Transpiration: types, factors and significance
Transpiration is the loss of water as vapour from the aerial parts of a plant, mainly through stomata. It cools the plant and drives the ascent of sap.
Types of transpiration
| Type | Water lost through | Share |
|---|---|---|
| Stomatal | Stomata, mostly on the lower surface of leaves | Most of the water lost |
| Cuticular | The waxy cuticle on leaves and young stems | A small amount; more in plants with thin cuticles |
| Lenticular | Lenticels, small pores in the bark of woody stems and on some fruits | Very little |
How stomata open and close
Each stoma is a pore between two bean-shaped guard cells. When the guard cells take in water, they become turgid and bend outwards, and the stoma opens. When they lose water, they become flaccid, and the stoma closes. Stomata usually open in light and close in the dark, or when the plant is short of water.
Factors affecting transpiration
| Factor | Effect on transpiration |
|---|---|
| Light (external) | Increases it, because stomata open in light |
| Temperature (external) | Increases it, because water evaporates faster when it is warm |
| Humidity (external) | Decreases it, because moist air takes up less water vapour |
| Wind (external) | Increases it, because wind sweeps away the moist air near the leaf |
| Soil water (external) | A shortage closes the stomata and reduces it |
| Leaf structure (internal) | Fewer or sunken stomata, a thick cuticle and small or spiny leaves reduce it, as in cactus and oleander |
Significance of transpiration
- Creates the transpiration pull that lifts water and minerals up the plant.
- Helps distribute minerals to all parts of the plant.
- Cools the leaves, as sweating cools our body.
- Removes excess water from the plant.
- Too much transpiration in hot, dry weather makes plants wilt, so it is called a necessary evil.
Activity: show that leaves give out water
- 1Water a potted plant, such as a balsam or tulsi plant. Cover one leafy shoot with a clear polythene bag and tie it at the stem.
- 2Cover the pot and soil with another bag, so water from the soil cannot reach the first bag.
- 3Keep the plant in sunlight for 2 to 3 hours.
- 4Observe: water droplets collect on the inside of the bag over the shoot.
- 5Conclusion: the leaves gave out water vapour, which condensed on the bag. This is transpiration.
A big neem or banyan tree gives cool shade partly because its leaves lose a lot of water by transpiration, which cools the air around them. In summer, farmers often water crops in the evening, when less water is lost.
1 marks: “What is the loss of water through lenticels called?” Lenticular transpiration.
2 marks: “Why is transpiration called a necessary evil?” It is needed for transpiration pull, mineral transport and cooling, but too much of it causes water loss and wilting.
4 marks: “Explain any four factors that affect transpiration.” Light (stomata open), temperature (faster evaporation), humidity (less loss in moist air), wind (removes moist air); also soil water and leaf structure.
Transpiration rises in 'Hot, Windy, Bright, Dry' weather: 'Hot Water Boils Dry'. It falls in humid air.