Science · Class 10 · Control and Coordination · Term 3

Coordination in Plants: Tropic Movements

Learn how plants respond to environmental triggers like light and gravity through directional growth movements.

TL;DR:Have you ever noticed how a plant kept indoors seems to lean towards the window? This topic explores the amazing, silent ways plants move and grow in response to their surroundings.

• CBSE Learning Outcomes
• NCERT: Class 10 - Chapter 7 - Coordination in Plants

About This Topic

This topic, 'Coordination in Plants: Tropic Movements', is a fundamental part of the Class 10 Science curriculum, specifically under the chapter 'Control and Coordination' as per the NCERT framework. It shifts the students' perspective from viewing plants as static organisms to understanding them as dynamic beings that actively respond to their environment for survival. The core concept revolves around how external stimuli like light, gravity, water, and touch trigger specific, directional growth responses. The mechanism, primarily driven by the plant hormone auxin, is a key focus. For instance, explaining how auxin accumulates on the shaded side of a stem to cause cell elongation and bending towards light (phototropism) provides a tangible example of hormonal coordination in plants.

Teaching this topic effectively involves moving beyond rote memorisation of definitions. It is crucial to connect these concepts to a plant's survival strategy. Positive phototropism ensures shoots get maximum sunlight for photosynthesis, while positive geotropism ensures roots anchor the plant firmly and find water and nutrients deep in the soil. By exploring different tropisms like hydrotropism (response to water), thigmotropism (response to touch), and chemotropism (response to chemicals), students gain a holistic understanding of plant adaptation. This topic lays the groundwork for more advanced concepts in plant physiology and ecology that students might encounter in higher classes.

Key Questions

1. Explain how phototropism helps a plant survive.
2. Compare geotropism in plant roots and shoots.
3. Identify the stimulus for hydrotropism and thigmotropism.

Learning Objectives

• Define tropism and differentiate between phototropism, geotropism, hydrotropism, and thigmotropism.
• Explain the role of the plant hormone auxin in mediating phototropism in plant shoots.
• Describe how tropic movements help plants survive by obtaining necessary resources like light and water.
• Design and interpret a simple experiment to demonstrate positive phototropism or positive geotropism.
• Compare and contrast the response of a plant's shoot and root to the stimuli of light and gravity.

Key Vocabulary

Tropism	A directional growth movement of a part of a plant in response to an external stimulus.
Stimulus	Any change in the external or internal environment of an organism that causes a response.
Phototropism	The growth of a plant part in response to a light stimulus. Shoots are positively phototropic, while roots are negatively phototropic.
Geotropism	The growth of a plant part in response to gravity. Roots are positively geotropic, while shoots are negatively geotropic.
Auxin	A key plant hormone that controls cell growth and elongation, playing a major role in tropic movements.

Watch Out for These Misconceptions

Common Misconception

Plants 'think' or 'decide' to bend towards the light.

What to Teach Instead

Plant movements are not conscious decisions. They are automatic growth responses controlled by chemical hormones, like auxin, which react to environmental stimuli.

Common Misconception

All plant movements are tropisms.

What to Teach Instead

Tropisms are directional growth movements. There are also nastic movements, which are non-directional, like the folding of leaves of a 'touch-me-not' plant, which is a response to touch but not related to the direction of the touch.

Common Misconception

Roots grow down only because they are heavy.

What to Teach Instead

While gravity pulls everything down, roots actively grow downwards in response to the stimulus of gravity. This is an active process called geotropism, not just a passive falling.

Active Learning Ideas

Experiential Learning

The Shoebox Maze for Light

Grow a small bean plant inside a shoebox with internal partitions forming a maze and a single hole for light at one end. Students will observe how the plant shoot navigates the maze over a week to grow towards the light source.

20 min·Small Groups

Experiential Learning

The Sideways Seedling Jar

Germinate a few seeds (like gram or bean) on moist cotton wool inside a transparent glass jar. Once the radicle and plumule appear, turn the jar on its side and observe for a few days how the root continues to grow downwards and the shoot bends to grow upwards.

15 min·Pairs

Experiential Learning

Tendril Touch Test

Provide students with a small climber plant, like a pea plant, and a thin stick or pencil. Students will gently touch a tendril to the support and observe over 2-3 days how it coils around the object, demonstrating thigmotropism.

10 min·Individual

Real-World Connections

• A houseplant on a windowsill bending towards the window to get more sunlight for photosynthesis.
• The roots of a large tree growing downwards to anchor it firmly in the soil, preventing it from falling over in strong winds.
• Climbing plants like money plants or grapevines using their tendrils to wrap around supports to climb higher and reach sunlight.
• Farmers ensuring seeds are planted in the correct orientation so that the root grows down and the shoot grows up efficiently.
• The roots of a plant growing towards a leaking underground water pipe, demonstrating hydrotropism in search of water.

Assessment Ideas

Quick Check

Give students diagrams of plants responding to different stimuli (e.g., a plant in a dark room with one window) and ask them to identify the stimulus, the type of tropism, and whether it is positive or negative.

Peer Assessment

Ask students to write a lab report on an experiment they conducted to demonstrate phototropism, including their hypothesis, method, observations, and conclusion explaining the role of auxin.

Quick Check

Provide a checklist with statements like 'I can define geotropism' or 'I can explain why shoots grow towards light'. Students rate their own understanding on a scale of 1 to 3.

Frequently Asked Questions

How does a plant know which way is up and which way is down?

Plants sense gravity through specialised cells. In response, the hormone auxin controls the direction of growth. It promotes shoot growth away from gravity (negative geotropism) and root growth towards gravity (positive geotropism).

Why do sunflowers face the sun all day?

This movement, called heliotropism, is a type of phototropism. It helps the sunflower maximise the amount of sunlight it receives for photosynthesis throughout the day, which helps it produce more energy and seeds.

Are tropic movements fast or slow?

Tropic movements are generally very slow because they involve actual growth and cell division. You can usually only notice the changes over several hours or days, unlike the fast movements of animals.