CoachA stoma is a pore in the epidermis of a leaf (or a green stem) bounded by two guard cells, kidney-shaped cells whose walls are thickened on the side facing the pore and whose cellulose microfibrils run around them like hoops. When the guard cells take up water and swell, the hoops prevent them from widening and force them to bow apart: the pore opens. When they lose water, it closes. A leaf carries a few hundred stomata per square millimetre, mostly on its lower surface; open, their pores are one or two percent of the leaf’s area and nearly all of its gas exchange passes through them.
