Chapter 13: Plant Growth and Development Long Questions | biology Class 11 CBSE

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Chapter 13: Plant Growth and Development Long Questions | biology Class 11 CBSE | ByteNotes.in

Long Answer

Medium difficulty • Structured explanation

Medium

Question 1

Long Form

Compare arithmetic and geometric growth in plants with respect to their pattern, mathematical expressions, and the type of growth curve each produces.

In arithmetic growth, only one daughter cell continues to divide after mitotic cell division while the other differentiates and matures; a root elongating at a constant rate is the simplest example.
The mathematical expression for arithmetic growth is Lt = L0 + rt, where Lt is the length at time t, L0 is the initial length, and r is the constant growth rate; plotting length against time yields a linear curve.
In geometric growth, both progeny cells following mitosis retain the ability to divide and continue to do so; growth is initially slow (lag phase) and then increases rapidly at an exponential rate (log phase) before levelling off in a stationary phase due to limited nutrients.
The exponential growth formula is W1 = W0 e^rt, where r is the relative growth rate (efficiency index) and W1 depends on the initial size W0; plotting growth parameter against time produces a sigmoid (S-shaped) curve.
The sigmoid curve is characteristic of living organisms in natural environments and is typical for all cells, tissues, and organs of a plant.
The key difference is that arithmetic growth leads to a constant, linear increase, while geometric growth leads to exponential increase followed by a plateau, reflecting real biological resource constraints.

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Question 2

Long Form

Describe the three phases of growth in a root tip with reference to the characteristics of cells in each phase.

The period of growth is divided into three phases: meristematic, elongation, and maturation, best illustrated by examining the root tip.
In the meristematic phase, cells at the root apex and shoot apex are constantly dividing; they are rich in protoplasm, possess large conspicuous nuclei, and have thin, cellulosic primary cell walls with abundant plasmodesmatal connections.
In the elongation phase, cells proximal to the meristematic zone (just behind the apex) show increased vacuolation, cell enlargement, and deposition of new cell wall material as their key characteristics.
In the maturation phase, cells further from the apex (more proximal) attain their maximal size through wall thickening and protoplasmic modifications; these cells are fully differentiated and represent the functional tissues of the plant.
The transition from one phase to the next is gradual and the three phases collectively define the growth of the entire root axis; most specialised tissues studied in plant anatomy belong to the maturation phase.
The rate at which cells pass through these phases varies widely: a single maize root apical meristem can produce more than 17,500 new cells per hour, illustrating the dynamic nature of meristematic activity.

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Question 3

Long Form

Explain the processes of differentiation, dedifferentiation, and redifferentiation in plants with suitable examples for each.

Differentiation is the process by which cells derived from meristems undergo structural and functional changes in their cell walls and protoplasm to perform specific roles; it represents the conversion of meristematic cells into mature, specialised cells.
A classic example of differentiation is the formation of tracheary elements: cells lose their protoplasm entirely and develop strong, elastic, lignocellulosic secondary cell walls, enabling them to transport water over long distances under tension.
Dedifferentiation is the phenomenon where living differentiated cells that have lost the capacity to divide regain this capacity under certain conditions; it represents a reversal of the differentiated state.
An example of dedifferentiation is the formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells in woody dicotyledonous plants, restoring meristematic activity.
Redifferentiation occurs when cells produced by dedifferentiated meristems once again lose the capacity to divide and mature to perform specific functions, such as the secondary xylem and phloem produced by the vascular cambium.
These three processes together demonstrate that plant development is 'open' and flexible — the fate of a cell is not permanently fixed and can be altered under appropriate intrinsic or extrinsic signals.

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Question 4

Long Form

Discuss the discovery, chemical nature, and physiological effects of auxin on plant growth and development.

The discovery of auxin began with Charles Darwin and his son Francis Darwin's observation that coleoptiles of canary grass bent towards a unilateral light source (phototropism); subsequent experiments showed the tip of the coleoptile was the source of a transmittable influence causing bending.
Auxin was first isolated from human urine; auxin was also isolated by F.W. Went from the tips of coleoptiles of oat seedlings; the term auxin is applied to indole-3-acetic acid (IAA) and other compounds with growth-regulating properties.
Natural auxins (IAA, IBA) are produced at growing apices of stems and roots; synthetic auxins include NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid), widely used in agriculture and horticulture.
Physiologically, auxins initiate rooting in stem cuttings, promote flowering in pineapples, prevent premature fruit and leaf drop in early stages, and promote abscission of older mature leaves and fruits.
Auxins cause apical dominance — the inhibition of lateral bud growth by the apical bud; decapitation removes this inhibition and is applied in tea plantations and hedge-making to promote bushy growth.
Auxins induce parthenocarpy in tomatoes, control xylem differentiation, help in cell division, and are used as herbicides — 2,4-D selectively kills dicotyledonous weeds without affecting mature monocots.

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Question 5

Long Form

Analyse the diverse physiological roles of ethylene in plants and explain how it is commercially exploited in agriculture.

Ethylene is a simple gaseous PGR synthesised in large amounts by tissues undergoing senescence and ripening fruits; it is unique among PGRs because it is a gas at room temperature.
Key physiological effects include: horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings; it also promotes senescence and abscission of leaves and flowers.
Ethylene is highly effective in fruit ripening; it enhances the respiration rate during ripening — a rise called the respiratory climacteric — making it one of the most agriculturally important PGRs.
Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, and promotes sprouting of potato tubers; it also promotes rapid internode/petiole elongation in deep water rice plants, helping leaves remain above water.
Ethylene promotes root growth and root hair formation, increasing the plant's absorption surface; it also induces female flower formation in cucumbers, thereby increasing yield.
Commercially, ethephon (an aqueous compound that slowly releases ethylene in plant tissues) is used to hasten ripening in tomatoes and apples, accelerate abscission in cotton, cherry, and walnut, and to initiate and synchronise flowering and fruit-set in pineapples and mangoes.

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Question 6

Long Form

Describe the characteristics of plant growth regulators (PGRs) and explain how their discovery came about through accidental observations.

PGRs are small, simple molecules of diverse chemical composition; they include indole compounds (IAA), adenine derivatives (kinetin), derivatives of carotenoids (ABA), terpenes (gibberellic acid, GA3), and gases (ethylene, C2H4); they are also referred to as phytohormones.
Discovery of auxin followed Darwin's phototropism experiments; F.W. Went isolated auxin from tips of oat coleoptiles, establishing that the tip produced a transmittable substance causing bending.
Gibberellin was discovered through the 'bakanae' disease of rice seedlings caused by the fungus Gibberella fujikuroi; Kurosawa (1926) showed sterile filtrates of the fungus produced disease symptoms, and the active substance was identified as gibberellic acid.
Kinetin was identified when F. Skoog found callus from tobacco stems proliferated only when the medium was supplemented with vascular tissue extract, yeast extract, coconut milk, or DNA; Miller et al. (1955) crystallised the active substance.
Abscisic acid was independently characterised in the mid-1960s by three research groups as inhibitor-B, abscission II, and dormin — all later proved to be chemically identical and named ABA.
Ethylene was confirmed by H.H. Cousins (1910) who showed a volatile substance released by ripened oranges hastened the ripening of stored unripened bananas; this volatile substance was later identified as ethylene.

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Question 7

Long Form

Compare the roles of auxins and cytokinins in overcoming apical dominance and in delaying leaf senescence.

Apical dominance is the inhibition of lateral (axillary) bud growth by the actively growing apical bud; auxin produced at the shoot apex is primarily responsible for suppressing lateral bud growth.
When the shoot tip is removed (decapitation), auxin levels drop and lateral buds are released from inhibition, resulting in branching — a principle used in tea plantations and hedge-making.
Cytokinins counteract apical dominance by promoting the growth of lateral buds even in the presence of the apical bud, thus opposing the inhibitory effect of auxin at the axillary nodes.
Regarding leaf senescence, auxins help to prevent premature leaf drop at early stages but promote abscission of older mature leaves — they do not generally delay senescence in leaves.
Cytokinins actively delay leaf senescence by promoting nutrient mobilisation: they direct nutrients towards the leaf, keeping it metabolically active and green for a longer duration.
Thus, while auxin maintains apical dominance and eventually promotes leaf abscission, cytokinin overcomes apical dominance and delays leaf senescence — demonstrating that PGRs can have complementary or antagonistic roles depending on the event and target tissue.

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Hard

Question 8

Long Form

Explain how development in plants is the outcome of both intrinsic and extrinsic factors with appropriate examples from the chapter.

Development in plants includes all changes from seed germination to senescence and is broadly defined as the sum of growth and differentiation; it is regulated by both intrinsic and extrinsic factors acting together.
Intrinsic factors include intracellular (genetic) factors — the genome of the plant which determines the basic developmental programme — and intercellular factors such as plant growth regulators (PGRs) that coordinate responses across cells and organs.
For example, PGRs like auxin, gibberellin, cytokinin, ABA, and ethylene are intrinsic chemical regulators that mediate developmental events such as flowering, dormancy, abscission, and senescence at the molecular and cellular level.
Extrinsic factors include light, temperature, water, oxygen, and nutrition; for example, light and temperature control initiation of flowering, and water status is directly linked to extension growth through cell turgidity.
Many extrinsic factors exert their effects on development via PGRs — for instance, temperature (vernalisation) and light (photoperiodism) regulate flowering through their interaction with hormonal pathways.
Plant plasticity — illustrated by heterophylly in buttercup and coriander — is a striking example of extrinsic (environmental) factors shaping developmental outcomes: the same genotype produces structurally different leaves depending on whether the plant grows in water or air.

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Question 9

Long Form

Analyse the physiological effects of gibberellins on plant growth and discuss their applications in agriculture and industry.

Gibberellins are acidic promotory PGRs, with more than 100 types reported from fungi and higher plants; gibberellic acid (GA3) was the first to be discovered and remains the most intensively studied.
A major physiological effect is the ability to increase the length of axis: GAs increase the length of grape stalks and cause apple fruits to elongate and improve in shape, with direct implications for fruit quality.
Gibberellins cause bolting — rapid internode elongation just prior to flowering — in plants with a rosette habit such as beet, cabbage, and many others; they also delay senescence, enabling fruits to remain on trees longer.
In agriculture, spraying sugarcane with gibberellins increases stem length, thereby raising carbohydrate yield by as much as 20 tonnes per acre — a major application in sugarcane cultivation.
Spraying juvenile conifers with GAs hastens the maturity period, leading to early seed production, which is valuable in forestry and breeding programmes.
In industry, GA3 is used to speed up the malting process in the brewing industry; it breaks seed dormancy and promotes germination, accelerating the conversion of starch to sugars in barley and other grains.

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Hard

Question 10

Long Form

Describe the roles of abscisic acid in seed development, dormancy, and stress responses, and explain why it is regarded as an antagonist to gibberellins.

Abscisic acid (ABA) was originally discovered for its role in regulating abscission and dormancy; it acts as a general plant growth inhibitor and inhibitor of plant metabolism.
ABA plays an important role in seed development and maturation: it promotes the maturation of seeds and induces dormancy in seeds and buds, preparing the plant for periods of unfavourable conditions.
By inducing dormancy, ABA helps seeds withstand desiccation and other factors unfavourable for growth; seeds in a dormant state do not germinate even in the presence of water and oxygen.
ABA is called the stress hormone because it stimulates stomatal closure in response to water deficit, reducing transpiration and helping the plant conserve water during drought stress; it also increases tolerance to various biotic and abiotic stresses.
ABA inhibits seed germination, directly opposing the germination-promoting effects of gibberellins; while GAs also promote bolting, growth, and break dormancy, ABA suppresses these processes.
In most situations ABA acts as an antagonist to GAs: for example, in seed dormancy, ABA promotes and maintains dormancy while GAs promote germination and seedling growth — making their balance critical for determining when a seed germinates.

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Chapter 13: Plant Growth and Development | Long Questions

Compare arithmetic and geometric growth in plants with respect to their pattern, mathematical expressions, and the type of growth curve each produces.

Describe the three phases of growth in a root tip with reference to the characteristics of cells in each phase.

Explain the processes of differentiation, dedifferentiation, and redifferentiation in plants with suitable examples for each.

Discuss the discovery, chemical nature, and physiological effects of auxin on plant growth and development.

Analyse the diverse physiological roles of ethylene in plants and explain how it is commercially exploited in agriculture.

Describe the characteristics of plant growth regulators (PGRs) and explain how their discovery came about through accidental observations.

Compare the roles of auxins and cytokinins in overcoming apical dominance and in delaying leaf senescence.

Explain how development in plants is the outcome of both intrinsic and extrinsic factors with appropriate examples from the chapter.

Analyse the physiological effects of gibberellins on plant growth and discuss their applications in agriculture and industry.

Describe the roles of abscisic acid in seed development, dormancy, and stress responses, and explain why it is regarded as an antagonist to gibberellins.

Unlock all Long Questions