Bacteria will exploit any opportunity to invade a new living space, in particular taking advantage of any easily-colonisable entrances into other living organisms. In plants one of these entrances is a doorway between the interior of the leaf and the outside air in order to exchange gases. Plants require carbon dioxide in order to carry out photosynthesis, and this has to be brought into the interior of the leaves. Similarly the excess oxygen produced must leave the cells before it builds up to toxic levels. In order to achieve this, plants have small holes in the leaves formed by two curved cells, known as guard cells, with a gap between them. These gaps are called stomata (singular stoma).
The stomata aren’t kept open all the time, but close and open at different times of the day depending on how much the plant needs them. This is controlled by the plant hormone abscisic acid which regulates the amount of water found in the guard cells. When the guard cells fill with water they expand and arch outwards – ballooning out to form two large curves with the pore between them. When they lose water they shrink back down and close the pore. Abscisic acid can regulate the water content of the guard cells by controlling ion channels in the guard cells, changing the osmotic pressure compared to surrounding cells and causing water to diffuse in or out.