Light micrograph of epidermal cells on the leaf of the onion, Allium cepa , showing four stomata, or pores on the surface. The pores allow the passage of gases through the otherwise impermeable epidermis to the interior of the leaf. On either side of the pores are 'guard' cells. These regulate the size of the pores, and thus the rate of gas transfer, by expanding or contracting due to internal hydrostatic pressure. Magnification x400 at 35mm size.