Biology

NIOS Class 12 Biology Chapter 20 Terminal Exercises

The NIOS Biology Growth and Development in Plants chapter explains how plants grow and respond to internal and external factors. Topics include growth hormones, tropisms, and stages of plant development. Students also learn about environmental influences on growth.

Solved terminal exercises provide structured answers for exam preparation. This chapter helps students link physiology with practical observations in plants, making it an important part of the NIOS Class 12 Biology curriculum.

NIOS Biology Growth And Development In Plants

1. State the different stages of cellular growth.

(i) Cell division: The number of cells increases due to mitosis.
(ii) Cell enlargement: The size of an individual cell increases after cell division due to an increase in the volume of its protoplasm.
(iii) Cell differentiation: In this stage, the structure of the cells changes to perform specific functions. And similar types of cells having the same functions form a group, which is known as a tissue.

2. Distinguish between growth and development.

Growth in living organisms may be defined as an irreversible increase in the number and size of a cell, organ, or whole organism.
Development is the whole series of qualitative and quantitative changes, such as growth, differentiation, and maturation, which an organism undergoes throughout its life cycle.

3. What is a sigmoid growth curve? State the different phases of the sigmoid curve.

The Sigmoid Growth Curve is the S-shaped curve obtained when the increase in cell number (growth rate) is plotted against time. It has three phases of growth.

(i) Lag Phase – This is the initial phase of growth when the rate of growth is very slow.
(ii) Log Phase – It shows rapid growth and is maximum during the entire life span.
(iii) Stationary Phase – Here, the rate of growth starts decreasing, and finally it stops.

4. Describe the various external factors that affect the growth of plants.

External factors are those factors present in the environment that affect the growth

of the plants directly or indirectly. These factors are (i) Light, (ii) Temperature, (iii) Water (iv) Mineral nutrients

(i) Light: Besides photosynthesis, light is also essential for seed germination, growth of the seedling, differentiation of various tissues and organs, and reproduction.
(ii) Temperature: Some plants grow in cold climates and some in hot climates. The optimum
temperature required for the growth of plants ranges between 28-30°C, but it may occur in the temperature range of 4-45°C. All metabolic activities of plants are directly affected by variations in temperature. A very low temperature causes injuries to the plant due to chilling and freezing, and a very high temperature stops its growth.
(iii) Water: You have already learnt that a plant absorbs water by its roots, uses it in photosynthesis and other biochemical processes, and some of it is lost through transpiration. For the proper growth of plants, a particular quantity of water is required. Both deficiency and excess of water retards the growth of plants.
(iv) Mineral Nutrients: All metabolic processes require inorganic nutrients. Plant growth is adversely affected by the deficiency of nutrients.

5. What is vernalisation?

Vernalisation is the low-temperature treatment that stimulates early flower formation in some plants. In other words, vernalisation is the process of accelerating the process of flowering by subjecting or exposing the plant to low temperatures.

Temperature is reduced to a particular point, then flowering occurs at an early stage. For example, by applying a temperature ranging between 1-10° C to a certain variety of wheat, rice, and cotton, the growth of seedlings is accelerated, and flowering occurs earlier.

6. Define the term Photoperiodism.

Photoperiodism is the response in growth, transpiration, photosynthesis, and reproduction (flowering) of a plant to the specific duration of light that falls on it per day.

7. What is auxin? What is its role in the growth of plants?

Auxin is a growth promoter, generally produced by the growing apex of the stem and root of the plants. It helps in the elongation of the shoot and root tips behind the apical meristem.

Functions of Auxin

(a) It promotes cell elongation;
(b) It suppresses the growth of the lateral bud and promotes the growth of only the apical cells.
(c) It delays the fall of leaves. (leaf abscission)
(d) NAA (Naphthalene acetic acid) prevents fruit drop in apples before they are ripe.
(e) 2, 4-D (2, 4-dichlorophenoxy acetic acid) acts as a dicot weedicide.

8. State any two functions of Gibberellin?

Functions of Gibberellins

(a) It helps in the elongation of stems in genetically dwarf plants. By using gibberellin, the height of the dwarf plants can be increased.
(b) It breaks dormancy of seeds and buds.
(c) It induces parthenocarpy. (Formation of seedless fruits without fertilization) or provides the stimulus received by pollination.

9. Explain the role of Cytokinins and Ethylene in the growth and development of plants.

Cytokinins

Cytokinins are synthesized in the root apex, endosperm of seeds, and young fruits, where

Cell division takes place continuously.

Functions of Cytokinins

(a) They stimulate cell division, cell enlargement, and cell differentiation.
(b) They prevent the aging of plant parts.
(c) They inhibit apical dominance and help in the growth of lateral buds into branches.

Ethylene

Ethylene is a gaseous hormone. It is found in ripening fruits, young flowers, and young leaves.

Functions of Ethylene

(a) It induces the ripening of fruits.
(b) It promotes senescence and abscission of leaves and flowers.
(c) In cells, it only increases the width, not the length.

10. Distinguish between epigeal germination and hypogeal germination.

(a) Epigeal Germination: Because of more growth of the hypocotyl, cotyledons come above the ground and form the first leaves of the new plant, e.g., in castor, neem, and bean, and the plumule forms the shoot.
(b) Hypogeal Germination: Because of poor growth of the hypocotyl, the cotyledons remain underground, and the plumule emerges from the soil to develop into the shoot system. e.g., maize and rice.

11. What is meant by seed germination? Describe the various factors responsible for seed germination.

Seed germination is the return of metabolic activities and growth by the seed tissue to give rise to a new plant through the development of the embryo.

Seed germination requires five factors: water, temperature, oxygen, light, and growth hormones.

(a) Water: The seed must swell up to rupture its seed coat. A ripe seed contains a very low quantity of water. So, for swelling to cause rupture of the seed coats supply of adequate water is essential. Biochemical reactions required for the growth and development of the seedling require water.
(b) Temperature: For the germination of seeds, a particular temperature is required. The degree of temperature required varies from species to species. Warmth accelerates chemical reactions inside.
(c) Oxygen: Oxygen is required in breaking down the reserve food of the seed and releasing energy for the metabolism of growth of the embryo.
(d) Light: In most of the seeds, light is not an essential factor for germination. But in some cases, like lettuce and tobacco, light is essential.
(e) Hormone: Besides the above external factors, hormones also control the germination of seeds.

12. What is senescence?

The deteriorative processes that ultimately lead to the complete loss of organization and functioning of the plant or its parts are known as senescence. It is due to a loss in the structure and function of an organ or the whole plant.

13. State any two practical utilities of growth hormones.

(i) With the help of auxins and gibberellins, seedless varieties of fruits can be produced. You might have seen seedless grapes and papayas in the market.
(ii) Early flowering in some plants is possible by applying growth regulators.
(iii) With the use of hormones, some fruits can be ripened at an early stage.
(iv) Germination in seeds can be possible by applying auxins.
(v) Germination of potatoes and onions can be stopped in storage by the application of growth inhibitors.

14. What is biological stress? Describe the different types of biological stress.

Any change in the environmental conditions that may adversely affect the growth or development of plants is called biological stress.

(a) Water Stress: Water stress includes both excess of water (flood) and scarcity of water (drought). Deficiency of water in the plant makes the leaves yellow, and they wilt. The various processes in plants, like photosynthesis and respiration, are reduced, cell enlargement is checked, cell size is deformed, and cell rigidity is lost due to a deficiency of water. Flooding or an excess amount of water in the soil reduces root and shoot growth and causes blackening of root tips and yellowing of leaves.
(b) Salt Stress: Salt stress occurs mainly due to the presence of an excess amount of calcium and sodium salts in the plant body. It causes dehydration of the cell, a change in the shape of the cell, and a disturbance in metabolic processes. Thus, cell growth as well as the growth and development of plants are retarded.

15. What is apical dominance? Name the hormone responsible for it.

Apical dominance is the phenomenon where the apical buds suppress the development of lateral buds. Auxin is the hormone responsible for the orthos phenomenon.

16. What is meant by plant movement? Describe any two types of movement of plants with examples.

Movement of plants, such as bending towards light, opening of buds, etc, in response to external stimuli is called plant movement. There are three types of such plant movements.

Tropic Movement

Movement in plants or in any part of the plants towards or away from some environmental factors is known as tropic (trope: turn) movement. The different types of tropic movements are,

(i) Phototropism: Induced by light, e.g., bending of stems towards light.
(ii) Geotropism: Induced by gravity, e.g., growth of roots towards gravity.
(iii) Thigmotropism: Movement caused by contact, e.g., twining stem and tendril, and the drooping of leaves of the sensitive plant by touch.
(iv) Hydrotropism: Induced by water, i.e., growth of roots towards the source of water.

Nastic Movement

The nastic (nastein: bending) movements are the growth movements resulting from to difference in the rate of growth on opposite sides of an organ, e.g., opening of petals, coiling of leaves, etc. When the upper side of an organ grows faster than the lower side, the movement is called epinasty. (e.g., downward curling of leaf, opening of sepals of goldmohur flower. When the lower side grows more rapidly than the upper side, it is called hyponasty. (e.g., upward curling of leaf blade)

Turgor Movements

These movements are due to a change in the volume of water inside the cell. When more water is present in the cell, it is fully expanded and becomes rigid or hard. Such a condition is called turgidity, and the cell is said to be turgid. When less water is present inside the cell, it is not fully expanded and remains soft. This is called a flaccid condition. The leaves bend in the hot summer due to excessive transpiration on account of the loss of turgidity of the cells of the leaf.