The stomata (plural, stoma= singular) are openings on the bottom of leaves that allow for gas exchange and water from the plant tissue can evaporate through them.
If the stomata are open all the time water from the plant will diffuse into the air based on humidity, basically if there is more water in the leaf than in the air, water will leave the leaf until the air around it has the same amount of water as in the leaf. SO if the air is dry the plant could lose a lot of water.
Factors affecting humidity around the leaves:
– temperature, the higher the temperature of the air the more water vapor (gas) it can hold so the more water will diffuse from the leaves to the air.
– wind, on windy days the water vapor around the leaf is ‘blown away’ so to speak and replaced with drier air so the leaves will be losing water faster than on an still day of the same temperature.
To protect itself against water loss the leaf has a waxy cuticle blocking water loss but it still need these openings to exchange Oxygen and Carbon Dioxide with the environment that it needs for photosynthesis.
To do this the leaf has specialized epidermal cells that surround the stoma called guard cells. The simplest way they work is shown in the diagram below.When the plant has enough water in its cells the guard cells swell up and open the stomata. When the plant isn’t getting enough water the guard cells can’t swell and the stomata stay closed, so the plant doen’s lose water. But if the stomata are closed photosynthesis can’t happen and eventually the plant might starve.
This is the most basic way plants deal with lack of water. There are hormones that can control the guard cells like abscisic acid to close or open the stomata to avoid losing water or dying of starvation. Some plants open the stomata only at night when it is cooler.