Samacheer Class 9 Science - Plant Physiology

• (a) Phototropism
• (b) Geotropism
• (c) Thigmotropism
• (d) Chemotropism

Answer:
(c) Thigmotropism

• (a) (CO_2) is reduced and water is oxidized
• (b) Water is reduced and (CO_2) is oxidized
• (c) Both are oxidized
• (d) Both are produced

Answer:
(a) (CO_2) is reduced and water is oxidized

• (a) Thigmonasty
• (b) Phototropism
• (c) Hydrotropism
• (d) Photonasty

Answer:
(c) Hydrotropism

• (a) Chemotropism
• (b) Geotropism
• (c) Phototropism
• (d) Thigmotropism

Answer:
(c) Phototropism

• (i) positively phototropic but negatively geotropic
• (ii) positively geotropic but negatively phototropic
• (iii) negatively phototropic but positively hydrotropic
• (iv) negatively hydrotropic but positively phototropic

Answer:
(b) (ii) and (iii)

• (a) Thermotropism
• (b) Thermonasty
• (c) Chemotropism
• (d) Thigmonasty

Answer:
(b) Thermonasty

• (a) Photosynthesis
• (b) Tropic movement
• (c) Transpiration
• (d) Nastic movement

Answer:
(a) Photosynthesis

False. Response to chemical stimulus is not called phototropism. Response to chemical stimulus is called chemotropism. Phototropism is the directional growth response of plants to light stimulus, where the shoot grows towards the light source. Chemotropism, on the other hand, is the growth response of plants to chemical stimuli, such as the growth of pollen tubes towards the ovule guided by chemical signals. These are distinct tropisms, each responding to different environmental stimuli.

True. Shoot is positively phototropic and negatively geotropic. This means that the shoot grows towards light (positive phototropism) and grows away from the direction of gravity (negative geotropism). The positive phototropic response helps the shoot maximize light exposure for photosynthesis. The negative geotropic response ensures that the shoot grows upward against gravity, allowing it to reach towards the light source. These responses are controlled by plant hormones, particularly auxins, which redistribute in response to light and gravity.

False. During hot weather, water evaporates more, not less. However, the statement about stomata is partially correct. During hot weather, water loss through transpiration increases because higher temperatures increase the rate of evaporation. To minimize excessive water loss and prevent desiccation, stomata partially close during hot weather, reducing the rate of transpiration. This is a protective mechanism that helps plants conserve water during periods of high temperature and potential drought stress. The partial closure of stomata reduces water loss while still allowing some gas exchange for photosynthesis.

False. Photosynthesis does not produce glucose and carbon dioxide. Photosynthesis produces glucose and oxygen. In this process, plants use light energy, water, and carbon dioxide to synthesize glucose, a simple sugar that serves as an energy source and building block for plant growth. Oxygen is released as a byproduct of photosynthesis. The overall equation for photosynthesis is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Carbon dioxide is a reactant, not a product of photosynthesis.

True. Photosynthesis helps maintain oxygen balance in the atmosphere. During photosynthesis, plants absorb carbon dioxide and release oxygen as a byproduct. This process is crucial for maintaining the atmospheric composition suitable for aerobic life on Earth. Plants, algae, and cyanobacteria are the primary producers of oxygen through photosynthesis. The oxygen released during photosynthesis is used by animals and plants for respiration. Additionally, photosynthesis also helps reduce atmospheric carbon dioxide levels, playing an important role in regulating the carbon cycle and mitigating climate change.

Answer: False. Correct Statement: Plants lose water mainly when stomata are open. When stomata are closed, the openings through which water vapour escapes are sealed, so water loss through transpiration is minimized. Stomata open during the day to allow gas exchange for photosynthesis, and this is when maximum water loss occurs. At night or during stress conditions, stomata close to conserve water.

Answer: Nastic movement is a non-directional movement of plant parts in response to a stimulus. Unlike tropic movements, nastic movements do not depend on the direction of the stimulus. The direction of movement is determined by the structure of the plant organ itself, not by where the stimulus comes from. Examples include the opening and closing of flowers in response to light and temperature, and the rapid closing of leaves in Mimosa pudica in response to touch. These movements are growth-independent and occur due to changes in turgor pressure in specialized cells.

Answer: (a) The plant part that bends towards gravity and away from light is the root system. Roots exhibit positive geotropism, meaning they grow downward in response to gravity, and negative phototropism, meaning they grow away from light sources. (b) The plant part that bends towards light and away from gravity is the shoot system. Shoots exhibit negative geotropism, growing upward against the pull of gravity, and positive phototropism, growing towards light sources to maximize photosynthesis.

Examples:

• Phototropism → stem growth towards light
• Photonasty → dandelion flower opening

(a) What are X and Y?

• X → Light energy
• Y → Chemical energy

(b) Why are Green Plants Autotrophic?

Answer:

Green plants prepare their own food by photosynthesis.

Answer: Transpiration is the loss of water in the form of water vapour from the aerial parts of plants, primarily through the stomata on leaves. It is an important physiological process that helps in the transport of minerals and nutrients from roots to leaves, maintains turgor pressure in cells, and cools the plant through evaporative cooling. Transpiration occurs mainly during the day when stomata are open for gas exchange, and the rate depends on factors such as temperature, humidity, wind speed, and light intensity.

Answer: Guard cells surround stomata. These are specialized cells that control the opening and closing of stomata by changing their turgor pressure. When guard cells are turgid, they swell and the stoma opens, allowing gas exchange and transpiration. When guard cells lose turgor and become flaccid, the stoma closes, reducing water loss. Guard cells contain chloroplasts and are sensitive to light, allowing stomata to open during the day and close at night.

Answer: (a) Growth-dependent movement is called tropic movement. Tropic movements are directional responses to stimuli where the direction of movement is determined by the direction of the stimulus. These movements involve cell elongation and are permanent. Examples include phototropism and geotropism. (b) Growth-independent movement is called nastic movement. Nastic movements are non-directional responses to stimuli where the direction of movement is independent of the stimulus direction. These movements occur due to changes in turgor pressure and are reversible. Examples include photonasty and thigmonasty.

Answer:

Pneumatophores are specialized breathing roots in mangrove plants.

They show negative geotropism because they grow upward, away from gravity.

Features:

• Grow upward from soil
• Possess lenticels
• Help gaseous exchange in waterlogged soil

Answer: Chlorophyll is a green pigment present in the chloroplasts of plant cells that is responsible for trapping light energy during photosynthesis. It absorbs light energy, particularly in the blue and red wavelengths, and converts this light energy into chemical energy in the form of ATP and NADPH. These energy-rich molecules are then used to synthesize glucose from carbon dioxide and water. Chlorophyll is essential for the light-dependent reactions of photosynthesis and gives plants their characteristic green colour.

Answer:

Roots show positive geotropism.

Reason

• Anchor plant firmly
• Absorb water and minerals
• Grow deep into soil

# Sunflower Movement

• Growth movement
• Directional response to light
• Phototropism

# *Mimosa pudica* Leaf Folding

• Non-growth movement
• Response to touch
• Thigmonasty

Answer:

Procedure

• Cover leafy branch with plastic bag.
• Tie tightly.
• Keep in sunlight.

Observation

Water droplets appear inside bag.

Conclusion

Leaves release water vapour by transpiration.

Answer: (a) The phenomenon exhibited by Plants A and B is photonasty, which is the non-directional movement of plant parts in response to light stimulus. (b) Examples of plants showing photonasty are Dandelion (Taraxacum officinale), whose flowers open in light and close in darkness, and Moon flower (Ipomoea alba), whose flowers open at night and close during the day. (c) The phenomenon exhibited by Plant C is thigmonasty, which is the non-directional movement of plant parts in response to touch or mechanical stimulus. (d) An example of a plant showing thigmonasty is Mimosa pudica, commonly known as the sensitive plant, whose leaves rapidly fold when touched.