TRANSPIRATION

WHAT IS TRANSPIRATION?

Transpiration is the loss of water in the upper parts of the plant- particularly in the leaves. 

Water enters the leaves in the xylem and passes to the mesophyll cells by osmosis. The water evaporates from the surface of the mesophyll cells to form water vapour. The spongy mesophyll cells have large air spaces between them that help the water diffuse through the leaf tissue. As water vapour collects in these air spaces, the water potential rises. Once the water potential in the leaf is higher than outside, water molecules will diffuse outside the leaf. Open stomata provide an easy route for the water vapour to leave the leaf. The stomat are open during the day to allow for gaseous exchange during photosynthesis. 

Transpiration involves three processes: 

• osmosis from the xylem to mesophyll cells
• evaporation from the surface of the mesophyll cells into the intercellular spaces
• diffusion of water vapour from the intercellular spaces out through the stomata

WHAT IS TRANSPIRATION STREAM? 

As water leaves the xylem in the leaf, it must be replaced from below. Water moves up the xylem from the roots to replace water lost. 

This movement of water up the stem is useful for a plant in a number of ways: 

• water is required in the leaves for photosynthesis
• water is required to enable cells to grow and elongate 
• water keeps the cells turgid
• the flow of water can carry useful minerals up the plant 
• evaporation of water can keep the plant cool 

HOW CAN WE MEASURE THE RATE OF TRANSPIRATION?

A piece of apparatus called a potometer can be used to estimate the rate…