Common stinging nettle (Urtica dioica) leaf, taken in polarised light with retarders. The leaf is at right and the petiole (leaf stem) is at left. This is a cleared leaf, one that has been treated to make the veins and other leaf structures more visible. The leaf is seen from the underside, showing the stinging and non-stinging trichomes (hair-like structures). The stinging trichomes are the long, sharp ones, and the non-stinging trichomes are the shorter ones.