NCERT Solutions for Class 11th BIOLOGY
Chapter 15 Plant Growth And Development
Question 1: Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
Answer (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 shoot apex) 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.
Answer (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 shoot apex) 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.
Question 2: Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Answer 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.
Answer 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.
Question 3: Describe briefly:
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates
Answer
(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 nutrient supply.
(c) Sigmoid growth curve
The growth of living organisms in their natural environment is characterised by an S-shaped curve called sigmoid rowth curve. This curve is divided into three phases – lag phase, log phase or exponential phase of rapid growth, and stationary phase.
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(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.
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates
Answer
(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 nutrient supply.
(c) Sigmoid growth curve
The growth of living organisms in their natural environment is characterised by an S-shaped curve called sigmoid rowth curve. This curve is divided into three phases – lag phase, log phase or exponential phase of rapid growth, and stationary phase.
(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.
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 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
(ii) Gibberellic acid
(iii) Cytokinins
(iv) Ethylene
(v) Abscisic acid
(i) Auxins
Discovery:
The first observations regarding the effects of auxins were made by Charles Darwin and Francis Darwin wherein they saw the coleoptiles of canary gross 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 prevent abscission of leaves and fruits.
Horticultural applications:
1. They are used as the rooting hormones in stem cuttings.
2. 2-4 D is used weedicide to kill broadleaf, dicotyledonous weeds.
3. They induce parthenocarpy in tomatoes.
4. They promote flowering in pineapple and litchi.
(ii) Gibberellic acid
Discovery:
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
Discovery:
Through their experimental observations, F. Skoog and his co-workers found that the tobacco callus differentiated when extracts of vascular tissues, yeast extract, coconut milk, or DNA were added to the culture medium. 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
Discovery:
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 and abscission of 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 the abscission of flowers and leaves in cotton, cherry, and walnut.
4. It promotes the number of female flowers in cucumbers.
(v) Abscisic acid
Discovery:
During the mid 1960s, inhibitor-B, abscission II, and dormin were discovered by three independent researchers. These were later on found to be chemically similar 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 induces abscission of leaves, fruits, and flowers. Horticultural application:
It induces seed dormancy in stored seeds.
Answer 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
(ii) Gibberellic acid
(iii) Cytokinins
(iv) Ethylene
(v) Abscisic acid
(i) Auxins
Discovery:
The first observations regarding the effects of auxins were made by Charles Darwin and Francis Darwin wherein they saw the coleoptiles of canary gross 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 prevent abscission of leaves and fruits.
Horticultural applications:
1. They are used as the rooting hormones in stem cuttings.
2. 2-4 D is used weedicide to kill broadleaf, dicotyledonous weeds.
3. They induce parthenocarpy in tomatoes.
4. They promote flowering in pineapple and litchi.
(ii) Gibberellic acid
Discovery:
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
Discovery:
Through their experimental observations, F. Skoog and his co-workers found that the tobacco callus differentiated when extracts of vascular tissues, yeast extract, coconut milk, or DNA were added to the culture medium. 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
Discovery:
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 and abscission of 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 the abscission of flowers and leaves in cotton, cherry, and walnut.
4. It promotes the number of female flowers in cucumbers.
(v) Abscisic acid
Discovery:
During the mid 1960s, inhibitor-B, abscission II, and dormin were discovered by three independent researchers. These were later on found to be chemically similar 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 induces abscission of 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.
Solution: The physiological mechanism for flower-ing is controlled by two factors: photoperiod or light period, i.e., photoperiodism and low temperature, i.e., vernalisation. Photoperiodism is defined as the flowering response of a plant to relative lengths of light/ dark period. Significance of photoperiodism is as follows:
Solution: The physiological mechanism for flower-ing is controlled by two factors: photoperiod or light period, i.e., photoperiodism and low temperature, i.e., vernalisation. Photoperiodism is defined as the flowering response of a plant to relative lengths of light/ dark period. Significance of photoperiodism is as follows:
- Photoperiodism determines the season in which a particular plant shall flower. For example, short day plants develop flowers in autumn-spring period (e.g., Dahlia, Xanthium) while long day plants produce flowers in summer (e.g., Amaranthus).
- Knowledge of photoperiodic effect is useful in keeping some plants in vegetative growth (many vegetables) to obtain higher yield of tubers, rhizomes etc. or keep the plant in reproductive stage to yield more flowers and fruits.
- A plant can be made to flower throughout the year by providing favourable photoperiod.
- Helps the plant breeders in effective cross-breeding in plants.
- Enable a plant to flower in different seasons.
Vernalisation is promotion or induction of flowering by exposing a plant to low temperature for some time. Significance of vernalisation is as follows :
(i) Crops can be grown earlier.
(ii)Plants can be grown in such regions where normally they do not grow.
(iii)Yield of the plant is increased.
(iv)Resistance to cold and frost is increased.
(v) Resistance to fungal diseases is increased.
6. Why is abscisic acid also known as stress hormone?
Solution: A fairly high concentration of abscisic acid (ABA) is found in leaves of plants growing under stress conditions, such as drought, flooding, injury, mineral deficiency etc. It is accompanied by loss of turgor and closure of stomata. When such plants are transferred to normal conditions, they regain normal turgor and ABA concentration decreases. Since the synthesis of ABA is accelerated under stress condition and the same is destroyed or inactivated when stress is relieved, it is also known as stress hormone.
Solution: A fairly high concentration of abscisic acid (ABA) is found in leaves of plants growing under stress conditions, such as drought, flooding, injury, mineral deficiency etc. It is accompanied by loss of turgor and closure of stomata. When such plants are transferred to normal conditions, they regain normal turgor and ABA concentration decreases. Since the synthesis of ABA is accelerated under stress condition and the same is destroyed or inactivated when stress is relieved, it is also known as stress hormone.
7. ‘Both growth and differentiation in higher plants are open’. Comment.
Solution: Plant growth is generally indeterminate. Higher plants possess specific areas called meristems which take part in the formation of new cells. The body of plants is built on a modular fashion where structure is never complete because the tips (with apical meristem) “are open ended – always growing and forming new organs to replace the older or senescent ones. Growth is invariably associated with differentiation. The exact trigger for differentiation is also not known. Not only the growth of plants are open- ended, their differentiation is also open. The same apical meristem cells give rise to different types of cells at maturity, e.g., xylem, phloem, parenchyma, sclerenchyma fibres, collenchyma, etc. Thus, both the processes are indeterminate, unlimited and develop into
different structures at maturity i.e., both are open.
Solution: Plant growth is generally indeterminate. Higher plants possess specific areas called meristems which take part in the formation of new cells. The body of plants is built on a modular fashion where structure is never complete because the tips (with apical meristem) “are open ended – always growing and forming new organs to replace the older or senescent ones. Growth is invariably associated with differentiation. The exact trigger for differentiation is also not known. Not only the growth of plants are open- ended, their differentiation is also open. The same apical meristem cells give rise to different types of cells at maturity, e.g., xylem, phloem, parenchyma, sclerenchyma fibres, collenchyma, etc. Thus, both the processes are indeterminate, unlimited and develop into
different structures at maturity i.e., both are open.
8. ‘Both a short day plant and a long day plant can produce flower simultaneously in a given place’. Explain.
Solution: A short day plant (SDP) flowers only when it receives a long dark period and short photoperiod, e.g., Xanthium, Dahlia etc. On the other hand, a long day plant (LDP) will flower only when it receives a long photoperiod and short dark period, e.g., wheat, oat etc. Thus critical photoperiod is that continuous duration of light which must not be exceeded in SDP and should always be exceeded in LDP in order to bring them to flower. Xanthium requires light for less than 15.6 hrs and Henbane requires light for more than 11 hrs. Xanthium (a SDP) and Henbane (DP) will flower simultaneously in light period between 11 to 15.6 hrs.
Solution: A short day plant (SDP) flowers only when it receives a long dark period and short photoperiod, e.g., Xanthium, Dahlia etc. On the other hand, a long day plant (LDP) will flower only when it receives a long photoperiod and short dark period, e.g., wheat, oat etc. Thus critical photoperiod is that continuous duration of light which must not be exceeded in SDP and should always be exceeded in LDP in order to bring them to flower. Xanthium requires light for less than 15.6 hrs and Henbane requires light for more than 11 hrs. Xanthium (a SDP) and Henbane (DP) will flower simultaneously in light period between 11 to 15.6 hrs.
Question 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.
Answer
(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
(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.
Answer
(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
Question 10: Would a defoliated plant respond to photoperiodic cycle? Why?
Answer 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.
Answer 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.
Question 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.
Answer (a) If GA3 is applied to rice seedlings, then the rice seedlings will exhibit internodeelongation 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.
(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.
Answer (a) If GA3 is applied to rice seedlings, then the rice seedlings will exhibit internodeelongation 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.
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