Plant Growth and Development
Welcome to HSLC Guru! This study guide covers ASSEB Class 11 Biology Chapter 15 — Plant Growth and Development. The chapter explores how plants grow, develop, and respond to internal and external signals throughout their life cycle. You will learn about growth phases, growth rates, plant growth regulators, photoperiodism, vernalisation, seed dormancy, senescence, and abscission. The notes, summary, question-answers, MCQs, fill-in-the-blanks, true/false statements, and glossary are aligned with the ASSEB syllabus to help students prepare effectively for the Class 11 final examination.
Summary
Growth is an irreversible permanent increase in size, mass, or volume of an organ, organism, or its parts due to the synthesis of new protoplasm. In plants, growth is indeterminate and continues throughout life because of the presence of meristems. Growth occurs in three distinct phases: the meristematic phase (active cell division at root and shoot apices), the elongation phase (cells expand, accumulate water in vacuoles, and lay down new cell wall material), and the maturation phase (cells attain their final shape and size, develop thickened walls, and acquire specific protoplasmic modifications). Growth can be expressed by two mathematical patterns: arithmetic growth, in which only one daughter cell continues to divide while the other differentiates (Lt = L0 + rt), and geometric growth, in which both daughter cells continue dividing, producing a sigmoid curve with lag, log (exponential), and stationary phases. Growth can be measured by increase in fresh weight, dry weight, length, area, volume, or cell number using instruments such as the auxanometer.
Differentiation is the process by which cells derived from meristems mature to perform specific functions, undergoing structural and biochemical changes such as loss of protoplasm in tracheary elements. Dedifferentiation is the regaining of dividing capacity by mature differentiated cells, as seen in the formation of interfascicular cambium and cork cambium. Redifferentiation occurs when these dedifferentiated cells once again lose their dividing capacity and mature to perform specific functions. Development is the sum total of growth and differentiation, encompassing all changes from seed germination to senescence. Plant development is regulated by intrinsic factors (genetic and chemical such as hormones) and extrinsic factors (light, temperature, water, oxygen, and nutrition). The phenomenon by which a plant exhibits different morphological structures in response to environmental conditions is called plasticity, exemplified by heterophylly in cotton, coriander, and aquatic plants like buttercup.
Plant growth regulators (PGRs) are small, simple organic molecules that influence growth and development. They are broadly grouped into growth promoters and growth inhibitors. Auxins (IAA, IBA — natural; NAA, 2,4-D — synthetic) were discovered by Charles Darwin and F.W. Went and promote cell elongation, apical dominance, root initiation, parthenocarpy, prevention of abscission, and are used as herbicides. Gibberellins (GA3), discovered by E. Kurosawa from Gibberella fujikuroi, cause stem elongation, bolting, breaking dormancy, increasing fruit length, and delaying senescence. Cytokinins, discovered by F. Skoog and Miller from herring sperm DNA (kinetin) and from corn kernels (zeatin), promote cell division, lateral bud growth, and delay leaf senescence (Richmond–Lang effect). Ethylene, discovered by Cousins, is a gaseous hormone that promotes fruit ripening, senescence, abscission, breaking of seed and bud dormancy, and triple response in seedlings. Abscisic acid (ABA), the stress hormone discovered by Addicott, induces stomatal closure during water stress, promotes seed dormancy, and induces abscission of leaves and fruits.
Photoperiodism is the response of plants to relative lengths of day and night for flowering. Plants are classified as short-day plants (SDP) which flower when exposed to a photoperiod less than the critical period (e.g., rice, soybean, tobacco, chrysanthemum), long-day plants (LDP) which flower when exposed to more than the critical period (e.g., wheat, radish, spinach, henbane), and day-neutral plants (DNP) in which flowering is not affected by photoperiod (e.g., tomato, cotton, sunflower, cucumber). Light is perceived by leaves through the pigment phytochrome, and a hypothetical hormone florigen is believed to induce flowering. Vernalisation is the qualitative or quantitative dependence of flowering on exposure to low temperature, ensuring that the plants do not produce mature flowers prior to attaining sufficient vegetative growth (e.g., winter varieties of wheat, biennials like sugar beet, cabbage, and carrot). Seed dormancy is a state in which seeds fail to germinate even under favourable conditions due to causes such as impermeable seed coats, immature embryos, or presence of inhibitors like ABA. Senescence is the age-dependent deterioration of cells and organs leading to death, while abscission is the natural shedding of leaves, flowers, and fruits triggered by ethylene and ABA at the abscission zone.
Question and Answer
1-Mark Questions
Q1. Define growth.
Answer: Growth is an irreversible permanent increase in size, mass, or volume of an organ or its parts due to the synthesis of new protoplasm.
Q2. Name the natural auxin found in plants.
Answer: Indole-3-acetic acid (IAA) is the natural auxin found in plants.
Q3. Who discovered gibberellins?
Answer: Gibberellins were discovered by E. Kurosawa from the fungus Gibberella fujikuroi.
Q4. What is the only gaseous plant hormone?
Answer: Ethylene is the only gaseous plant hormone.
Q5. Define photoperiodism.
Answer: Photoperiodism is the response of plants to the relative lengths of day and night for flowering.
Q6. Name the hormone called the stress hormone.
Answer: Abscisic acid (ABA) is called the stress hormone.
Q7. What is vernalisation?
Answer: Vernalisation is the promotion of flowering by exposure of plants to low temperature for a definite period.
Q8. Give one example of a short-day plant.
Answer: Rice (Oryza sativa) is an example of a short-day plant.
Q9. What does PGR stand for?
Answer: PGR stands for Plant Growth Regulator.
Q10. Name the instrument used to measure plant growth.
Answer: Auxanometer is the instrument used to measure plant growth.
2/3-Mark Questions
Q1. Differentiate between arithmetic growth and geometric growth.
Answer: In arithmetic growth, only one of the daughter cells continues to divide while the other differentiates and matures. The growth rate is constant and is represented by the equation Lt = L0 + rt, producing a linear curve. Example — root elongation at constant rate. In geometric growth, both daughter cells retain their ability to divide, producing exponential increase in cell number. The curve is sigmoid (S-shaped) with lag, log, and stationary phases. Example — embryo growth and population growth of cells.
Q2. Explain dedifferentiation and redifferentiation with examples.
Answer: Dedifferentiation is the phenomenon in which permanent (mature, differentiated) plant cells regain their power of cell division. Examples include the formation of interfascicular cambium and cork cambium from parenchyma cells. Redifferentiation is the phenomenon by which dedifferentiated cells once again lose the capacity to divide and become mature to perform specific functions. Example — secondary xylem and phloem produced by vascular cambium become mature conducting tissues.
Q3. Write any three physiological effects of auxins.
Answer: Three physiological effects of auxins are: (i) Cell elongation — auxins promote elongation of stem and coleoptile cells by loosening cell walls; (ii) Apical dominance — auxin produced at the shoot apex inhibits the growth of lateral buds; (iii) Root initiation — auxins induce the formation of adventitious roots in stem cuttings, helpful in vegetative propagation; they also promote parthenocarpy and prevent premature fruit drop.
Q4. What are short-day plants and long-day plants? Give one example of each.
Answer: Short-day plants (SDP) are those plants that flower when the photoperiod is less than the critical period, i.e., they require a long continuous dark period. Example — rice, chrysanthemum, soybean. Long-day plants (LDP) are those that flower when the photoperiod is more than the critical period, i.e., they require a longer light period. Example — wheat, radish, spinach, henbane.
Q5. Mention any three applications of cytokinins.
Answer: Three applications of cytokinins are: (i) They promote cell division in callus tissue and are widely used in tissue culture to induce shoot formation; (ii) They delay leaf senescence (Richmond–Lang effect) by retaining chlorophyll and protein content; (iii) They overcome apical dominance by promoting growth of lateral buds, increasing branching in ornamental plants.
Q6. What is seed dormancy? List two causes.
Answer: Seed dormancy is the condition in which viable seeds fail to germinate even under favourable environmental conditions of moisture, temperature, and oxygen. Two causes of seed dormancy are: (i) Impermeable and hard seed coats that prevent entry of water and oxygen (e.g., legume seeds); (ii) Presence of chemical inhibitors such as abscisic acid (ABA), phenolic acids, and coumarins inside the seed which prevent germination.
5/7-Mark Questions
Q1. Describe the three phases of plant growth in detail.
Answer: Plant growth occurs in three distinct phases at the apex of root and shoot:
(i) Meristematic phase / Phase of cell formation: This phase occurs at the root and shoot apices in the meristematic region. Cells in this region are small, isodiametric, thin-walled, with dense cytoplasm and prominent nuclei. They divide actively by mitosis and possess abundant plasmodesmata. Cells produced here form the source of all new tissues.
(ii) Elongation phase / Phase of cell enlargement: The cells produced in the meristematic region move into this zone, where they undergo enlargement. The phase is characterised by enlargement of cell volume due to vacuolation, increased water uptake, deposition of new cell wall material, and elongation of cells along the axis. The cells become longer and acquire a definite shape.
(iii) Maturation phase / Phase of cell differentiation: Cells of this zone attain their maximum size in terms of wall thickening and protoplasmic modifications. They differentiate into specific tissues such as xylem, phloem, parenchyma, collenchyma, and sclerenchyma to perform specialised functions. The cells lose their ability to divide and the cell walls become rigid through deposition of secondary materials.
Q2. Discuss the discovery, physiological effects, and applications of auxins.
Answer: Discovery: Charles Darwin and his son Francis Darwin (1880) observed that the coleoptile of canary grass bends towards a unilateral source of light and concluded that some “influence” was transmitted from the tip to the lower region. F.W. Went (1928) isolated this substance from oat coleoptile tips using agar blocks and named it “auxin” (from Greek auxein = to grow). Kogl and Haagen-Smit (1931) chemically isolated it as IAA (Indole-3-acetic Acid).
Types: Natural auxins include IAA and IBA (Indole-3-butyric acid); synthetic auxins include NAA (Naphthalene acetic acid) and 2,4-D (2,4-Dichlorophenoxyacetic acid).
Physiological effects: (i) Promote cell elongation in stems and coleoptiles; (ii) Cause apical dominance by suppressing growth of lateral buds; (iii) Initiate rooting in stem cuttings; (iv) Induce parthenocarpy (production of seedless fruits); (v) Prevent premature abscission of leaves, flowers, and fruits; (vi) Induce xylem differentiation; (vii) Promote tropic curvatures (phototropism, geotropism).
Applications: (i) Used in vegetative propagation through stem cuttings; (ii) 2,4-D is used as a selective herbicide to kill broad-leaved dicot weeds in cereal fields; (iii) NAA prevents fruit drop in apples and pears; (iv) Used to produce seedless fruits commercially; (v) Used in tissue culture for callus initiation.
Q3. Explain photoperiodism. Discuss the categories of plants based on photoperiodic response with examples.
Answer: Photoperiodism is defined as the response of plants to the relative lengths of day and night with respect to flowering. It was discovered by Garner and Allard (1920) while working on the Maryland Mammoth variety of tobacco. The site of perception of light is the leaves, and the pigment involved is phytochrome. A hypothetical flowering hormone called florigen is believed to translocate from leaves to the apical bud where it induces flowering.
On the basis of photoperiodic response, plants are classified into three categories:
(i) Short-Day Plants (SDP): They flower when the photoperiod is less than a critical period (i.e., the dark period must exceed a critical length). Examples — rice, soybean, chrysanthemum, tobacco, cotton, Xanthium.
(ii) Long-Day Plants (LDP): They flower when the photoperiod is more than the critical period. Examples — wheat, barley, oat, radish, spinach, sugar beet, henbane (Hyoscyamus niger).
(iii) Day-Neutral Plants (DNP): Flowering in these plants is not affected by photoperiod and they can flower throughout the year. Examples — tomato, cotton (some varieties), sunflower, cucumber, maize.
It is the duration of the dark (night) period rather than the light period that is critical, as proven by night-break experiments.
Q4. Describe vernalisation and its significance.
Answer: Vernalisation is the qualitative or quantitative dependence of flowering on exposure to low temperature for a specific duration. The term was coined by T.D. Lysenko (1928). It ensures that the plants do not produce mature flowers prior to attaining sufficient vegetative growth and reproductive maturity. The site of perception of vernalisation stimulus is the shoot apex and meristematic regions of the embryo, while the active growing buds, root tips, and developing leaves can also perceive the stimulus.
Examples: (i) Winter varieties of wheat, barley, and rye are sown in autumn; they germinate and remain as small seedlings through winter, then resume growth in spring to flower in summer. If sown in spring, they fail to flower or produce mature grains, but pre-treatment of moist seeds at low temperature (1–10 °C) for some weeks induces flowering. (ii) Biennials like sugar beet, cabbage, and carrot normally take two years to flower; vernalisation can stimulate them to flower in the first year.
Significance: (i) Vernalisation shortens the vegetative phase and induces early flowering; (ii) It allows winter crops to be raised in regions with severe winters; (iii) It increases cold resistance of crops; (iv) It enhances yield as crops mature within a specific period; (v) It is useful in plant breeding to produce more crops in a year. The stimulus produced is called vernalin.
Q5. Compare the physiological effects of gibberellins, cytokinins, and ethylene.
Answer: Gibberellins (GA): (i) Cause spectacular elongation of internodes (e.g., bolting in cabbage); (ii) Promote stem elongation in dwarf plants and convert them to tall plants; (iii) Break dormancy of seeds and buds; (iv) Induce flowering in long-day plants under non-inductive conditions; (v) Increase length of grape stalks; (vi) Increase fruit size in apples; (vii) Delay senescence; (viii) Promote alpha-amylase synthesis in malting industry.
Cytokinins: (i) Promote cell division (cytokinesis); (ii) Promote lateral bud growth, overcoming apical dominance; (iii) Delay leaf senescence (Richmond–Lang effect); (iv) Promote nutrient mobilisation in leaves; (v) Help in chloroplast maturation; (vi) Used in tissue culture to induce shoot formation; (vii) Break seed dormancy in some species.
Ethylene: (i) Promotes ripening of fruits (climacteric ripening); (ii) Induces senescence and abscission of leaves and flowers; (iii) Breaks dormancy of seeds and buds (e.g., potato tubers, peanuts); (iv) Initiates flowering in pineapple and mango; (v) Promotes femaleness in flowers of cucurbits; (vi) Induces triple response in seedlings (reduced elongation, increased thickness, horizontal growth); (vii) Promotes root hair formation. Ethephon is widely used commercially as a source of ethylene.
Multiple Choice Questions (MCQs)
Q1. The natural auxin found in plants is —
(a) NAA (b) 2,4-D (c) IAA (d) IBA
Answer: (c) IAA
Q2. Gibberellins were first isolated from —
(a) Aspergillus niger (b) Gibberella fujikuroi (c) Penicillium (d) Rhizopus
Answer: (b) Gibberella fujikuroi
Q3. The only gaseous plant hormone is —
(a) Auxin (b) Cytokinin (c) Ethylene (d) ABA
Answer: (c) Ethylene
Q4. The hormone that induces stomatal closure during water stress is —
(a) IAA (b) GA (c) Cytokinin (d) ABA
Answer: (d) ABA
Q5. The pigment that perceives photoperiod in plants is —
(a) Chlorophyll (b) Phytochrome (c) Carotenoid (d) Xanthophyll
Answer: (b) Phytochrome
Q6. Which of the following is a short-day plant?
(a) Wheat (b) Spinach (c) Rice (d) Tomato
Answer: (c) Rice
Q7. Vernalisation is the induction of flowering by —
(a) Long photoperiod (b) Short photoperiod (c) Low temperature (d) High temperature
Answer: (c) Low temperature
Q8. Apical dominance is caused by —
(a) Auxin (b) Gibberellin (c) Cytokinin (d) Ethylene
Answer: (a) Auxin
Q9. The hypothetical hormone responsible for inducing flowering is —
(a) Florigen (b) Vernalin (c) Auxin (d) Phytochrome
Answer: (a) Florigen
Q10. Cytokinin was first isolated from —
(a) Coconut milk (b) Herring sperm DNA (c) Corn kernels (d) Tomato fruits
Answer: (b) Herring sperm DNA
Fill in the Blanks
Q1. The instrument used to measure plant growth is called __________.
Answer: auxanometer
Q2. Geometric growth is represented by a __________ shaped curve.
Answer: sigmoid (S-shaped)
Q3. __________ is known as the stress hormone in plants.
Answer: Abscisic acid (ABA)
Q4. Plants which flower irrespective of day length are called __________ plants.
Answer: day-neutral
Q5. The promotion of flowering by exposure to low temperature is called __________.
Answer: vernalisation
True or False
Q1. Growth is a reversible process in plants.
Answer: False
Q2. Ethylene is widely used in fruit ripening.
Answer: True
Q3. Long-day plants flower when the dark period exceeds the critical length.
Answer: False
Q4. Cytokinins delay leaf senescence.
Answer: True
Q5. 2,4-D is a synthetic auxin used as a herbicide.
Answer: True
Glossary
| Term | Meaning |
|---|---|
| Growth | Irreversible permanent increase in size, mass, or volume of an organ or its parts. |
| Meristem | Group of actively dividing cells responsible for plant growth. |
| Differentiation | Maturation of meristematic cells to perform specific functions. |
| Dedifferentiation | Regaining of dividing capacity by mature, differentiated cells. |
| Redifferentiation | Loss of dividing capacity again by dedifferentiated cells. |
| Development | Sum total of growth and differentiation in a plant. |
| Plasticity | Ability of plants to follow different pathways of development under different environments. |
| Auxanometer | Instrument used for measuring plant growth. |
| Arithmetic Growth | Growth in which only one daughter cell continues to divide. |
| Geometric Growth | Growth in which both daughter cells retain dividing capacity, giving sigmoid curve. |
| PGR | Plant Growth Regulator — small organic molecule regulating growth and development. |
| Auxin | Hormone causing cell elongation, apical dominance, root initiation (e.g., IAA). |
| Gibberellin | Hormone promoting stem elongation, bolting, breaking dormancy. |
| Cytokinin | Hormone promoting cell division and delaying senescence. |
| Ethylene | Gaseous hormone promoting fruit ripening and abscission. |
| Abscisic Acid (ABA) | Stress hormone that promotes seed dormancy and stomatal closure. |
| Photoperiodism | Response of plants to relative lengths of day and night for flowering. |
| SDP | Short-Day Plant — flowers when photoperiod is below critical length. |
| LDP | Long-Day Plant — flowers when photoperiod is above critical length. |
| DNP | Day-Neutral Plant — flowers irrespective of photoperiod. |
| Phytochrome | Pigment that perceives photoperiod in plants. |
| Florigen | Hypothetical flowering hormone produced in leaves. |
| Vernalisation | Promotion of flowering by exposure to low temperature. |
| Vernalin | Hypothetical chemical stimulus produced during vernalisation. |
| Seed Dormancy | Inability of viable seeds to germinate even under favourable conditions. |
| Senescence | Age-dependent deterioration of cells leading to death. |
| Abscission | Natural shedding of leaves, flowers, and fruits at the abscission zone. |
| Apical Dominance | Suppression of lateral bud growth by the apical bud due to auxin. |
| Parthenocarpy | Development of fruits without fertilisation, induced by auxins. |
| Bolting | Sudden elongation of internodes prior to flowering, induced by gibberellins. |
| Richmond–Lang Effect | Delay of leaf senescence by cytokinins. |
| Triple Response | Effect of ethylene on seedlings — reduced elongation, increased thickness, horizontal growth. |