Lower leaf epidermis of the stonecrop plants showing puzzle shaped epidermal cells with scattered stomata. Stomata (singular: stoma) are like little mouths on the leaf surface, specialized in gas-exchange – CO2 enters a plant through them. The pairs of sausage-shaped cells, like the lips of these mouths, are “guard cells”, which guard the opening and closure of the stomata. In many flowering plants, the stomata seem to randomly distribute on a leaf surface, but actually there is a “one-cell-spacing rule” in their developmental program: two stomata will not be immediately adjacent to each other, and they will be separated by at least one other cell. Such spacing is achieved through communication between the cells (e.g. molecule exchange), so the location of each stoma is known by others.