Transpiration is the process by which plants lose water in the form of water vapour into the atmosphere.

  • Water is lost through stomata, cuticle and lenticels.
  • Stomatal transpiration:
  • This accounts for 80-90% of the total transpiration in plants.
  • Stomata are found on the leaves.

Cuticular transpiration:

  • The cuticle is found on the leaves, and a little water is lost through it.
  • Plants with thick cuticles do not lose water through the cuticle.

Lenticular transpiration

  • Is loss’ of water through lenticels.
  • These are found on stems of woody plants.
  • Water lost through the stomata and cuticle by evaporation leads to evaporation of water from surfaces of mesophyll cells.
  • The mesophyll cells draw water from the xylem vessels by osmosis.
  • The xylem in the leaf is continuous with xylem in the stem and root.

Structure and function of Xylem

  • Movement of water is through the xylem.
  • Xylem tissue is made up of vessels and tracheids.

Xylem Vessels

  • Xylem vessels are formed from cells that are elongated along the vertical axis and arranged end to end.
  • During development, the cross walls and organelles disappear and a continuous tube is formed.
  • The cells are dead and their walls are strengthened by deposition of lignin.
  • The lignin has been deposited in various ways.
  • This results in different types of thickening
  • Simple spiral.
  • Double spiral.

The bordered pits are areas without lignin on xylem vessels and allow passage of water in and out of the lumen to neighbouring cells.


  • Tracheids have cross-walls that are perforated.
  • Their walls are deposited with lignin.
  • Unlike the xylem vessels, their end walls are tapering or chisel-shaped.
  • Their lumen is narrower.
  • Besides transport of water, xylem has another function of strengthening the plant which is provided by xylem fibres and xylem parenchyma.

Xylem fibres:

  • Are cells that are strengthened with lignin.
  • They form wood.

Xylem parenchyma:

  • These are cells found between vessels.
  • They form the packing tissue.

Forces involved in Transportation of Water and Mineral Salts

Transpiration pull

  • As water vaporises from spongy mesophyll cells into sub-stomatal air spaces, the cell sap of mesophyll cells develop a higher osmotic pressure than adjacent cells.
  • Water is then drawn into mesophyll cells by osmosis from adjacent cells and finally from xylem vessels.
  • A force is created in the leaves which pull water from xylem vessels in the stem and root.
  • This force is called transpiration pull.

Cohesion and Adhesion:

  • The attraction between water molecules is called cohesion.
  • The attraction between water molecules and the walls of xylem vessels is called adhesion.
  • The forces of cohesion and adhesion maintain a continuous flow of water in the xylem from the root to the leaves.


  • Is the ability of water to rise in fine capillary tubes due to surface tension.
  • Xylem vessels are narrow, so water moves through them by capillarity.

Root Pressure:

  • If the stem of a plant is cut above the ground level, it is observed that cell sap continues to come out of the cut surface.
  • This shows that there is a force in the roots that pushes water up to the stem.
  • This force is known as root pressure.

Importance of Transpiration

  • Transpiration leads to excessive loss of water if unchecked.

Some beneficial effects are:

  • Replacement of water lost during the process.
  • Movement of water up the plant is by continuous absorption of water from the soil.
  • Mineral salts are transported up the plant.
  • Transpiration ensures cooling of the plant in hot weather.
  • Excessive loss of water leads to wilting’ and eventually death if water is not available in the soil.

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