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What are phototropism and geotropism?

Class 10 ScienceNCERT chapter: Control and Coordinationइसे हिंदी में पढ़ें

Exam-ready answer

Phototropism is the directional growth movement of a plant part in response to light. Shoots grow towards light (positive phototropism) and roots grow away from light (negative phototropism). Geotropism is the directional growth movement in response to gravity. Roots grow downwards towards gravity (positive geotropism) and shoots grow upwards away from gravity (negative geotropism).

Explanation

Plants are fixed, so they cannot move from place to place. Instead, they respond to external stimuli such as light, gravity, water and touch by growing in a particular direction. This growth movement is called tropism. If the growth is towards the stimulus, it is called positive tropism; if it is away from the stimulus, it is called negative tropism.

Phototropism is the response of a plant part to light. The shoot tip grows towards the direction of light, so shoots show positive phototropism. The root grows away from light, so roots show negative phototropism. This helps leaves get maximum light for photosynthesis. The stimulus here is light, and the response is the bending of the growing part.

Geotropism (also called gravitropism) is the response of a plant part to gravity. The root grows downwards, in the direction of gravity, so roots show positive geotropism. The shoot grows upwards, against gravity, so shoots show negative geotropism. This helps roots fix the plant in soil and absorb water and minerals, while shoots get light and air.

Points to remember

  1. Tropism: directional growth movement of a plant part in response to an external stimulus.
  2. Phototropism: response to light. Shoot → positive phototropism; root → negative phototropism.
  3. Geotropism: response to gravity. Root → positive geotropism; shoot → negative geotropism.
  4. Positive tropism = growth towards the stimulus; negative tropism = growth away from the stimulus.
  5. Examples: shoot bending towards a window (phototropism); roots growing downwards into soil (geotropism).

Exam tip

Draw a simple labelled diagram of a germinating seed showing the root growing downwards (positive geotropism) and the shoot growing upwards (negative geotropism), and label light direction for phototropism. Write the definition of tropism first, then explain both terms with the words 'positive' and 'negative' clearly.

Questions students also ask

What is the difference between phototropism and geotropism?
Phototropism is the growth response of a plant part to light, while geotropism is the growth response to gravity. In phototropism, shoots bend towards light; in geotropism, roots grow downwards and shoots grow upwards.
Why do roots show negative phototropism?
Roots grow away from light because they do not need light for photosynthesis. Growing away from light and towards gravity helps roots go deeper into the soil to absorb water and minerals and to anchor the plant firmly.
Give one example of geotropism in plants.
When a seed germinates, its root always grows downwards into the soil and its shoot grows upwards. The downward growth of the root is an example of positive geotropism, and the upward growth of the shoot is negative geotropism.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.