Light micrograph of epidermal cells on the underside of a leaf of beetroot, Beta vulgaris, showing several doughnut-shaped stomata or pores. 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 unknown.