15.1 Coordination and Response in Plants

2026 Syllabus Objectives

By the end of this subtopic, you should be able to:

  1. Describe gravitropism as a response in which parts of a plant grow towards or away from gravity
  2. Describe phototropism as a response in which parts of a plant grow towards or away from light
  3. Explain the role of auxin in controlling shoot growth, including:
    • (a) where auxin is made
    • (b) how auxin spreads through the plant
    • (c) how auxin is unequally distributed in response to light and gravity
    • (d) how auxin stimulates cell elongation
  4. Investigate gravitropism and phototropism in shoots and roots

Main Content

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1. What Is a Tropism?

Plants cannot move from place to place the way animals can. However, they are still able to respond to stimuli (things in their environment that affect them). One of the most important ways plants respond is by growing in a particular direction. This kind of directional growth response is called a tropism.

  • If a plant grows towards a stimulus, the response is called positive.
  • If a plant grows away from a stimulus, the response is called negative.

There are two main tropisms you need to know:

  • Gravitropism — a response to gravity
  • Phototropism — a response to light

2. Gravitropism

Gravitropism (sometimes called geotropism) is the response in which parts of a plant grow either towards or away from the force of gravity.

Shoots and Gravity

  • Shoots (the above-ground stems of a plant) grow away from gravity — this means they grow upwards.
  • This is called a negative gravitropic response (negative = away from the stimulus).
  • This makes sense because growing upward puts the leaves in sunlight, where they can carry out photosynthesis.

Roots and Gravity

  • Roots grow towards gravity — this means they grow downwards into the soil.
  • This is called a positive gravitropic response (positive = towards the stimulus).
  • This is useful because it anchors the plant in the soil and allows the roots to absorb water and minerals.

Simple summary: Shoots grow up (away from gravity = negative). Roots grow down (towards gravity = positive).

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