All plants obtain the water they need by absorbing it from the soil through their roots. A small amount of this water gets used, however, the rest evaporates from the surface of the leaves. This process is called transpiration.
When water is absorbed by the roots, it passes through to the xylem. Its movement throughout a plant is driven by the process of transpiration. There is a thin layer of water on the surface of spongy mesophyll cells.
As water evaporates from the mesophyll layer, new water is drawn from the cells below because water molecules are attracted to other water molecules. This creates a pull that drags water up the xylem, which is useful as it helps the plant transport water against the force of gravity. This steady movement of water through the xylem is called the transpiration stream.
Transpiration also has other functions. For example:
Stomata are tiny holes in the epidermis of a leaf (the leaf’s surface). They control gas exchange by opening and closing and are involved in the evaporation of water from the leaves. Stomata allow water vapour and oxygen to flow out of the leaf and carbon dioxide to move into the leaf.
Where conditions are drier, plants have adapted to have fewer or smaller stomata to preserve as much water as possible. This reduces their surface area to volume ratio.
There are typically more stomata on the underside of the leaf than on the top side. This allows gases to be exchanged while minimising water loss by evaporation, as the lower surface is shaded and cooler.
In low light conditions, the guard cells lose water and become flaccid (soft and floppy), causing the stomata to close. However, during the day, guard cells take in water by osmosis and become plump and turgid.
The size of the stomatal opening plays an essential part in keeping the plant healthy. The size of the stoma helps the plant to control the rate of transpiration and therefore limit the levels of water loss from the leaf. This means the plant is less likely to wilt.