cause the plant's sting. The tips are poisonous, causing the stinging sensation when touching the leaf.