Transpiration and the Transpiration Stream

Transpiration is the loss of water from a plant by evaporation.

The leaves of a plant are adapted for efficient photosynthesis. As a result of this, they have a large surface area to absorb more sunlight. It is the energy from this sunlight that changes water in the leaves from a liquid state to a vapour state. The leaves also contain tiny pores called stomata which allow carbon dioxide from the atmosphere to enter the leaf for use in photosynthesis. These pores also allow for water vapour to exit the leaves.

Most of the evaporation occurs in the spongy mesophyll cells in the leaf. The water vapour produced fills the empty spaces between the mesophyll cells and exits the leaf through the stomata via the process of diffusion.

Diffusion is the net movement of molecules from an area of higher concentration to an area of lower concentration. So in the case of transpiration, there is a high concentration of water vapour in the leaf compared to the air outside. Thus water vapour diffuses out of the leaf into the air.



The water lost from the mesophyll cells increases the concentration of the cell sap within the cell. This results in water from the leaf xylem being transported to the cells via osmosis. Water drawn from the leaf xylem is in turn replaced by the water that flows upwards from the stem xylem, and this is replaced by the water from root xylem which is replaced by the water drawn from the root hairs. This movement of water through the xylem is called the transpiration stream.

Thus transpiration explains how water travels against the pull of gravity to reach the upper regions of plants without the use of a pump.

 

The animation below explains the transpiration stream: