PLANT PHYSIOLOGY Notes 9th Science Lesson 22 Notes in English

PLANT PHYSIOLOGY Notes 9th Science Lesson 22 Notes in English

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.

Tropism in Plants

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

Types of Tropism

• Based on the nature of stimuli, tropism can be classified as follows.
• Phototropism: Movement of a plant part towards light. e.g. shoot of a plant.
• Geotropism: Movement of a plant in response to gravity. e.g. root of a plant.
• Hydrotropism: Movement of a plant or part of a plant towards water. e.g root of a plant.
• Thigmotropism: Movement of a plant part due to touch. e.g. climbing vines.
• Chemotropism: Movement of a part of plant in response to chemicals. e.g 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.

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 follows.

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.

Photonasty:

• Movement of a part of a plant in response to light. e.g.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. e.g. Mimosa pudica, folds leaves and droops when touched. It is also known as Seismonasty.

Thermonasty:

• Movement of part of a plant is associated with change in temperature. e.g. Tulip flowers bloom as the temperature increases.
• 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.

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:

Activity 3

• Pluck a variegated leaf from Coleus plant kept in sunlight.
• De- starch 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 de- starch 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

Transpiration

• The loss of water in the form of water vapour from the aerial parts 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.

Types of Transpiration

• There are three types of transpiration:
• Stomatal transpiration: Loss of water from plants through stomata. It accounts for 90- 95% 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?

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.

MORE TO KNOW:

Halophytes

• Some halophytes produce negatively geotropic roots (e.g. Rhizophora).
• These roots turn 180° upright for respiration.

Nastic movement

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

Xanthopterin

• 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.

EXTRA POINTS:

• 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.