Plant Physiology

Class 9 | Science — Biology | Unit 19

Class 9 Unit 19 Science

UNIT 19 PLANT PHYSIOLOGY

Learning Objectives

After completing this lesson, students will be able to:

• know that plants too have certain autonomic movements.
• understand different types of movement in plants.
• differentiate tropic movement from nastic movement.
• gain knowledge on transpiration.
• understand that plants produce their food through the process of photosynthesis.
• understand the process of transpiration.

Introduction

Animals move in search of food, shelter and for reproduction. Do plants show such movement? Have you observed the leaves of Mimosa pudica (touch-me-not plant) closes on touching, whereas Helianthus annuus (sunflower) follows the path of the sun from dawn to dusk, (from east to west). These movements are triggered by an external stimuli. Unlike animals, plants do not move on their own from one place to another, but can move their body parts for getting sunlight, water and nutrients. They are sensitive to external factors like light, gravity, temperature etc. In this lesson, we will study about various movements in plants, photosynthesis and transpiration.

19.1 Tropism in Plants

Tropism is a unidirectional movement of a whole or part of a plant towards the direction of stimuli.

19.1.1 Types of Tropism

Based on the nature of stimuli, tropism can be classified as follows.

Types of Tropism

Phototropism: Movement of a plant part towards light. Example shoot of a plant.

Geotropism: Movement of a plant in response to gravity. Example root of a plant.

Hydrotropism: Movement of a plant or part of a plant towards water. Example root of a plant.

Thigmotropism: Movement of a plant part due to touch. Example climbing vines.

Chemotropism: Movement of a part of plant in response to chemicals. Example growth of a pollen tube in response to sugar present on the stigma.

Tropism is generally termed positive if growth is towards the signal and negative if it is away from the signal. Shoot of a plant moves towards the light, the roots move away. Thus the shoots are positively phototropic.

Usually shoot system of a plant is positively phototropic and negatively geotropic and root system is negatively phototropic and positively geotropic.

Do You Know?

Some halophytes produce negatively geotropic roots (Example Rhizophora). These roots turn 180 degree upright for respiration.

19.2 Nastic Movements

Nastic movements are non-directional response of a plant or part of a plant to stimulus. Based on the nature of stimuli, nastic movements are classified as Photonasty and Thigmonasty.

Types of Nastic Movements

Photonasty: Movement of a part of a plant in response to light. Example Taraxacum officinale, blooms in morning and closes in the evening. Similarly, Ipomea alba (Moon flower), opens in the night and closes during the day.

Thigmonasty: Movement of a part of plant in response to touch. Example Mimosa pudica, folds leaves and droops when touched. It is also known as Seismonasty.

Activity 1

Take a glass trough and fill it with sand. Keep a flower pot containing water, plugged at the bottom at the centre of the glass trough. Place some soaked pea or bean seeds around the pot in the sand. What do you observe after 6 or 7 days? Record your observation.

Activity 2

Take pea seeds soaked in water overnight. Wait for the pea seeds to germinate. Once the seedling has grown put it in a box with an opening for light on one side. After few hours, you can clearly see how the stem has bent and grown towards the light.

Do You Know?

The Venus Flytrap (Dionaea muscipula) presents a spectacular example of thigmonasty. It exhibits one of the fastest known nastic movement.

Table 19.1 Differences between Tropic and Nastic movements

Tropic movements	Nastic movements
Unidirectional response to the stimulus.	Non-directional response to the stimulus.
Growth dependent movements.	Growth independent movements.
More or less permanent and irreversible.	Temporary and reversible.
Found in all plants.	Found only in a few specialized plants.
Slow action.	Immediate action.

Thermonasty: Movement of part of a plant is associated with change in temperature. Example Tulip flowers bloom as the temperature increases.

19.3 Photosynthesis

'Photo' means 'light' and 'synthesis' means 'to build'. Thus photosynthesis literally means 'building up with the help of light'. During this process, the light energy is converted into chemical energy. Green plants are autotrophic in their mode of nutrition because they prepare their food materials through a process called photosynthesis. The overall equation of photosynthesis can be given as below:

Photosynthesis Equation:

6CO₂ + 12H₂O (Carbon dioxide and Water) in the presence of Sunlight and Chlorophyll → C₆H₁₂O₆ + 6H₂O + 6O₂ (Glucose and Water and Oxygen gas)

Do You Know?

Do the insects also trap solar energy? Tel Aviv University Scientists have found out that Vespa orientalis (Oriental Hornets) have similar capabilities to trap solar energy. They have a yellow patch on its abdomen and an unusual cuticle structure which is a stack of 30 layers thick. The cuticle does not contain chlorophyll but it contains the yellow light sensitive pigment called xanthopterin. This works as a light harvesting molecule transforming light energy into electrical energy.

The end product of photosynthesis is glucose which will be converted into starch and stored in the plant body. Plants take in carbon dioxide for photosynthesis; but for its living, plants also need oxygen to carry on cellular respiration.

19.3.1 Requirements for Photosynthesis

Activity 3

Pluck a variegated leaf from Coleus plant kept in sunlight. Destarch it by keeping in dark room for 24 hours. Draw the picture of this leaf and mark the patches of cholorphyll on the leaf. Immerse the leaf in boiling water followed by alcohol and test it for starch using iodine solution. Record your observation.

Activity 4

Place a potted plant in a dark room for about 2 days to destarch its leaves. Cover one of its leaves with the thin strip of black paper as shown in the picture. make sure that the leaf is covered on both sides. Keep the potted plant in bright sunlight for 4 to 6 hours. Pluck the selected covered leaf and remove the black paper. Immerse the leaf in boiling water for a few minutes and then in alcohol to remove chlorophyll. Test the leaf now with iodine solution for the presence of starch. The covered part of the leaf does not turn blue-black whereas the uncovered part of the leaf turns blue-black colour. Why are the changes in colour noted in the covered and uncovered part of the leaf?

These activities show that certain things are necessary for photosynthesis. They are:

1. Chlorophyll Green pigment in leaves
2. Water
3. Carbon dioxide (from air)
4. Sun light

19.4 Transpiration

The loss of water in the form of water vapour from the leaf of the plant body is called as transpiration. The leaves have tiny, microscopic pores called stomata. Water evaporates through these stomata. Each stomata is surrounded by guard cells. These guard cells help in regulating the rate of transpiration by opening and closing of stomata.

19.4.1 Types of Transpiration

There are three types of transpiration:

Types of Transpiration

Stomatal transpiration: Loss of water from plants through stomata. It accounts for 90 to 95 percent of the water transpired from leaves.

Cuticular transpiration: Loss of water in plants through the cuticle.

Lenticular transpiration: Loss of water from plants as vapour through the lenticels. The lenticels are tiny openings that protrude from the barks in woody stems and twigs as well as in other plant organs.

But transpiration is necessary for the following reasons.

1. It creates a pull in leaf and stem.
2. It creates an absorption force in roots.
3. It is necessary for continuous supply of minerals.
4. It regulates the temperature of the plant.

Activity 5

Take a plastic bag and tie it over a leaf and place the plant in light. You can see water condensing inside the plastic bag. The water is let out by the leaves. Why does this occur?

19.4.2 Exchange of Gases

How does the plant get air? The leaves have minute pores called stomata through which the exchange of air takes place. These minute pores can be seen through a microscope. Air exchange takes place continuously through the stomata. Plants exchange gases (CO₂ to O₂) continuously through these stomata. You will study more about these physiological process in your higher classes.

Points to Remember

1. Growth movement whose direction is determined by the direction of the stimulus is called Tropism.
2. Non-directional, response of a plant part to stimulus is called nastic movement.
3. The process by which plants prepare their food material is called photosynthesis.
4. The loss of water in the form of water vapour from the aerial parts of the plant body is called transpiration.
5. Stomata are minute opening on the leaves.

GLOSSARY

Phototropism Unidirectional movement of a plant part to light stimulus. Geotropism Response of a plant part to gravity stimulus. Hydrotropism Response of a plant part to water stimulus. Thigmotropism Response of a plant part to touch stimulus. Chemotropism Response of a plant part to chemical stimulus. Thigmonasty Non-directional movement of a plant part in response to touch of an stimulus. Photonasty Non-directional movement of a plant part in response to light stimulus.

TEXT BOOK EXERCISES

Exercise 1. Choose the correct answer.

1. The tropic movement that helps the climbing vines to find a suitable support is

a. Phototropism b. Geotropism c. Thigmotropism d. Chemotropism

2. The chemical reaction occurs during photosynthesis is

a. CO₂ is reduced and water is oxidized b. water is reduced and CO₂ is oxidized c. both CO₂ and water are oxidized d. both CO₂ and water are produced

3. The bending of root of a plant in response to water is called

a. Thigmonasty b. Phototropism c. Hydrotropism d. Photonasty

4. A growing seedling is kept in the dark room. A burning candle is placed near it for a few days. The tip part of the seedling bends towards the burning candle. This is an example of

a. Chemotropism b. Geotropism c. Phototropism d. Thigmotropism

5. The root of the plant is

(1) positively phototropic but negatively geotropic

(2) positively geotropic but negatively phototropic

(3) negatively phototropic but positively hydrotropic

(4) negatively hydrotropic but positively phototropic

a. (1) and (2) b. (2) and (3) c. (3) and (4) d. (1) and (4)

6. The non-directional movement of a plant part in response to temperature is called

a. Thermotropism b. Thermonasty c. Chemotropism d. Thigmonasty

7. Chlorophyll in a leaf is required for

a. photosynthesis b. tropic movement c. transpiration d. nastic movement

8. Transpiration takes place through

a. fruit b. seed c. flower d. stomata Exercise 2. Fill in the blanks.

1. The shoot system grows upward in response to .

2. is positively hydrotropic as well as positively geotropic.

3. The green pigment present in the plant is .

4. The solar tracking of sunflower in accordance with the path of sun is due to .

5. The response of a plant part towards gravity is .

6. Plants take in carbon di oxide for photosynthesis but need for their living.

Exercise 3. Match column A with column B.

Column A	Column B
Roots growing downwards into soil.
Shoots growing towards the light.
Shoots growing upward.
Roots growing downwards away from light.

Options: Positive phototropism, Negative geotropism, Negative phototropism, Positive geotropism

Exercise 4. State whether true or false. If false, correct the statement. 1. The response of a part of plant to the chemical stimulus is called phototropism. 2. Shoot is positively phototropic and negatively geotropic. 3. When the weather is hot, water evaporates lesser which is due to opening of stomata. 4. Photosynthesis produces glucose and carbon dioxide. 5. Photosynthesis is important in releasing oxygen to keep the atmosphere in balance. 6. Plants lose water when the stomata on leaves are closed. Exercise 5. Answer very briefly.

1. What is nastic movement?

2. Name the plant part a) Which bends in the direction of gravity but away from the light. b) Which bends towards light but away from the force of gravity.

3. Differentiate phototropism from photonasty.

4. Photosynthesis converts energy X into energy Y. a) What are X and Y? b) Green plants are autotrophic in their mode of nutrition. Why?

5. Define transpiration.

6. Name the cell that surrounds the stoma.

Exercise 6. Answer briefly.

1. Give the technical terms for the following: a) Growth dependent movement in plants. b) Growth independent movement in plants.

2. Explain the movement seen in Pneumatophores of Avicennia.

3. Fill in the blanks:

4. What is chlorophyll?

5. Name the part of plant which shows positive geotropism. Why?

6. What is the difference between movement of flower in sunflower plant and closing of the leaves in the Mimosa pudica?

7. Suppose you have a rose plant growing in a pot, how will you demonstrate transpiration in it?

8. Mention the differences between stomatal and lenticular transpiration

9. To which directional stimuli do (a) roots respond (b) shoots respond?

Exercise 7. Answer in detail.

1. Differentiate between tropic and nastic movements

2. How will you differentiate the different types of transpiration?

Exercise 8. Higher Order Thinking Skills.

1. There are 3 plants A, B and C. The flowers of A open their petals in bright light during the day but closes when it gets dark at night. On the other hand, the flowers of plant B open their petals at night but closes during the day when there is bright light. The leaves of plant C fold up and droop when touched with fingers or any other solid object. a) Name the phenomenon shown by the flowers of plant A and B. b) Name one plant each which behaves like the flowers of plant A and B. c) Name the phenomenon exhibited by the leaves of plant C. d) Name the plant which behaves like the leaves of plant 'C'?

2. Imagine that student A studied the importance of certain factors in photosynthesis. He took a potted plant and kept it in dark for 24 hours. In the early hours of the next morning, he covered one of the leaves with dark paper in the centre only. Then he placed the plant in sunlight for a few hours and tested the leaf which was covered with black paper for starch. a) What aspect of photosynthesis was being investigated? b) Why was the plant kept in the dark before the experiment? c) How will you prove that starch is present in the leaves? d) Name the raw materials needed for photosynthesis.

REFERENCE BOOKS

1. Devlin and Witham, 1986. Plant Physiology, 1st edition
2. B.P. Pandey 2003 Modern Practical Botany Vol. II.
3. V.K. Jain 2003 Plant physiology.

INTERNET RESOURCES

• http://web.mit.edu/esgbio
• http://www.bioedonline.org/
• http://www.biology.arizona.edu/default.html

📝 Summary

1. Plants exhibit movements triggered by external stimuli, such as Mimosa pudica closing its leaves when touched and sunflowers following the sun's path from dawn to dusk.
2. Tropism is a unidirectional movement of plants toward stimuli, including phototropism (toward light), geotropism (response to gravity), hydrotropism (toward water), thigmotropism (due to touch), and chemotropism (response to chemicals).
3. Shoots are typically positively phototropic and negatively geotropic, while roots are negatively phototropic and positively geotropic, with movements termed positive when toward the signal and negative when away from it.
4. Nastic movements are non-directional responses to stimuli, classified as photonasty (response to light) like Taraxacum officinale blooming in morning and closing in evening, and thigmonasty (response to touch) like Mimosa pudica folding leaves when touched.
5. Plants are sensitive to external factors like light, gravity, and temperature, moving their body parts to obtain sunlight, water, and nutrients rather than moving from place to place like animals.