Transport Of Water In The Xylem

MOVEMENT OF WATER OUT THROUGH STOMATA:

• The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.
• As a result there is a water potential gradient from the air spaces throught the stomata to the air.
• Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air.
• Water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
• By changing the size of the stomatal pores, plants can control their rate of transpiration.

MOVEMENT OF WATER ACROSS THE CELLS OF A LEAF:

• Water is lost from mesophyll cells by evaporation fom their cell walls to the air spaces of the leaf.
• This is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm.
• In the case of the cytoplasmic route the water movement occurs because:
  • Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun.
  • These cells now have a lower water potential and so water enters by osmosis from neighbouring cells.
  • The loss of water from these neighbouring cells lowers their water potential.
  • They, in turn, take in water from their neighbours by osmosis.
• In this way, a water…