Plant Growth and Development

CBSE Class 11 Biology

NCERT Solutions
Chapter 15
Plant Growth and Development

1. Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.

Ans. (a) Growth: It is an irreversible and permanent process, accomplished by an increase in the size of an organ or organ parts or even of an individual cell.

(b) Differentiation: It is a process in which the cells derived from the apical meristem (root and shootapex) and the cambium undergo structural changes in the cell wall and the protoplasm, becoming mature to perform specific functions.

(c) Development: It refers to the various changes occurring in an organism during its life cycle - from the germination of seeds to senescence.

(d) De-differentiation: It is the process in which permanent plant cells regain the power to divide under certain conditions.

(e) Re-differentiation: It is the process in which de-differentiated cells become mature again and lose their capacity to divide.

(f) Determinate growth: It refers to limited growth. For example, animals and plant leaves stop growing after having reached maturity.

(g) Meristem In plants, growth is restricted to specialised regions where active cell divisions take place. Such a region is called meristem. There are three types of meristems - apical meristem, lateral meristem, and intercalary meristem.

(h) Growth rate: It can be defined as the increased growth in plants per unit time.

2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?

Ans. In plants, growth is said to have taken place when the amount of protoplasm increases. Measuring the growth of protoplasm involves many parameters such as the weight of the fresh tissue sample, the weight of the dry tissue sample, the differences in length, area, volume, andcell numbermeasured during the growth period. Measuring the growth of plants using only one parameter does not provide enough information and hence, is insufficient for demonstrating growth.

3. Describe briefly:

(a) Arithmetic growth

(b) Geometric growth

(d) Absolute and relative growth rates

Ans. (a) Arithmetic growth: In arithmetic growth, one of the daughter cells continues to divide, while the other differentiates into maturity. The elongation of roots at a constant rate is an example of arithmetic growth.

(b) Geometric growth: Geometric growth is characterised by a slow growth in the initial stages and a rapid growth during the later stages. The daughter cells derived from mitosis retain the ability to divide, but slow down because of a limited nutrientsupply.

(c) Sigmoid growth curve: The growth of living organisms in their natural environment is characterised by an S-shaped curve called sigmoid growth curve. This curve is divided into three phases â€“ lag phase, log phase or exponential phase of rapid growth, and stationary phase. Exponential growth can be expressed as:

$W_{1} = W_{0} e^{n}$

(d) Absolute and relative growth rates: Absolute growth rate refers to the measurement and comparison of total growth per unit time.Relative growth rate refers to the growth of a particular system per unit time, expressed on a common basis.

4. List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.

Ans. Plant growth regulators are the chemical molecules secreted by plants affecting the physiological attributes of a plant. There are five main plant growth regulators. These are

(i) Auxins : The first observations regarding the effects of auxins were made by Charles Darwin and Francis Darwin wherein they saw the coleoptiles of canary grass bending toward a unilateral source of light. It was concluded after a series of experiments that some substance produced at the tip of coleoptiles was responsible for the bending. Finally, this substance was extracted as auxins from the tips of coleoptiles in oat seedlings.

Physiological functions:

1. They control plant cell-growth.

2. They cause the phenomenon of apical dominance.

3. They control division in the vascular cambium and xylem differentiation.

4. They induce parthenocarpy and preventabscissionof leaves and fruits.

Horticultural applications:

1. They are used as the rooting hormones in stem cuttings.

2. 2-4 D is used weedicide to killbroadleaf, dicotyledonous weeds.

3. They induce parthenocarpy in tomatoes.

4. They promote flowering in pineapple and litchi.

(ii) Gibberellic acid: Bakane or the "foolish rice seedling" disease was first observed by Japanese farmers. In this disease, rice seedlings appear to grow taller than natural plants, and become slender and pale green. Later, after several experiments, it was found that this condition was caused by the infection from a certain fungus Gibberella fujikuroi.The active substance was isolated and identified as gibberellic acid.

Physiological functions:

1. It causes elongation of internodes.

2. It promotes bolting in rosette plants.

3. It helps in inducing seed germination by breaking seed dormancy and initiating the synthesis of hydrolases enzymes for digesting reserve food.

Horticultural applications:

1. It helps in increasing the sugar content in sugarcane by increasing the length of the internodes.

2. It increases the length of grape stalks.

3. It improves the shape of apple.

4. It delays senescence.

5. It hastens maturity and induces seed-production in juvenile conifers.

(iii) Cytokinins: Through their experimental observations, F. Skoog and his co-workers found that the tobaccocallusdifferentiated when extracts of vascular tissues, yeastextract, coconut milk, or DNA were added to the culturemedium. This led to the discovery of cytokinins.

Physiological functions:

1. They promote the growth of lateral branches by inhibiting apical dominance.

2. They help in the production of new leaves,chloroplasts, and adventitious shoots.

3. They help in delaying senescence by promoting nutrient mobilisation.

Horticultural applications:

1. They are used for preventing apical dominance.

2. They are used for delaying senescence in leaves.

(iv) Ethylene: It was observed that unripe bananas ripened faster when stored with ripe bananas. Later, the substance promoting the ripening was found to be ethylene.

Physiological functions:

1. It helps in breaking seed and bud dormancy.

2. It promotes rapid internode-elongation in deep-water rice plants.

3. It promotes root-growth and formation of root hairs.

4. It promotes senescence andabscissionof leaves and flowers.

5. It hastens the respiration rate in fruits and enhances fruit ripening.

Horticultural applications:

1. It is used to initiate flowering and synchronising the fruit set in pineapples.

2. It induces flowering in mango.

3. Ethephon is used to ripen the fruits in tomatoes and apples, and accelerate theabscissionof flowers and leaves in cotton, cherry, and walnut.

4. It promotes the number of female flowers in cucumbers.

(v) Abscisic acid: During the mid 1960s, inhibitor-B,abscissionII, and dormin were discovered by three independent researchers. These were later on found to bechemicallysimilar and were thereafter called ABA (Abscisic acid).

Physiological functions:

1. It acts as an inhibitor to plant metabolism.

2. It stimulates stomatal closure during water stress.

3. It induces seed dormancy.

4. It inducesabscissionof leaves, fruits, and flowers.

Horticultural application:

It induces seed dormancy in stored seeds.

5. What do you understand by photoperiodism and vernalisation? Describe their significance.

Ans. Photoperiodism refers to the response of plants with respect to the duration of light (i.e., period of day and night). On the basis of its response to the duration of light, a plant is classified as a short-day plant, a long-day plant, or a day-neutral plant. Short-day plants flower when they are exposed to light for a period less than the critical day-length. Long-day plants flower when they are exposed to light for a period more than the critical day-length (for example: radish). When no marked correlation is observed between the duration of exposure to light and the flowering response, plants are termed as day-neutral plants (for example: tomato).It is hypothesised that the hormonal substance responsible for flowering is formed in the leaves, subsequently migrating to the shoot apices and modifying them into flowering apices. Photoperiodism helps in studying the response of flowering in various crop plants with respect to the duration of exposure to light.

Vernalisation is the cold-induced flowering in plants. In some plants (such as the winter varieties of wheat and rye and biennials such as carrot and cabbage), exposure to low temperature is necessary for flowering to be induced. The winter varieties of rye and wheat are planted in autumn. They remain in the seedling stage during winters and flower during summers. However, when these varieties are sown in spring, they fail to flower. Similar response is seen in cabbage and radish.

6. Why is abscisic acid also known as stress hormone?

Ans. Abscisic acid is called stress hormones as it induces various responses in plants against stress conditions.It increases the tolerance of plants toward various stresses. It induces the closure of the stomata during water stress. It promotes seed dormancy and ensures seed germination during favourable conditions. It helps seeds withstand desiccation. It also helps in inducing dormancy in plants at the end of the growing season and promotes abscission of leaves, fruits, and flowers.

7. ‘Both growth and differentiation in higher plants are open’. Comment.

Ans. Growth and development in higher plants is referred to as being open. This is because various meristems, having the capacity for continuously dividing and producing new cells, are present at different locations in these plant bodies.

8. ‘Both a short day plant and a long day plant can produce can flower simultaneously in a given place’. Explain.

Ans. The flowering response in short-day plants and long-day plants is dependent on the durations for which these plants are exposed to light. The short-day plant and long-day plant can flower at the same place, provided they have been given an adequate photoperiod.

9. Which one of the plant growth regulators would you use if you are asked to:

(a) induce rooting in a twig

(b) quickly ripen a fruit

(d) induce growth in axillary buds

(e) ‘bolt’ a rosette plant

(f) induce immediate stomatal closure in leaves.

Ans. (a) Auxine

(b) Ethylene

(d) Cytokinins

(e) Gibberellic acid

(f) Abscisic acid

10. Would a defoliated plant respond to photoperiodic cycle? Why?

Ans. A defoliated plant will not respond to the photoperiodic cycle.

It is hypothesised that the hormonal substance responsible for flowering is formed in the leaves, subsequently migrating to the shoot apices and modifying them into flowering apices. Therefore, in the absence of leaves, light perception would not occur, i.e., the plant would not respond to light.

11. What would be expected to happen if:

(a) GA3 is applied to rice seedlings

(b) dividing cells stop differentiating

(d) you forget to add cytokinin to the culture medium.

Ans. (a) Rice seedlings will grow extremely tall.

(b) An undifferentiated mass of cells is formed.

(d) The callus will not develop shoot buds.

Plant Growth and Development - Solutions