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NCERT Solutions for Class 9th Science Chapter 3 Epidermis, xylem and stomata — Pause and Ponder

Book page 34 Updated on2026-09-08

Q2.
Why do you think that a thick cuticle on the outer wall of epidermis is advantageous for a plant living in the desert but disadvantageous for a plant living underwater?
Answer

Because the cuticle blocks the movement of water and gases — and a desert plant needs that block, while a submerged plant is killed by it.

In the desert. Water is the limiting factor. A thick waxy cuticle:

  • cuts down evaporation from the epidermal surface, so transpiration is restricted almost entirely to the stomata, which the plant can close;
  • reflects some sunlight and reduces heating;
  • protects against abrasion by blowing sand and against parasites.

Underwater. Water is never in short supply, so there is nothing to conserve — and the cuticle now only does harm:

  • A submerged plant absorbs water, minerals, dissolved CO₂ and O₂ directly through its whole surface, because there is no transpiration pull to drive a root-to-leaf stream. A waterproof cuticle would seal off that entire absorbing surface.
  • Gases diffuse about 10 000 times more slowly in water than in air, so the plant is already short of CO₂ and O₂. Adding a diffusion barrier would starve it further.
Why it happens: a structure is never good or bad on its own — it is good only for a particular environment. The cuticle costs the plant its gas and water exchange across the surface and buys it water conservation. In the desert that trade is worth making; underwater the plant pays the cost and gains nothing. That is why submerged leaves have a very thin cuticle or none at all, and usually no stomata either.
Q3.
Once water is absorbed by plant roots, it has to travel against gravity through xylem. How do the ‘dead’ cells of the xylem work together with the living cells of leaves at the top to keep the water moving?
Answer

The dead cells supply the pipe; the living cells of the leaf supply the pull. Neither works without the other.

Leaf (living cells) Water vapour out of stomata = transpiration Xylem: dead, hollow, lignified tubes — no cross walls, no cytoplasm Unbroken water column pulled upward Root hair
The living leaf pulls; the dead xylem is the pipe that carries the pull down to the root.

Step by step:

  1. In the leaf, living mesophyll cells lose water vapour through the stomata — transpiration.
  2. Losing water makes those cells draw water from the nearest xylem vessel.
  3. Water molecules stick to one another (cohesion), so the column of water in the xylem does not break. Pulling at the top therefore pulls the whole column.
  4. This tension — the transpiration pull — is transmitted right down to the root, where water enters through the root hair and replaces what was lost.

Why the tube must be dead: tracheids and vessels lose their cytoplasm and their end walls at maturity, so a vessel is a continuous open pipe with nothing inside to obstruct flow. Their walls are thick and lignified, which is essential — a column under tension tends to collapse the tube inwards, and only a stiff lignified wall can resist that.

The key idea: the energy for lifting water does not come from the plant at all — it comes from the Sun, which evaporates water at the leaf. The plant simply provides a strong, unbroken pipe and a wet surface at the top. That is why water can be lifted tens of metres up a tall tree with no pump anywhere in the system.
Q4.
What do you think will happen if there were no stomata in the epidermis of the stem or leaves?
Answer

The plant would seal itself in: gas exchange would nearly stop, transpiration would stop, and with it the upward pull that carries water and minerals to the leaves.

ProcessWhat goes wrong without stomataConsequence
PhotosynthesisCO₂ cannot get in through the waxy cuticleFood production falls sharply; the plant starves
RespirationO₂ cannot enter, CO₂ cannot leaveCells cannot release energy efficiently
TranspirationNo water vapour escapesNo transpiration pull — the xylem column is not dragged upwards
Mineral supplyWater no longer streams from root to leafMinerals dissolved in that stream never reach the leaves
TemperatureNo evaporative cooling from the leaf surfaceLeaves overheat in the sun and enzymes are damaged
Waste removalWastes normally eliminated with transpired water stay behindWastes accumulate in the plant body
Why plants keep stomata even though they lose water through them: a stoma is a controlled hole. Every gain the plant makes — CO₂ in, cooling, the transpiration pull — depends on that hole being open, and every gram of water lost is the price. The plant manages the trade-off by opening and closing the stomata, which is far better than not having them at all. A plant with no stomata is like a house with no doors or windows: nothing enters, nothing leaves, nothing works.
Did you know? Some desert plants come close to this: they keep their stomata shut all day and open them only at night, when the air is cool and moist, so that they lose the least water for the CO₂ they take in.
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