IGCSE Biology Paper-4: Specimen Questions with Answers 246 - 246 of 279

Question 246

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Auxins control the growth responses of seedlings.

Explain how auxins control the growth response of the seedlings.

Explanation

Phototropism’s

In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. The plant does NOT bend towards the light. It grows because the auxin causes the cells to elongate on the shaded side, so this side grows more. This unequal growth of the two sides, results in the growth of the stem towards the light.

Gravitropism’s

Phototropism is a response to the stimulus of light, whereas gravitropism (also called geotropism) is a response to the stimulus of gravity.

Plants responses to gravity:

  • when the stem grows against the force of gravity, this is known as a negative gravitropism
  • when a root grows in the direction of the force of gravity, this is known as a positive gravitropism

In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity. The opposite happens in a stem. When a stem is placed horizontally, the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity.

Roots and Stems Grow Downwards