medium importancemedium difficulty6 min

Tropism

Tropism is the directional growth movement of a plant part in response to a stimulus.

In one line: Tropism means growth turned by a stimulus.

Practice This Concept

Understand it

Tropism happens when a plant grows towards or away from a stimulus.
The movement is directional, so the direction of the stimulus matters.
Plant parts can show positive tropism when they grow towards the stimulus, or negative tropism when they grow away from it.
This is different from fast movement in animals because it is mainly a growth response in plants.

Example

A shoot growing towards light shows positive phototropism, and roots growing downward show positive geotropism.

Rule to remember

Rule: tropism is directional growth, and it may be positive or negative depending on movement towards or away from the stimulus.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1A plant kept near a window bends toward the light source.
2The shoot shows positive phototropism because it grows in the direction of the stimulus.

Method you can reuse

1Identify the stimulus.
2Check whether the movement is growth based.
3Note the direction of growth.
4Decide positive or negative tropism.
5Name the correct type such as photo or geo tropism.

Visual tip

A directional growth arrow diagram works well, especially for shoot toward light and root toward gravity.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What makes tropism different from a simple plant movement?

Think first, then show answer

Tropism is different because it is a directional growth response to a stimulus. The plant part grows towards or away from the stimulus, so the direction of the stimulus matters.

Common wrong answer

A common wrong answer is to describe any bending as tropism even when growth direction is not involved. Tropism must be directional growth.

Don't mix this up

Students often call every plant movement tropism. Not every movement is tropic; tropism specifically involves directional growth.

Exam tip

In exam answers, always mention the stimulus and the direction of growth. That is the heart of tropism.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask for the meaning of tropism, the type of tropism in a given situation, or positive versus negative movement.

1
Enough for 1 mark
Tropism is the directional growth movement of a plant part in response to a stimulus.
2
Add for 2 marks
Example: A shoot growing towards light shows positive phototropism, and roots growing downward show positive geotropism.
3
Add for 3 marks
Tropism happens when a plant grows towards or away from a stimulus.

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