If you peel off a thin layer of cells from the surface of a leaf and mount them in a drop of water on a microscope slide, this is what you see – the leaf breathing pores, or stomata. When they open they allow carbon dioxide in and oxygen out. They are, without doubt, the most important portals on the planet. The carbon dioxide that they let in is turned into sugars that form the basis of our food, either directly from plants or indirectly through the domesticated animals that eat plants. The oxygen that they release allows us to breathe. Somehow they have to balance the passage of gases with conserving water, so they open and close depending on how much water is available. Each stoma is made of two lip-shaped guard cells, that bend apart to create a pore when they inflate with water, but collapse to close the pore when they wilt. The one in the top picture (x400) is open, and in the middle picture (x200) one is open and one is closed. The horizontal rows of narrow cells in the bottom picture (x100) are the veins of the leaf, which in this case came from the garden plant spiderwort Tradescantia virginiana.