Read the below statements about transpiration: 1. Transpiration is highest in dry and windy conditions. 2. It helps in the ascent of sap through cohesion-tension. 3. Transpiration always results in wilting. Which of the above statements are correct?
- (a)Only 1 and 2 are correct
- (b)Only 1 and 3 are correct
- (c)Only 2 and 3 are correct
- (d)1, 2 and 3 are correct
Answer
Why
Correct — A. Statement 1 is true. Dry air and wind both keep the water-vapour gradient between leaf and air steep, so water escapes faster.
Statement 2 is true. Water evaporating at the leaf pulls on the column behind it, held together by cohesion, and that tension lifts sap up the xylem.
Statement 3 is false. Wilting sets in when water loss outruns what the roots take up, not whenever a plant transpires. That leaves 1 and 2: option (a).
Why the others are wrong
- (b)Only 1 and 3 are correct — Keeps the false statement 3. Transpiration does not always cause wilting, and this option also drops the true cohesion-tension statement.
- (c)Only 2 and 3 are correct — Drops statement 1 and keeps statement 3. Dry, windy air speeds transpiration by steepening the vapour gradient, and wilting follows only when loss exceeds uptake.
- (d)1, 2 and 3 are correct — The word always breaks statement 3. A well-watered plant transpires through the day and stays turgid, so all three cannot be correct.
Concept
Transpiration is the loss of water as vapour from the aerial parts of a plant, mostly through the stomata.
Its rate depends on the gradient between the moist leaf and the air around it. Dry air and wind keep that gradient steep, so both speed transpiration up.
The same evaporation drives the cohesion-tension mechanism: water molecules cling together, so a pull at the leaf passes down an unbroken column in the xylem.
NCERT Class 10 notes that root pressure matters more at night, while in the day, with stomata open, the transpiration pull becomes the major force moving water in the xylem.
Wilting is a loss of turgor. It follows when loss outruns uptake, so transpiration causes it only under water stress.
Key facts
- Transpiration is the loss of water as vapour from the aerial parts of the plant (NCERT Class 10, Life Processes).
- Dry air and wind both raise the rate of transpiration by steepening the water-vapour gradient at the leaf.
- In the day, with stomata open, transpiration pull is the major force moving water up the xylem.
- A plant wilts when its water loss exceeds the water taken up by its roots.
Study next
Common traps
- Letting 'always' pass: one well-watered plant that transpires all day without wilting is enough to break statement 3.
- Thinking humid air speeds transpiration: humid air flattens the vapour gradient and slows it.
Here three statements test the rate, the mechanism and a supposed side-effect of transpiration, and the false one carries the absolute word always.
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