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What is transpiration, and why is it important for plants?

Class 7 ScienceNCERT chapter: Life Processes in Plantsइसे हिंदी में पढ़ें

Exam-ready answer

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through tiny pores called stomata on the leaves. It is important because it creates a suction pull that draws water and minerals up from the roots, it cools the plant, and it releases water vapour into the air.

Explanation

Leaves have tiny openings called stomata, mostly on their lower surface. Each stoma is surrounded by two guard cells that open and close the pore. Stomata let carbon dioxide in for photosynthesis and let oxygen out. When they are open, water from inside the leaf also escapes as water vapour. Most of the water absorbed by the roots leaves the plant in this way.

To see transpiration, cover a leafy twig of a potted plant with a transparent polythene bag and tie its mouth. Keep the plant in sunlight. After a few hours, tiny water droplets appear on the inner side of the bag. This water came out of the leaves as vapour and condensed on the cooler bag. A similar bag tied over a twig without leaves collects very little water.

Transpiration is useful in three ways. As water evaporates from the leaves, more water is pulled up from below, so water and minerals rise through the xylem even to the top of tall trees. Evaporation also takes away heat, which keeps leaves cool on hot days. The water vapour released adds moisture to the air and is a part of the water cycle that leads to rain.

Points to remember

  1. Transpiration: loss of water as vapour from the aerial parts of a plant, mainly the leaves.
  2. Stomata: tiny pores on leaves, opened and closed by guard cells, through which transpiration mostly occurs.
  3. Transpiration creates a suction pull that lifts water and minerals through the xylem.
  4. It cools the plant by evaporation, much as sweating cools our body.
  5. It adds water vapour to the air and is a part of the water cycle.

Exam tip

Define transpiration using the words 'water vapour', 'aerial parts' and 'stomata', then list at least two uses. Support the answer with a labelled diagram of the polythene-bag activity showing droplets inside the bag.

Questions students also ask

On which kind of day is transpiration faster?
Transpiration is faster on a hot, sunny, dry and windy day, because water evaporates quickly from the leaves under these conditions. It is slower on humid, cool or still days, and at night when most stomata are closed.
How are stomata useful besides transpiration?
Stomata allow exchange of gases. During photosynthesis, carbon dioxide enters the leaf through them and oxygen goes out. During respiration, oxygen enters and carbon dioxide leaves. Guard cells open or close the stomata to control both gas exchange and water loss.
Why do plants wilt on a hot afternoon?
On a hot afternoon, leaves may lose water by transpiration faster than the roots can absorb it from the soil. The cells lose water and become soft, so leaves and stems droop. Once enough water is available again, the plant usually recovers.

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