Plant Growth and DevelopmentClass 11 Biology NCERT Solutions
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Q1Exercises
Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
Solution
Growth: An irreversible and permanent increase in the size of an organ, its parts, or an individual cell. It is generally accompanied by metabolic processes and an increase in dry weight.
Differentiation: The process by which cells derived from meristems mature and undergo structural changes in their cell walls and protoplasm to perform specific functions. For example, cells losing their protoplasm to form tracheary elements.
Development: The entire sequence of events in an organism's life cycle, from seed germination to senescence. It is the sum of two processes: growth and differentiation.
Dedifferentiation: The process by which living, differentiated cells that have lost the ability to divide, regain the capacity for cell division under certain conditions. For example, the formation of interfascicular cambium from parenchyma cells.
Redifferentiation: The process by which cells formed through dedifferentiation (like those of the cambium) divide and then lose their ability to divide again as they mature to perform specific functions. For example, the formation of secondary xylem and phloem from cambium cells.
Determinate growth: A type of growth in which an organ or the whole plant stops growing after reaching a certain size. Leaves, flowers, and fruits exhibit determinate growth.
Meristem: A region in a plant containing actively dividing, undifferentiated cells that have the capacity to divide and self-perpetuate. Meristems are responsible for the continuous or indeterminate growth of a plant.
Growth rate: The increased growth per unit time. It can be expressed mathematically and can be of two types: arithmetic and geometric.
Q2Exercises
Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Solution
Growth in a plant is a complex process involving increases in various aspects, and no single parameter is sufficient to measure it throughout the plant's life. Different parameters are suitable for measuring growth at different stages or in different parts of the plant.
For example:
- Cell number: This is a good measure for early embryonic development or growth in a culture, such as a maize root apical meristem producing over 17,500 new cells per hour.
- Cell size: For a watermelon, the growth is primarily due to a massive increase in the size of its cells (up to 3,50,000 times), making cell size a better parameter here.
- Length: This is a suitable parameter for measuring the growth of a pollen tube or the elongation of a stem or root.
- Surface area: The growth of a dorsiventral leaf is best measured by the increase in its surface area.
- Fresh or dry weight: Increase in fresh or dry weight is a common measure for the overall growth of a plant or its organs, like fruits. Dry weight is often considered more reliable as it excludes the variable water content.
Since a flowering plant undergoes all these types of growth in different parts and at different times (e.g., stem elongation, leaf expansion, fruit development), using only one parameter would provide an incomplete and inaccurate picture of its overall growth.
Q3Exercises
Describe briefly:
(a)
Arithmetic growth
(b)
Geometric growth
(c)
Sigmoid growth curve
(d)
Absolute and relative growth rates
Solution
(a) Arithmetic growth: In this type of growth, following mitotic division, only one of the two daughter cells continues to divide, while the other differentiates and matures. This results in a constant rate of growth. When the length of the organ is plotted against time, a linear curve is obtained. It is mathematically expressed as:
Where,
= length at time 't'
= length at time 'zero'
= growth rate
An example is a root elongating at a constant rate.
(b) Geometric growth: In this type of growth, the initial growth is slow (lag phase), followed by a rapid exponential growth phase (log phase) where both daughter cells retain the ability to divide. However, as nutrient supply becomes limited, the growth slows down and enters a stationary phase. The mathematical expression for the exponential phase is:
Where,
= final size
= initial size
= relative growth rate
= time of growth
= base of natural logarithms
This is typical for the growth of organisms in a natural environment.
(c) Sigmoid growth curve: A sigmoid or S-shaped curve is obtained when a parameter of growth is plotted against time for an organism growing in a natural environment with limited resources. It is characteristic of geometric growth. The curve has three distinct phases:
- Lag phase: The initial phase where growth is slow.
- Log phase (Exponential phase): A period of rapid growth where the organism has adapted to the new environment.
- Stationary phase: The phase where growth slows down and eventually stops as resources become limited or due to the accumulation of toxic materials.
(d) Absolute and relative growth rates:
- Absolute growth rate: It is the measurement and comparison of total growth per unit time. For example, if a leaf grows by 5 in one week, its absolute growth rate is 5 /week.
- Relative growth rate: It is the growth of a system per unit time expressed on a common basis, such as per unit initial parameter. It measures the efficiency of growth. For example, if a leaf of 5 grows by 5 and another leaf of 50 also grows by 5 in the same time, their absolute growth is the same. However, the relative growth rate of the smaller leaf is much higher (100%) compared to the larger leaf (10%).
Q4Exercises
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.
Solution
The five main groups of natural plant growth regulators (PGRs) are:
- Auxins
- Gibberellins
- Cytokinins
- Abscisic acid (ABA)
- Ethylene
A Note on Auxins:
Discovery:
The discovery of auxins began with the observations of Charles Darwin and his son Francis Darwin. They observed that the coleoptiles of canary grass bent towards a unilateral light source (phototropism). Through experiments, they concluded that a transmittable influence from the tip of the coleoptile caused this bending. Later, F.W. Went (1928) isolated this substance from the tips of oat coleoptiles and named it 'auxin' (from the Greek 'auxein': to grow).
Physiological Functions:
- Apical Dominance: The growing apical bud inhibits the growth of lateral (axillary) buds. Auxin produced in the apical bud is responsible for this phenomenon.
- Cell Elongation: Auxins promote the elongation of cells in stems and coleoptiles.
- Root Initiation: They help to initiate rooting in stem cuttings.
- Flowering: They promote flowering in certain plants like pineapples.
- Parthenocarpy: Auxins can induce the development of fruit without fertilization, e.g., in tomatoes.
- Abscission Control: They prevent the dropping of young leaves and fruits but promote the abscission of older, mature leaves and fruits.
- Xylem Differentiation: Auxins play a role in controlling the differentiation of xylem tissue.
Agricultural/Horticultural Applications:
- Plant Propagation: Auxins like IBA (indole butyric acid) and NAA (naphthalene acetic acid) are widely used to induce rooting in stem cuttings for vegetative propagation.
- Herbicide: Synthetic auxins like 2, 4-D (2, 4-dichlorophenoxyacetic acid) are used as selective herbicides to kill broad-leaved dicot weeds without affecting mature monocot plants. This is commonly used to maintain weed-free lawns.
- Flowering: Spraying auxins on pineapple plants helps in synchronizing flowering and fruit set.
- Preventing Fruit Drop: They are used to prevent the premature dropping of fruits like apples and oranges.
- Producing Seedless Fruits: They are used to induce parthenocarpy and produce seedless fruits like tomatoes.
Q5Exercises
Why is abscisic acid also known as stress hormone?
Solution
Abscisic acid (ABA) is known as the stress hormone because it plays a crucial role in helping plants cope with various adverse environmental conditions or stresses, such as drought, high salinity, and extreme temperatures.
Its primary function during stress is to conserve water. When a plant experiences water scarcity (drought stress), the synthesis of ABA increases. ABA then stimulates the closure of stomata on the leaf surface. By closing the stomata, the plant reduces water loss through transpiration, thus conserving its water content and increasing its tolerance to the dry conditions. Because of its role in mediating plant responses to these stresses, it is aptly called the 'stress hormone'.
Q6Exercises
'Both growth and differentiation in higher plants are open'. Comment.
Solution
The statement 'Both growth and differentiation in higher plants are open' refers to the unique ability of plants to grow and develop new cell types and structures throughout their life.
Open Growth: Plant growth is described as 'open' or 'indeterminate' because plants possess meristems (e.g., apical and lateral meristems) at specific locations. These meristems contain cells that are perpetually in a state of division. They continuously add new cells to the plant body throughout its life, allowing for unlimited growth. This is unlike animals, where growth is determinate and stops once adulthood is reached.
Open Differentiation: Differentiation in plants is also 'open'. This means that cells arising from the same meristem can differentiate into different types of cells and tissues depending on their final location within the plant body. The final structure a cell attains is determined by its position. For example, cells positioned away from the root apical meristem differentiate into root-cap cells, while those pushed to the periphery of the root mature into the epidermis. This flexibility, where the developmental fate of a cell is not rigidly determined early on, is what makes differentiation 'open'.
Q7Exercises
'Both a short day plant and a long day plant can produce can flower simultaneously in a given place'. Explain.
Solution
Yes, it is possible for a short-day plant (SDP) and a long-day plant (LDP) to flower simultaneously in a given place. Flowering in these plants is not determined by the absolute length of day or night, but rather by whether the photoperiod is longer or shorter than a specific critical duration.
- Short-day plants (SDPs) flower when the day length is shorter than their critical photoperiod, or more accurately, when the uninterrupted dark period is longer than a critical length.
- Long-day plants (LDPs) flower when the day length is longer than their critical photoperiod, or when the uninterrupted dark period is shorter than a critical length.
The critical photoperiod is different for different plants.
Example Scenario:
Let's assume we have:
- A short-day plant (like Xanthium) that requires a dark period longer than 8.5 hours to flower (meaning a day length less than 15.5 hours).
- A long-day plant (like Spinach) that requires a dark period shorter than 11 hours to flower (meaning a day length more than 13 hours).
If these two plants are grown in a place where the day length is 14 hours, the conditions for both to flower are met:
- For the SDP: The dark period is 10 hours (24 - 14), which is longer than its critical requirement of 8.5 hours. So, it will flower.
- For the LDP: The dark period is 10 hours, which is shorter than its critical requirement of 11 hours. So, it will also flower.
Thus, under a photoperiod that satisfies the specific critical requirements of both types of plants, they can flower simultaneously in the same location.
Q8Exercises
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.
Solution
(a) Auxins: They are widely used to initiate rooting in stem cuttings.
(b) Ethylene: It is highly effective in promoting fruit ripening.
(c) Cytokinins: They promote nutrient mobilization, which helps in delaying leaf senescence.
(d) Cytokinins: They help overcome apical dominance and promote the growth of lateral (axillary) buds.
(e) Gibberellins: They promote bolting, which is the internode elongation just before flowering in rosette plants like beet and cabbage.
(f) Abscisic acid (ABA): It is known as the stress hormone and stimulates the closure of stomata.
Q9Exercises
Would a defoliated plant respond to photoperiodic cycle? Why?
Solution
No, a defoliated plant (a plant from which all leaves have been removed) would not respond to the photoperiodic cycle.
This is because the perception of the light and dark stimulus for flowering occurs in the leaves. The leaves contain the photoreceptor pigment (phytochrome) that detects the duration of the photoperiod. Once the appropriate light/dark stimulus is perceived, the leaves produce a floral hormone or stimulus (hypothesized as 'florigen') which is then transported to the apical meristems to induce the formation of floral buds. Without leaves, the plant cannot perceive the photoperiodic signal, and therefore, flowering cannot be initiated in response to it.
Q10Exercises
What would be expected to happen if:
(a)
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.
Solution
(a) If (Gibberellic acid) is applied to rice seedlings, they would exhibit excessive elongation of their stems. This phenomenon is similar to the symptoms of the 'bakanae' or 'foolish seedling' disease in rice, which is caused by the fungus Gibberella fujikuroi, the natural source from which gibberellins were first discovered. The seedlings would become abnormally tall and weak.
(b) If dividing cells stop differentiating, the plant would not be able to form specialized tissues and organs. The cells would continue to divide, forming an unorganized mass of undifferentiated cells called a callus or a tumor. Without specialized tissues like xylem for water transport, phloem for food transport, and epidermis for protection, the plant cannot function and would not develop into a mature, organized organism.
(c) If a rotten or ripening fruit gets mixed with unripe fruits, the unripe fruits will ripen much faster. Ripening fruits produce and release ethylene, a gaseous plant hormone that promotes the ripening process. This ethylene gas will diffuse and affect the nearby unripe fruits, triggering their ripening and potentially leading to their rapid spoilage.
(d) If cytokinin is forgotten to be added to the culture medium (containing auxin), cell division will be severely hampered. Cytokinins are essential for promoting cytokinesis (cell division). In plant tissue culture, a proper balance of auxin and cytokinin is required for the growth of the callus and its subsequent differentiation into shoots and roots. Without cytokinin, the callus would likely fail to grow, and shoot formation would not be induced.