NCERT Class 11 Biology Plant Growth and Development Important Notes

Growth: A permanent and irreversible increase in size of an organ or its part or even of an individual cell is called growth. Growth is usually accompanied by metabolic process; which occur at the expense of energy.

Indeterminate Nature of Plant Growth: Plants retain the capacity for unlimited growth throughout their life. This becomes possible because of the presence of meristems at certain locations in the plant body. This type of growth in which new cells are always being added is called the open form of growth.

Phases of Growth: The growth can be divided into three phases, viz. meristematic, elongation and maturation.

a. Meristematic Phase: The constantly dividing cells of the root apex and the shoot apex represent the meristematic phase of growth. The cells of meristematic region are rich in protoplasm and posses large conspicuous nuclei. The cell walls of these cells are primary in nature, thin and cellulosic; with abundant plasmodesmatal connections.

b. Elongation Phase: The proximal cells which are just next to the meristematic zone represent the phase of elongation. In this phase, there is increased vacuolation, cell enlargement and new cell wall deposition.

c. Maturation Phase: Next to the phase of elongation lies the maturation zone. The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications. 

Growth Rate: The increased growth per unit time is called growth rate. The growth rate can be arithmetic or geometrical.

Arithmetic Growth Rate: 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:

Lt = L0 + rt

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

Geometrical Growth Rate: 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

Conditions for Growth 

• Water: Plant growth is closely linked to water status of the plant. Water is required for cell enlargement and also for turgidity. Turgidity helps in extension growth in cells. Moreover, water provides the medium for enzymatic activities needed for growth.

• Oxygen: Oxygen is another important factor for growth. Oxygen helps in releasing energy which is utilised in growth activities.

• Nutrients: Various nutrients are also required by plants for synthesis of protoplasm.

• Temperature: A range of optimum temperature is also necessary for growth in a plant. Any deviation from the optimum range can be detrimental for the survival of plant.

DIFFERENTIATION, DEDIFFERENTIATION AND REDIFFERENTIATION

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. Let us take example of tracheary element. The cells of a tracheary element lose their protoplasm and develop a very strong, elastic, lignocellulosic secondary cell walls. These changes help the tracheary element to carry water to long distances even under extreme tension.

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.

Plasticity: Some plants show different growth pathways in response to environment or to phases of life to form different types of structures. This ability of plants is called plasticity. For example; the leaves of coriander are of different shape at a younger stage than at a mature stage. This phenomenon is called heterophylly. Heterophylly can also be seen in cotton and larkspur. Leaves of buttercup are of different shapes when they grow in water than when they grow in air. This is another example of plasticity.

PLANT GROWTH REGULATORS

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.

• Apical Dominance: In most of the higher plants, the growing apical bud inhibits the growth of lateral buds. This phenomenon is called apical dominance. Farmers remove shoot tips to ensure the growth of lateral buds. This practice is widely used in tea plantations and in hedge-making.

• Auxins induce parthenocarpy, e.g. in tomatoes. Auxins are widely used as herbicides, e.g. 2, 4-D is widely used to kill dicotyledonous weeds. It is also used to prepare seed-free lawns by gardeners. Auxins also control xylem differentiation and help in cell division. 

Gibberellins: There are more than 100 gibberellins. They are denoted as GA1, GA2, GA3 and so on. Giberellic Acid (GA3) was one of the first gibberellins to be discovered. All gibberellins are acidic.

Functions of Gibberellins:

• Gibberellins cause an increase in length of axis. They cause fruit elongation and also delay senescence. Thus, gibberellins can be helpful in keeping the fruits for a longer duration on tree. In brewing industry, GA3 is used to speed up the malting process.

• Spraying sugarcane crop with gibberellins increases the length of stem. This helps in increasing the yield by as much as 20 tonnes per acre.

• Gibberellins are sprayed on juvenile conifers to hasten the maturity period. This leads to early seed production. Gibberellins also promote bolting in beet, cabbages and many plants with rosette habit. Internode elongation just prior to flowering is called bolting. 

Cytokinins: Cytokinins have specific effects on cytokinesis. Kinetin was discovered from autoclaves of herring sperm DNA. Kinetin does not occur naturally in plants. Zeatin is a naturally occurring cytokinin which was isolated from corn-kernels and coconut milk.

Cytokinins are synthesized in the regions of rapid cell division. It helps to produce new leaves, chloroplast in leaves, lateral shoot growth and adventitious shoot formation. Cytokinins help in overcoming the apical dominance. Cytokinins promote nutrient mobilization which helps in the delay of leaf senescence.

Ethylene: Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening fruits.

• Horizontal growth of seedlings, swelling of axis and apical hook formation (in dicot seedlings) are some of the examples of activities of ethylene.

• Ethylene promotes senescence and abscission; especially of leaves and flowers. It is highly effective in fruit ripening.

• Ethylene breaks seed and bud dormancy. It initiates germination in peanut seeds. It initiates sprouting of potato tubers.

• Ethylene promotes rapid internode/petiole elongation in deep water rice plants.

• Ethylene also promotes root growth and root hair formation.

• Ethylene is one of the most widely used PGR in agriculture. It is used to initiate flowering and for synchronizing fruit-set in pineapples. It also induces flowering in mango. It is used for hastening fruit ripening in tomatoes and apples. It accelerates abscission in flower and fruits. 

Abscisic Acid: ABA is a plant growth inhibitor. It plays a major role in seed development, maturation and dormancy. ABA stimulates closure of stomata and increases the tolerance of plants to various types of stresses. Hence, it is also called the stress hormone.

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 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. 

Many important crops; like wheat, barley, rye, etc. also have spring varieties. The spring variety is normally planted in the spring and come to flower and produce grain before the end of the growing season. But winter varieties of these plants would normally fail to bear flower if they are planted in spring. Such plants are planted in autumn so that they can flower during winter and bear seeds during spring. Such plants are usually harvested during mid-summer. Biennial plants also show vernalisation.

NCERT Solution for Class 11 Biology Plant Growth and Development

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

(c) Sigmoid growth curve

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 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

(c) A rotten fruit gets mixed with unripe fruits

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.