Light micrograph of the epidermal cells on the surface of the leaf of the Trinity Flower Tradescantia x andersoniana. In the centre of this image is a stoma, or pore. It is through pores such as these that gases may reach the inside of the leaf: the rest of the epidermal cells being impermeable. Two 'guard' cells surround the pore itself. Changes in hydrostatic pressure within these cells govern the pore's size and thus the rate of gas transfer across the epidermis. Magnification x400 at 35mm size.