So her skin has hairs on it that sting. The hairs are very sharp and they are hollow. There is a poisonous juice inside, something the protoplasm has made; and when the sharp end of a hair sticks into your finger, the little turned-up end breaks off, and the poisonous juice gets into the wound and irritates and causes the finger to swell a little.
There is a way to take hold of a nettle so that it cannot sting. The little poison-filled hairs all point up, as you see in the picture. So if you stroke the nettle or draw your hand over it from root to tip, it cannot hurt you. Your hand presses the hairs flat against the stem and they cannot stick into you.
Sometimes hairs branch and make a thick network, like felt, over the leaf. They do this in the mullein, and here is a picture of mullein hairs very highly magnified.
Prickles and scales are made of cells as hairs are.
All parts of the plant above ground and sometimes the roots are covered with skin, but only the parts above ground are covered with hairs or prickles. Some plants are abundantly supplied with these protections; others manage to get along without them.
Plants very often have glands in their skins. These glands are merely cells which take certain things from the sap and pour them out on the outside of the plant.
Glands secrete their fluids inside the skin cells, and these fluids finally break through the outer wall of the skin cell and so get to the surface, or else they pass through stomata specially provided for them. They sometimes cover the surface of the plant with a sticky substance, as is the case with young birch twigs.
Glands also secrete the gum or resin which covers up the winter buds and keeps out the rain, and which makes the young leaves of the cherry shine so.