Plant Growth

NCERT Solution

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

Answer: Growth: A permanent and irreversible increase in size of an organ or its part or even of an individual cell is called growth.

Differentiation: The process which leads to maturation of cells is called differentiation. During differentiation, a few or major changes happen in protoplasm and cell walls of the cells.

Development: All the changes which an organism goes through during its life cycle become parts of development. In case of a flowering plant, right from seed germination to seed bearing, each stage is a part of the development process.

Dedifferentiation: A differentiated cell can regain its capacity for cell division under certain conditions. This phenomenon is called dedifferentiation. Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells is an example of dedifferentiation.

Redifferentiation: A dedifferentiated plant cell once again loses its capacity to divide and becomes mature. This phenomenon is called redifferentiation.

Determinate Growth: When growth stops after a certain phase, this type of growth is called determinate growth.

Meristem: The plant tissue which has the ability to divide is called meristematic tissue. The region with such tissues is called meristem.

Growth Rate: The increased growth per unit time is called growth rate.

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

Answer: Different parts of a plant grow in different ways. There could be one way of measuring growth for a stem, while there could be a different way of measuring growth in a leaf. Different plants can be of different sizes and measurement of growth should be done in different ways in them. Hence, there cannot be one parameter to measure growth in different plants or even at different stages of the life cycle of a flowering plant.

Question 3: Describe briefly:

(a) Arithmetic growth

Answer: In this type of growth, only one daughter cell continues to divide after the mitosis. Another daughter cell differentiates and matures. Elongation of root at a constant rate is an example of arithmetic growth. Mathematically, the arithmetic growth rate can be shown as follows:

L_t=L₀ + rt

Here, L_t is length at time ‘t’, L₀ is length at time 0 and r is the rate per unit time.

(b) Geometric growth

Answer: In most of the cases, the initial growth is slow and is called the lag phase. After this, the growth is quite rapid and at an exponential rate. This phase is called the log or exponential phase. In this phase, both the daughter cells (formed after mitosis) continue to divide. The last phase marks a slowed down growth. This happens because of limited nutrient supply. This phase is called the stationary phase. The graph of the geometric growth gives a sigmoid curve.

The exponential growth can be mathematically represented as follows:

W₁ = W₀ert

Here, W₁ = final size (weight, height, number etc.), W₀ = initial size at the beginning of the period, r = growth rate, t = time of growth and e = base of natural logarithms

Answer: The S-shaped curve on graph; to show geometric growth is called the sigmoid growth curve.

(d) Absolute and relative growth rates

Answer: When growth is measured in absolute terms, e.g. in terms of length or weight, it is called absolute growth. When growth is measure in terms of comparative terms; like percentage growth; it is called relative growth.

Question 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.

Answer: Auxins, gibberellins, cytokinins, ethylene and abscisic acid are the main groups of PGR.

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.

Functions of Auxins:

Auxins help to initiate rooting in stem cuttings. This property is widely used for plant propagation by stem cuttings.
Auxins promote flowering. Auxins help to prevent fruit and leaf drop at early stages but promote abscission of older and mature leaves and fruits.

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

Answer: Photoperiodism: Flowering in certain plants depends on a combination of light and dark exposures and also on the relative duration of light and dark periods. This response of plants to variable duration of sunlight is called photoperiodism.

Flowering is an important step towards seed formation. Hence, phtoperiodism plays an important role in plant evolution.

Vernalisation: In some plants, flowering is quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is called vernalisation. Flowering is promoted during the period of low temperature because of vernalisation.

Question 6: Why is abscisic acid also known as stress hormone?

Answer: Abscisic acid helps plants to withstand stress and hence they are also known as stress hormone. For example; ABA closes stomata and thus helps in reducing water loss due to transpiration.

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

Answer: In higher plants, some of the cells retain their capacity of cell division. Hence, growth is of open type. Moreover, some of these cells always undergo differentiation after some rounds of cell division. Hence, the scope of differentiation is also open. Thus, it can be said the both growth and differentiation in higher plants are open.

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

Answer: Flowering in some plants depends on relative durations of light and dark periods. A short day plant requires a longer dark period, while a long day plant requires a longer light period. But flowering can happen simultaneously in them.

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

(a) Induce rooting in a twig

Answer: Auxins

(b) Quickly ripen a fruit

Answer: Ethylene

(c)Delay leaf senescence

Answer: Cytokinin

(d) Induce growth in axillary buds

Answer: Cytokinin

(e)‘Bolt’ a rosette plant

Answer: Gibberellins

(f) Induce immediate stomatal closure in leaves.

Answer: Abscisic Acid

Question 10: Would a defoliated plant respond to photoperiodic cycle? Why?

Answer: Leaves are the sites of perception of light/dark duration. Hence, a defoliated plant would not respond to photoperiodic cycle.

Question 11: What would be expected to happen if:

(a) GA3 is applied to rice seedlings

Answer: Hasten the growth of stem

(b) Dividing cells stop differentiating

Answer: Growth hormone is more prominent

Answer: The rotten fruit would release ethylene which can hasten the ripening of unripe fruits.

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

Answer: Cell division would be slower.