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NIOS Class 12 Biology Chapter 20 Growth and Development in Plants
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Growth and Development in Plants
Chapter: 20
MODULE – III: REPRODUCTION AND HEREDITY
NIOS TEXTBOOK QUESTIONS ANSWERS
INTEXT QUESTIONS 20.1
1. Distinguish between growth and development.
Ans: (i) Growth: It refers to “the increase in number and size of a cell, organ or organism”.
(ii) Development: It is “the series of changes including growth differentiation and maturation.”
2. What is differentiation?
Ans: The process of change in cells, tissues or organs in order to carry out various functions.
3. What role does it play in plant growth and development?
Ans: The similar cells organise to make a group known as the tissue to perform a specific function.
INTEXT QUESTIONS 20.2
1. Name the plant hormones concerned with the following:
(i) Elongation of cell ____________.
Ans: Auxin.
(ii) Shedding of Leaves __________.
Ans: Eethylene.
(ii) Breaking seed dormancy __________.
Ans: Abscisic Acid.
2. Mention two functions of Auxin.
Ans: (i) Cell elongation.
(ii) Delays fall of leaves.
(iii) Suppresses growth of lateral bud.
3. What is the difference between dedifferentiation and redifferentiation?
Ans: Dedifferentiation: It refers to a process by which precursor cells become distinct cell types to perform a specific function.
Redifferentiation: It is the process by which plant cells while undergoing dedifferent-iation lose their capacity to divide once again but they become mature to perform specific functions.
4. Which two hormones are essential for vascular tissue differentiation?
Ans: (i) Auxim.
(ii) Cytokinins.
INTEXT QUESTIONS 20.3
1. Distinguish between Phototropism and Geotropism.
Ans: Movement induced by light-photo-tropism
Movement induced by gravity-Geotropism.
2. Give two examples of turgor movement.
Ans: (i) Closure of leaves on fall of darkness.
(ii) dropping of leaves on touch.
(ii) closing leaves of Venus fly trap to catch a landing insect. (Note: Write any two of these.)
TERMINAL EXERCISES
1. State the different stages of cellular growth.
Ans: (i) Cell division.
(ii) Cell enlargement.
(iii) Coll maturation.
2. Distinguish between growth and development.
Ans. (i) Growth: It can be defined “as an irreversible increase in volume which is usually accompanied by an increase in size and weight of an organism; usually growth is associated with increase in the amount of protoplasm in an organism. The amount of protoplasm increases when anabolic activities are more compared to catabolic activities. True growth involves the production of more protoplasm and in best expressed as an increase in dry mass.
(ii) Development: This is not growth. An organism can grow without developing. Often it develops without growing, but as a rule they go together. A typical organism does not grow at a constant rate.
3. What is a sigmoid growth curve? State the different phases of sigmoid curve.
Ans: Sigmoid Curve: Rate of growth of a plant or its part is not always same during its life span. It may be slow and at other times rapid. If you plot increase in cell number (growth rate) against time, we get a typical S-shape curve. It is known as growth curve or sigmoid curve.
S-shaped growth curve has three phases, namely:
(a) Lag phase: It is “initial phase of growth when rate of growth is very slow”.
Fig. 20.4. Sigmoid curve.
(b) Log phase: It exhibits “rapid growth and is maximum for entire life span.”
(c) Stationary phase: The rate of growth begins to decrease and at last it stopsTotal time period during which all these phases, occur is known as the grand period of growth.
4. Describe the various external factors that affect the growth of plants.
Ans: External Growth Factors: They include light, water, temperature and mineral nutrients.
(a) Light is essential for photosynthesis. Plants that grow in dark, become pale, tall, yellowish as well as weak. The leaves remain small.
(b) Temperature: 28°C to 30°C is optimum temperature for plant growth but it can range from 4°C to 45°C. Temperature affects metabolism and enzyme activity.
(c) Water: Plants absorb water by root system. It is essential for photosynthesis and biochemical processes of plant. In excess and deficiency of water, plant growth is decreased.
(d) Mineral nutrient: Nutrients are needed for growth and development. Their deficiency leads to hunger signs.
5. What is vernalisation?
Ans: Vernalisation: It is “the method of inducing early flowering in plants by keeping their at low temperature.
6. Define the term Photoperiodism.
Ans: The photoperiodism is defined “as the response in growth, reproduction (flowering) of a plant to duration of light, that falls on it per day”.
7. What is auxin? What is its role in the growth of plants?
Ans: Auxin: It is a phytohormone (plant growth regulator). It is produced at root up and stem tip. It is helpful in elongation of shoot and root tips behind apical meristem, e.g. indole 3. Acetive Acid, Synthetic auxins include 2BA, NAA and 2, 4-D. Auxins promote cell division, delay leaf abscission and suppress growth of lateral bud (apical dominance).
8. State any two functions of Gibberellin.
Ans: Gibberellins:
(i) Cause stem elon-gation.
(ii) Break dormancy of seeds.
(iii) Induce parthenocarpy. (Note: Write any two of these)
9. Explain the role of Cytokinins and Ethylene in growth and development of plants.
Ans: (i) Role of Cytokinins: They help in growth of lateral buds. They stimulate cell division, cell enlargement and cell differen-tiation. It checks ageing in plants.
(ii) Role of Ethylene: It increases cell width and promotes senescence. Ethylene promotes abscission of leaf as well as the flowers.
10. Distinguish between epigeal germination and hypogeal germination.
Ans: Difference between Epigeal and Hypogeal Germination:
| Epigeal Germination (Epi = above; geo = soil) | Hypogeal Germination (Hypo = below; geo = soil) |
| 1. It occurs in gram, pea, french beans and mustard. | It occurs in rice, wheat and maize (monocots). |
| 2. The cotyledons come above the ground/soil. | The cotyledons remain under soil/ground. |
Fig. 20.5. (A) Epigeal Germination and (B) Hypogeal Germination
11. What is meant by seed germination? Describe the various factors responsible for seed germination.
Ans: Seed-germination: It is the first part of plant growth. It refers “to the return of metabolic activities and the growth by seed tissue to give rise to a new plant”.
Factors affecting seed germination: Water, temperature, O2, light and hormone influence seed-germination. Water is necessary for seed to germinate. Seed swells up after absorbing water. Seed coat bursts. Warmth affects chemical reactions. Temperature requirement is different from species to species. Breakdown of reserved food occurs in presence of O2 and energy is produced. Light is necessary for seeds to geminate in tobacco and lettuce plants. Abscisic acid decreases seed germi-nation but gibberellins induce it. Auxin, ethylene and cytokinins (plant hormones) break seed dormancy, initiate germination of the seeds.
12. What is senescence?
Ans: The senescence is defined “as a process during which any part of plant or whole plant completely loses its function and finally dies”.
13. State any two practical utilities of growth hormones.
Ans: Two Practical Utilities of Growth Hormones:
(i) By the application of growth regulators such as auxins and gibberellins, many seedless varieties of fruits are produced e.g., papaya.
(ii) The ripening of fruits may occur at an early stage in some plants.
14. What is biological stress? Describe the different types of biological stress.
Ans: Biological stress: It refers “to any change in environmental conditions which may adversely influence growth or development in plants. This stress takes place chiefly deficiency due to temperature, water, salt, various pollutants, shade, light, etc.
(a) Water: In deficiency of water the leaves of plants become pale and they wilt but in excess of water they become pale, root system is reduced. Water content reduces respiration and synthesis of food.
(b) Salt stress: Excess of Ca, Na salts cause salt strees in plants. Other effects are dehydration of cell’s shape of cell in changed, metabolism is disturbed.
15. What is apical dominance? Name the hormone responsible for it.
Ans: Apical Dominance: When the apical bud, at the apex of shoot suppresses the development of lateral buds into branches, the phenomenon is termed as apical dominance. Auxins are responsible for apical dominance in plants.
16. What is meant by plant move-ment? Describe any two types of movement of plants with example.
Ans: Movements in Plants: The plants exhibit movement of their body parts as bending towards light, opening of flowers etc. The movements caused by the changes in the position or organs due to the enlargement of cells or increase in number of cells are called growth movements. They are also known as tropic movements.
Two types of movement of plants are:
(i) Tropic e.g. geotropism and phototropism.
(ii) Nastic-growth movements resulting due to growth rate difference on opposite sides of an organ, e.g. epinasty, hyponasty, etc.
(i) Geotropism: It is induced by gravity. Example roots are positively geotropic while shoot run away from the gravity (negatively. geotropic). (See Fig.)
Fig. 20.6. Geotropism.
(ii) Phototropism: (Heliotropism). The tropical movements which occur in response of an external unilateral light stimulus are called as phototropic movements or phototropism. The stems generally show a curvature towards the source of light and they are called positively phototropic. The roots grow away from the source of light and they are called negatively phototropic.
