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
- 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.
Capillarity
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.