Stomata - an overview
As plants colonised drier terrestrial habitats, they developed water-resistant surfaces to conserve water, but these impede the movement of gasses - essential for respiration and photosynthesis - in and out of tissues.
A stomate is a microscopic pore on the surface of a leaf (or stem, petal or developing fruit) which facilitates the exchange of gases between the atmosphere and the inner tissues, but it's far more than a simple fixed opening. Because they regulate the movement of carbon dioxide, oxygen and water vapour, they're involved in several critical plant processes and therefore survival, growth and yield.
A "guard cell" lies on each side of the stomatal opening. The specialised cell wall architecture means that when the guard cells swell with water their shape alters. They pull apart, creating an aperture. When these cells lose turgidity, the pore closes.
Water loss through the stomata is a compromise the plant must make in order to photosynthesise (although this transpiration can serve other purposes). Unfortunately, pathogenic fungi and bacteria can sometimes take advantage of these openings to invade the plant, too.
The mechanism of stomatal function and how it's regulated is complex and the subject of ongoing research. Flux of potassium ions plays an important part. Stomata can respond to water supply, carbon dioxide concentration, light, hormones and the presence of pathogens. Stomata also exhibit circadian rhythm.
Typically, they open in the day and close at night, although some plants adapted to arid environments have a special way of collecting CO2 at night so that they can keep stomata closed during the day.
Another water-conserving adaptation seen in some species is the sinking of stomata below the main leaf surface and/or protection of the opening with hairs, to slow the rate of diffusion of water vapour.
Because the underside of the leaf is less susceptible to desiccation, it's not surprising that they are usually most concentrated on that surface on broadleaf plants. However, the number and placement of stomates typical of a species varies as an adaptation to its habitat. The floating leaves of some aquatic plants, for example, have stomates on the upper side. The environment in which an individual plant is grown (e.g amount of sun/shade) will also affect development of stomata.
Plants can undergo a certain amount of wilting and recover, but it should be realised that once the stomates close, there will be little photosynthesis occurring. This might be tolerable in the case of perennial landscape plants (and some desert-adapted species have special photosynthetic mechanisms to cope with this problem). However, it will be an issue when maximum yield and quality of vegetables, fruit and flowers is the goal.