Unit 2

Plant Physiology

Chapter 2.5

Plant Growth and Movement

Class 12 Biology – Plant Growth and Movement Notes PDF

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Introduction

DefinitionPlant growth is an irreversible increase in size, mass, volume or cell number, while plant movement is a change in position or orientation of a plant organ in response to internal or external stimuli.

This chapter includes seed germination, dormancy, photoperiodism, vernalization, senescence and plant movements, especially tropic and nastic responses.

1. Seed Germination

DefinitionSeed germination is the resumption of active growth of the embryo in a seed, resulting in emergence of the seedling under favourable conditions.

Essential Conditions

Water: activates metabolism, softens seed coat and allows stored food to become available.
Oxygen: required for aerobic respiration and ATP production.
Suitable temperature: permits enzyme-controlled reactions to proceed efficiently.
Viable seed: the embryo must be alive and capable of growth.

Main Events

Imbibition of water.
Activation of enzymes and respiration.
Mobilization of stored food.
Radicle emerges first and forms the young root.
Plumule develops into the shoot.

Fig. 1 – Main Stages of Seed Germination

2. Seed Dormancy

DefinitionDormancy is a temporary condition in which a viable seed fails to germinate even when apparently favourable environmental conditions are available.

Causes

Hard or impermeable seed coat.
Immature embryo.
Presence of chemical inhibitors.
Requirement for a specific temperature or light treatment.

Breaking Dormancy

Scarification: weakening or breaking a hard seed coat.
Stratification: exposure to moist chilling or another required temperature regime.
Washing away inhibitors.
Suitable light or temperature treatment depending on species.

Fig. 2 – Dormant Seed to Germinating Seed

3. Photoperiodism

DefinitionPhotoperiodism is the physiological response of plants to the relative lengths of day and night, especially in relation to flowering.

Short-Day Plants

Flower when the day length is shorter than a critical value, usually when the uninterrupted night is sufficiently long.

Long-Day Plants

Flower when the day length exceeds a critical value or when the night is sufficiently short.

Day-Neutral Plants

Flowering is comparatively independent of day length within normal environmental ranges.

Important Idea

In many plants, the duration of the dark period is especially important in controlling flowering.

Fig. 3 – Photoperiodism

4. Vernalization

DefinitionVernalization is the induction or acceleration of flowering by exposing seeds, seedlings or growing plants to a suitable period of low temperature.

It is important in many winter cereals and biennial plants. The cold treatment allows the plant to acquire the competence to flower when later conditions become favourable.

Fig. 4 – Concept of Vernalization

5. Senescence

DefinitionSenescence is the genetically regulated process of ageing that leads to progressive deterioration and eventual death of cells, organs or the whole plant.

Chlorophyll breaks down and leaves may yellow.
Photosynthetic activity decreases.
Proteins and other cellular components are degraded.
Nutrients may be transferred from ageing organs to seeds or growing tissues.
Senescence may affect a single organ, a group of organs or the entire plant.

6. Plant Movements

6.1 Tropic Movements

DefinitionA tropic movement is a directional growth response in which the direction of movement depends on the direction of the external stimulus.

Tropism	Stimulus	Typical Response
Phototropism	Light	Young shoots usually bend toward light.
Gravitropism	Gravity	Roots usually grow with gravity; shoots against gravity.
Hydrotropism	Water	Roots grow toward a region of greater moisture.
Thigmotropism	Touch	Tendrils coil around a support.
Chemotropism	Chemicals	Pollen tube grows toward the ovule under chemical guidance.

Fig. 5 – Positive Phototropism of a Shoot

Fig. 6 – Gravitropism of Root and Shoot

6.2 Nastic Movements

DefinitionA nastic movement is a non-directional response in which the direction of movement does not depend on the direction from which the stimulus comes.

Photonasty: response to changes in light intensity.
Nyctinasty: sleep movement associated with day-night changes.
Thigmonasty / seismonasty: rapid response to touch or mechanical shock, as in Mimosa pudica.

Fig. 7 – Nastic Movement in Mimosa

7. Quick Revision & Exam Points

Important Questions

Define seed germination and state its essential conditions.
What is seed dormancy? Mention methods of breaking it.
Define photoperiodism and classify plants according to photoperiodic response.
Define vernalization and state its significance.
Define senescence.
Differentiate tropic and nastic movements.
Explain phototropism and gravitropism.
Give examples of nastic movement.

Germination → resumption of embryo growth.
Water + oxygen + suitable temperature are essential.
Dormancy prevents immediate germination.
Photoperiodism → response to day/night length.
Vernalization → cold-induced flowering competence.
Senescence → ageing process.
Tropism is directional.
Nastic movement is non-directional.
Shoot → positive phototropism.
Root → positive gravitropism.

Source handling: The original handwritten PDF remains embedded above. The typed section is a syllabus-aligned reconstruction designed to follow the same chapter scope while providing clearer searchable text and mobile-friendly labelled diagrams.

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