Structure of Stomata

A stoma is a microscopic pore found in the epidermis of leaves and green stems, typically about 10-40 micrometres long and 3-10 micrometres wide, and a single mature leaf can carry anywhere from around 50 up to 500 stomata in every square millimetre of its surface. Each stoma is bordered by a pair of specialised guard cells - living epidermal cells with a distinctive semi-lunar or kidney shape - joined firmly together at both of their ends but free to separate in the middle, opening a pore between them; the guard cells are, in turn, supported and surrounded by ordinary neighbouring epidermal cells called subsidiary or accessory cells. A defining structural feature of every guard cell is that its inner wall, facing directly onto the pore, is noticeably thicker than its outer wall - this asymmetry is what allows turgor changes to physically open and close the pore, since a swelling guard cell stretches its thinner, more elastic outer wall further than its thicker, less elastic inner wall, bowing the pair of cells apart. Internally, each stoma opens into a sub-stomatal cavity, an air space that stays connected to the …