All plants need water and they obtain this by absorbing it through the soil, through the 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 and the movement of water throughout a plant is driven by. Water builds up on the surface of spongy mesophyll cells, as there are large spaces between the 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:

  • Keeping the cells turgid, to support the plant’s structure
  • Providing water to the leaves for photosynthesis
  • Transporting mineral ions
  • Cooling the plant – Converting water to water vapour (evaporation) requires heat energy. So during transpiration, the heat energy gets used up, which cools down the plant.

The Structure and Function of Stomata

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 out of the leaf and carbon dioxide into the leaf.

The diagram above shows a stoma (the singular of stomata) both open and closed.

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. This allows gases to be exchanged while minimising water loss by evaporation, as the lower surface is shaded and cooler.

  • Guard cells, which are a pair of sausage-shaped cells, surround the stoma to regulate its size.

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.