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.
- 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.
- 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 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.
- Are cells that are strengthened with lignin.
- They form wood.
- These are cells found between vessels.
- They form the packing tissue.
Forces involved in Transportation of Water and Mineral Salts
- 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.
- 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.