Definition
Transpiration is the loss of water in the form of water vapour through stomata.
Types of Transpiration
- Stomatal – Occurs through stomata
- Cuticular – Occurs through surface of stem and leaves
- Lenticular – Occurs through lenticels
- Also Read Photosynthesis
Differences between Evaporation and Transpiration
| Evaporation | Transpiration |
| Loss of water from the surface in the form of water vapours | Loss of water from aerial parts of plants in the form of water vapours |
| Fast process | Slow process |
| A physical change controlled by temperature and pressure | A partially physical and vital process controlled by various internal and external factors |
Factors affecting Transpiration
External Factors:
- Light
- Wind Speed
- Humidity
- Temperature
- Atmospheric pressure
- Carbon dioxide
Internal Factors:
Water content of the leaves
Adaptations in Plants To Reduce Excessive Transpiration
- Sunken stomata
- Less stomata
- Narrower leaves
- Loss of leaves
- Reduced exposed surface
- Thick cuticle
Significance
- Helps in ascent of sap
- Cools the internal temperature of plants
- Distribution of water and minerals in plants
- Also read Absorption by roots
Experiments for Demonstration
Cobalt chloride paper is used to check transpiration. It is a blue coloured paper which on exposure to moisture, changes its colour to pink.
Measurement of Transpiration
Two methods for measuring the process are :
- Weighing Method
- Potometer Method
Potometer is a device used to measure water intake by a plant; this intake is equal to the water loss through the process. There are a number of designs of potometer that are used to measure the process. Some examples include Farmer’s potometer and Ganong’s potometer (to measure rate of water intake), Darwin’s potometer (to demonstrate the suction force created by the process ), Garreau’s potometer (to demonstrate unequal transpiration from both the surfaces of a dorsiventral leaf), etc
Limitations of using potometer:
- Introducing the air bubble is not an easy task.
- The twig can die after sometime.
- Changes in outside temperature can affect the position of air bubble in the capillary tube.
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