NCERT Solutions for Class 11 Biology Chapter 15: Plant Growth and Development

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

Ans. Growth: It is a permanent, irreversible rise in the size of an organ, its parts, or even a single cell. Growth is aided by metabolic activities that occur as a result of the energy available.

Differentiation: Root apical and shoot-apical meristems, as well as cambium cells, develop and mature to execute specialised roles. Differentiation is the process that leads to maturation.

Development: Development refers to all of an organism’s changes throughout the course of its life.

Dedifferentiation: Plants that have lost their ability to divide can regain it under certain circumstances. Dedifferentiation is the term for this phenomenon. Meristem formation is an example.

Redifferentiation: Redifferentiated cells have lost their ability to divide but have matured to perform certain roles as a result of dedifferentiation.

Determinate growth: Determinate growth refers to a cell’s, tissue’s, or organism’s ability to grow for a set amount of time. Most plants’ growth is indefinite, with some plants reaching a plateau and subsequently ceasing to grow.

Meristem: Meristem is a type of plant tissue that contains undifferentiated cells (meristematic cells).

Growth rate: A growth rate is the amount of growth per unit of 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, and cell number measured 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

(c) Sigmoid growth curve.

(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 slow growth in the initial stages and rapid growth during the later stages. The daughter cells derived from mitosis retain the ability to divide but slow down because of a limited nutrient supply.

(c) Sigmoid growth curve: The growth of living organisms in their natural environment is characterised by an S-shaped curve called the sigmoid growth curve. This curve is divided into three phases – lag phase, log phase or exponential phase of rapid growth, and stationary phase.

Plant Growth and Developmentans3

Exponential growth can be expressed as:
W1 = W0 ert

Where,
W1= Final size
W0= Initial size
r = Growth rate
t = Time of growth
e = Base of natural logarithms

(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. The five main groups of natural plant growth regulators are Auxins, gibberellins, cytokinins, ethylene and abscisic acid.

Auxins: Auxin was first isolated from human urine. The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds which have certain growth regulating properties. Auxins are usually produced by the growing apices. IAA and IBA (Indole Butyric Acid) have been isolated from plants. Naphthalene Acetic Acid (NAA) and 2, 4 – D (2, 4-dichlorophenoxyacetic) are synthetic auxins.

Physiological functions of Auxins:

(a) Auxins help to initiate rooting in stem cuttings. This property is widely used for plant propagation by stem cuttings.

(b) Auxins promote flowering. Auxins help to prevent fruit and leaf drop at early stages but promote abscission of older and mature leaves and fruits.

Horticultural/Agricultural importance:

1. They induce parthenocarpy in plants like tomatos.

2. They are used as weedicides that kill dicotyledonous weeds.

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

Ans. Photoperiodism: It can be termed as the response of plants to periods of day/night. It is theorized that the hormonal substance that is responsible for flowering, is formed in the leaves which subsequently migrates to the shoot apices and alters them into flowering apices.

Significance: The process of photoperiodism helps in studying the response of flowering in different crop plants when the duration of exposure of light is considered.

Vernalisation: It is the phenomena where the process of flowering in some plants is either quantitatively or qualitatively dependent on the exposure to lower temperatures. In particular, it refers to promoting the flowering process by a period of lower temperatures.

Significance: The process prevents precocious reproductive development late in the growing season which thereby enables the plant to have sufficient time to attain maturity.

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

Ans. Abscisic acid is called stress hormones because of following reasons:

(a) It induces various responses in plants against stress conditions.

(b) It increases the tolerance of plants toward various stresses.

(c) It induces the closure of the stomata during water stress.

(d) It promotes seed dormancy and ensures seed germination during favourable conditions.

(e) It helps seeds withstand desiccation.

(f) 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. The higher plants possess the ability for unlimited growth and this ability is due to the presence of meristems at several locations of their body. These meristems have the ability to self-perpetuate and divide and therefore, growth in higher plants is open. In addition, some of the cells in such plants always undergo differentiation after some rounds of cell division and therefore, differentiation is also open.

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

Ans. The exposure to light duration controls the flowering response in short-day plants and long-day plants. Therefore, 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

(c) delay leaf senescence

(d) induce growth in axillary buds

(e) ‘bolt’ a rosette plant

(f) induce immediate stomatal closure in leaves.

Ans. (a) Induce rooting in a twig – Auxins

(b) Quickly ripen a fruit – Ethylene

(c) Delay leaf senescence – Cytokinins

(d) Induce growth in axillary buds – Cytokinins

(e) Bolt a rosette plant – Gibberellic acid

(f ) Induce immediate stomatal closure in leaves – Abscisic acid

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

Ans. A plant that has lost its leaves (defoliated) will not respond to the photoperiodic cycle. This is due to the fact that the leaves are the areas where dark or light duration is perceived. Plants  would not respond to light if they did not have leaves.

11. What would be expected to happen if:

(a) GA3 is applied to rice seedlings

(b) dividing cells stop differentiating

(c) a rotten fruit gets mixed with unripe fruits

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

Ans. (a) If GA3 is applied to rice seedlings, the rice seedlings will exhibit internode-elongation and increase in height.

(b) If dividing cells stop differentiating, then the plant organs such as leaves and stem will not be formed. The mass of undifferentiated cell is called callus.

(c) If a rotten fruit gets mixed with unripe fruits, then the ethylene produced from the rotten fruits will hasten the ripening of the unripe fruits.

(d) If you forget to add cytokinin to the culture medium, then cell division, growth, and differentiation will not be observed.