NCERT Class 10 Science — Chapter 2: Control and Coordination

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Assertion (A): Auxin is synthesised at the shoot tip and helps the cells to grow longer. Reason (R): When light is coming from one side of the plant, auxin diffuses towards the illuminated side of the shoot.

Official previous-year question

Verified 1 Aug 2026.

Options

ABoth A and R are true and R is the correct explanation of A
BBoth A and R are true but R is NOT the correct explanation of A
CA is true but R is false
DA is false but R is true

Did you get this right?

Solution

The correct option is

A is true but R is false

Explanation

Auxins are a class of plant hormones (phytohormones) that play a crucial role in the regulation of plant growth. They are primarily responsible for cell elongation and are involved in directional growth responses known as tropisms, specifically phototropism (response to light).

Assertion (A) is Correct:
Auxin is synthesized primarily at the growing apices of the plant, such as the shoot tip. From the tip, it is transported downwards to the region of elongation. Auxin promotes the elongation of cells by increasing the plasticity of the cell wall, allowing the cells to expand and grow longer.
Reason (R) is Incorrect:
The mechanism of phototropism involves the unequal distribution of auxin. When light comes from one side of the plant (unilateral illumination), auxin diffuses towards the shady (dark) side of the shoot, not the illuminated side. This higher concentration of auxin on the shady side stimulates the cells in that region to grow longer than the cells on the illuminated side. Consequently, the shoot bends towards the light source.

Key Takeaway:

Auxin accumulates on the shaded side of a shoot during phototropism, causing cells on that side to elongate more rapidly than those on the lit side, resulting in the bending of the plant towards the light.

Your note

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The correct option is

A is true but R is false

Explanation

Assertion (A) is Correct:
Auxin is synthesized primarily at the growing apices of the plant, such as the shoot tip. From the tip, it is transported downwards to the region of elongation. Auxin promotes the elongation of cells by increasing the plasticity of the cell wall, allowing the cells to expand and grow longer.
Reason (R) is Incorrect:
The mechanism of phototropism involves the unequal distribution of auxin. When light comes from one side of the plant (unilateral illumination), auxin diffuses towards the shady (dark) side of the shoot, not the illuminated side. This higher concentration of auxin on the shady side stimulates the cells in that region to grow longer than the cells on the illuminated side. Consequently, the shoot bends towards the light source.

Key Takeaway: