Revision: Plant Physiology >> Transpiration Biology (English Medium) ICSE Class 10 CISCE

A potometer is a device that measures the rate of water intake by a plant and this water intake is almost equal to the water lost through transpiration.

Some plants may lose water or other fluids along with dissolved substances directly in liquid form and not as water vapour. This is known as exudation.

Hydathodes are specialised structures that facilitate the discharge of excess water from the tips of veins in the leaf. They facilitate the release of superfluous water in liquid form by alleviating hydrostatic pressure within the plant.

1. Stomatal transpiration is the loss of water vapour through minute openings (stomata) on the leaf surface, mainly for photosynthesis and cooling.
2. Process: Water evaporates from mesophyll cells → enters intercellular spaces → moves to sub-stomatal cavity → diffuses out through stomata.
3. Direction of Movement: Water vapour moves from a region of higher concentration inside the leaf to a lower concentration outside, by diffusion.
4. Transpirational Pull: Continuous evaporation creates a pull that draws water upwards from roots through xylem, even in tall trees.
5. Regulation: Opening and closing of stomata is regulated by turgidity of guard cells—open when turgid, close when flaccid. More transpiration occurs from the lower surface of dicot leaves.

1. Cuticular transpiration is the loss of water vapour through the waxy cuticle covering the leaf surface.
2. Factors: Thicker cuticle → less transpiration.
3. Adaptation: Desert plants have thick cuticles to reduce water loss.

1. Lenticular transpiration is the loss of water vapour through lenticels—small openings on the bark of older stems.
2. Structure: Lenticels are always open and not surrounded by guard cells.
3. Function: They allow gaseous exchange and minimal water loss by evaporation from exposed cell surfaces.
4. Comparison: The Least amount of transpiration occurs via lenticels compared to stomatal and cuticular transpiration.
5. Location: Found on woody stems, not leaves.

• Transpiration is a necessary process that helps in maintaining turgidity, shape, and structure of plant cells.
• It helps in cooling the plant by reducing leaf temperature through evaporation.
• Transpiration aids in the absorption and upward movement (ascent of sap) of water and minerals.
• It facilitates gaseous exchange needed for photosynthesis and respiration as stomata remain open.
• Excessive transpiration is harmful as it causes wilting, injury, and may even lead to the death of the plant; hence, it is called a “necessary evil.”

1. Guttation is the loss of water droplets through hydathodes at leaf margins in uninjured plants.
2. It occurs early in the morning or at night, especially in humid conditions when transpiration is low.
3. The exudate in guttation is mainly water with dissolved mineral salts.
4. Bleeding is the flow of plant sap (rich in sugars) from cut or injured parts.
5. Bleeding occurs in all plants after injury, aided by root pressure.