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Plant Growth and Development

NCERT Class 11 · Biology Based on NCERT Class 11 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores how plants grow, differentiate, and develop from a zygote into a mature organism. It explains the unique open form of growth in plants due to meristems, the measurable parameters and phases of growth, and the processes of differentiation, dedifferentiation, and redifferentiation. The one big idea a student must walk away with is that plant growth and development are controlled by both intrinsic factors (plant growth regulators) and extrinsic factors (light, temperature, water, nutrients), and that these processes are highly plastic and indeterminate.
Key Concepts (28)
Growth
An irreversible permanent increase in size of an organ or its parts or even of an individual cell, accompanied by metabolic processes (both anabolic and catabolic) that occur at the expense of energy.
e.g. Expansion of a leaf is growth; swelling of wood in water is not growth because it is reversible.
Meristem
A region of plant tissue where cells retain the capacity to divide and self-perpetuate, contributing to the open form of growth.
e.g. Root apical meristem and shoot apical meristem are responsible for primary growth.
Open form of growth
A form of growth in plants wherein new cells are always being added to the plant body by the activity of the meristem, allowing unlimited growth throughout life.
e.g. A tree continues to increase in height and girth year after year due to meristematic activity.
Primary growth
Growth that occurs due to the activity of root apical meristem and shoot apical meristem, contributing to elongation of the plant along its axis.
e.g. Elongation of a stem tip in a young seedling.
Secondary growth
Growth that occurs due to the activity of lateral meristems (vascular cambium and cork-cambium), causing an increase in the girth of organs.
e.g. Increase in thickness of a tree trunk over years.
Phases of growth
The period of growth divided into three phases: meristematic, elongation, and maturation.
e.g. In root tips, the region of cell division (meristematic), region of elongation, and region of maturation are clearly visible.
Arithmetic growth
A type of growth where after mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures, resulting in a linear increase over time.
e.g. A root elongating at a constant rate, expressed as Lt = L0 + rt.
Geometric growth
A type of growth where both progeny cells following mitotic cell division retain the ability to divide, leading to an exponential increase, often showing a sigmoid curve.
e.g. Growth of cells in culture or many higher plants and plant organs.
Sigmoid growth curve
A typical S-shaped curve obtained when growth parameter is plotted against time, showing lag phase, exponential (log) phase, and stationary phase.
e.g. Growth of a plant in a natural environment typically follows a sigmoid curve.
Absolute growth rate
Measurement and comparison of total growth per unit time.
e.g. If leaf A and leaf B both increase in area by 5 cm² in a given time, their absolute growth rate is the same.
Relative growth rate
Growth of a given system per unit time expressed on a common basis, e.g., per unit initial parameter.
e.g. If leaf A (10 cm²) and leaf B (50 cm²) both increase by 5 cm², leaf A has a much higher relative growth rate.
Differentiation
The act leading to maturation of cells derived from meristems, during which cells undergo structural changes in their cell walls and protoplasm to perform specific functions.
e.g. Formation of a tracheary element involves loss of protoplasm and development of lignocellulosic secondary cell walls.
Dedifferentiation
The phenomenon where living differentiated cells that have lost the capacity to divide regain the capacity of division under certain conditions.
e.g. Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
Redifferentiation
The process where cells produced by dedifferentiated meristems once again lose the capacity to divide but mature to perform specific functions.
e.g. Cells of cork cambium redifferentiate into cork cells.
Development
The sum of growth and differentiation, including all changes an organism goes through during its life cycle from germination of the seed to senescence.
e.g. A seed germinating, growing into a seedling, then a mature plant, flowering, fruiting, and finally senescing.
Plasticity
The ability of plants to follow different pathways in response to environment or phases of life to form different kinds of structures.
e.g. Heterophylly in cotton, coriander, and larkspur where juvenile leaves differ in shape from mature leaves.
Heterophylly
A phenomenon where leaves of the same plant show different shapes, either due to age (juvenile vs mature) or environment (air vs water).
e.g. Buttercup produces different shaped leaves in air and in water.
Plant growth regulators (PGRs)
Small, simple molecules of diverse chemical composition that control plant growth and development; also called plant hormones or phytohormones.
e.g. Auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
Auxins
A group of PGRs (e.g., IAA, IBA, NAA, 2,4-D) that promote cell division, cell enlargement, tropic growth, flowering, fruiting, and seed formation; also used in rooting cuttings, apical dominance, and as herbicides.
e.g. 2,4-D is used to kill dicotyledonous weeds in lawns.
Gibberellins
A group of promotory PGRs (e.g., GA3) that cause increase in length of axis, delay senescence, promote bolting, and speed up malting process.
e.g. Spraying sugarcane with gibberellins increases stem length and yield by up to 20 tonnes per acre.
Cytokinins
PGRs (e.g., kinetin, zeatin) that promote cytokinesis, produce new leaves and chloroplasts, overcome apical dominance, and delay leaf senescence.
e.g. Kinetin was discovered from autoclaved herring sperm DNA and promotes cell division in callus.
Ethylene
A simple gaseous PGR that promotes senescence, abscission, fruit ripening, and breaks seed and bud dormancy; also induces flowering in mango and pineapples.
e.g. Ethephon, a source of ethylene, is used to hasten fruit ripening in tomatoes and apples.
Abscisic acid (ABA)
A PGR that acts as a general growth inhibitor, stimulates stomatal closure, increases stress tolerance, induces seed dormancy, and acts as an antagonist to gibberellins.
e.g. ABA is called the stress hormone because it helps plants tolerate drought and other stresses.
Apical dominance
The phenomenon where the growing apical bud inhibits the growth of lateral (axillary) buds.
e.g. Removal of shoot tips (decapitation) results in growth of lateral buds, used in tea plantations and hedge-making.
Parthenocarpy
Development of fruit without fertilisation, induced by auxins.
e.g. Auxins induce parthenocarpy in tomatoes.
Bolting
Internode elongation just prior to flowering, promoted by gibberellins.
e.g. Gibberellins promote bolting in beet, cabbages, and many plants with rosette habit.
Respiratory climactic
The rise in rate of respiration during ripening of fruits, enhanced by ethylene.
e.g. Ethylene enhances respiration rate during fruit ripening.
Ethephon
A compound that in aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly.
e.g. Ethephon is used to hasten fruit ripening and accelerate abscission in flowers and fruits.
Formulas & Equations (2)
Lt = L0 + rt
Reads as: L at time t equals L naught plus r times t
Variables: Lt = length at time 't'; L0 = length at time 'zero'; r = growth rate / elongation per unit time
Use when: Use this formula when calculating arithmetic growth, e.g., root elongation at a constant rate.
⚠ Students forget that 'r' is the growth rate per unit time and not the total growth; they also confuse L0 with the final length.
W1 = W0 e^(rt)
Reads as: W one equals W naught times e to the power of r times t
Variables: W1 = final size (weight, height, number etc.); W0 = initial size at the beginning of the period; r = growth rate (relative growth rate); t = time of growth; e = base of natural logarithms
Use when: Use this formula when calculating geometric (exponential) growth, e.g., cell number increase in culture.
⚠ Students often forget that 'r' is the relative growth rate (efficiency index) and not the absolute growth rate; they also misuse natural logarithms.
How It Works
Phases of Growth in a Root Tip
  1. Step 1: Meristematic phase — cells at root apex and shoot apex divide constantly; cells are rich in protoplasm, have large nuclei, thin cellulosic primary walls with abundant plasmodesmatal connections.
  2. Step 2: Elongation phase — cells proximal to the meristematic zone undergo increased vacuolation, cell enlargement, and new cell wall deposition.
  3. Step 3: Maturation phase — cells further away from the apex attain maximal size, with wall thickening and protoplasmic modifications; tissues and cell types are formed.
💡 Students often confuse the order: meristematic → elongation → maturation. Also, the elongation zone is detected by the parallel line technique (Figure 13.3).
Arithmetic Growth
  1. Step 1: After mitotic cell division, only one daughter cell continues to divide.
  2. Step 2: The other daughter cell differentiates and matures.
  3. Step 3: The root elongates at a constant rate, giving a linear curve when length is plotted against time.
  4. Step 4: Mathematically expressed as Lt = L0 + rt.
💡 In arithmetic growth, only one daughter cell retains the ability to divide; the other differentiates. This is different from geometric growth where both daughter cells divide.
Geometric Growth
  1. Step 1: Initial growth is slow (lag phase).
  2. Step 2: Growth increases rapidly at an exponential rate (log or exponential phase).
  3. Step 3: With limited nutrient supply, growth slows down leading to a stationary phase.
  4. Step 4: Plotting growth against time gives a typical sigmoid or S-curve.
  5. Step 5: Mathematically expressed as W1 = W0 e^(rt).
💡 The sigmoid curve is characteristic of living organisms growing in a natural environment. The exponential phase is also called the log phase.
Differentiation of a Tracheary Element
  1. Step 1: Cells derived from root apical meristem, shoot apical meristem, or cambium begin to differentiate.
  2. Step 2: The cells lose their protoplasm.
  3. Step 3: They develop a very strong, elastic, lignocellulosic secondary cell wall.
  4. Step 4: The mature tracheary element functions to carry water to long distances even under extreme tension.
💡 Students often forget that differentiation involves loss of protoplasm in tracheary elements. The secondary cell wall is lignocellulosic, not just cellulosic.
Dedifferentiation and Redifferentiation
  1. Step 1: Living differentiated cells (e.g., parenchyma) that have lost the capacity to divide regain the capacity of division under certain conditions — this is dedifferentiation.
  2. Step 2: Examples include formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
  3. Step 3: The dedifferentiated meristems divide and produce cells.
  4. Step 4: These new cells once again lose the capacity to divide but mature to perform specific functions — this is redifferentiation.
💡 Dedifferentiation is the regain of division capacity; redifferentiation is the subsequent maturation. Students often reverse the two terms.
Discovery of Auxin (Darwin and Went)
  1. Step 1: Charles Darwin and Francis Darwin observed that coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism).
  2. Step 2: They concluded that the tip of the coleoptile was the site of a transmittable influence that caused bending of the entire coleoptile.
  3. Step 3: F.W. Went later isolated auxin from tips of coleoptiles of oat seedlings.
💡 The Darwins did not isolate auxin; they only demonstrated the existence of a transmittable influence. Went isolated auxin.
Discovery of Gibberellins (Kurosawa)
  1. Step 1: The 'bakanae' (foolish seedling) disease of rice seedlings was caused by a fungal pathogen Gibberella fujikuroi.
  2. Step 2: E. Kurosawa (1926) reported that symptoms of the disease appeared in rice seedlings when treated with sterile filtrates of the fungus.
  3. Step 3: The active substances were later identified as gibberellic acid.
💡 Gibberellins were discovered from a fungal pathogen, not from plants initially. The disease is called 'foolish seedling' because infected seedlings grow abnormally tall and thin.
Discovery of Cytokinins (Skoog and Miller)
  1. Step 1: F. Skoog and co-workers observed that callus from internodal segments of tobacco stems proliferated only if the nutrient medium was supplemented with extracts of vascular tissues, yeast extract, coconut milk, or DNA, in addition to auxins.
  2. Step 2: Miller et al. (1955) identified and crystallised the cytokinesis-promoting active substance, which they termed kinetin.
  3. Step 3: Kinetin is a modified form of adenine (a purine) and does not occur naturally in plants.
  4. Step 4: Natural cytokinins like zeatin were later isolated from corn-kernels and coconut milk.
💡 Kinetin is synthetic (from autoclaved herring sperm DNA), not natural. Zeatin is a natural cytokinin.
Discovery of Abscisic Acid (ABA)
  1. Step 1: During mid-1960s, three independent researches reported purification and chemical characterisation of three different inhibitors: inhibitor-B, abscission II, and dormin.
  2. Step 2: All three were later proved to be chemically identical.
  3. Step 3: It was named abscisic acid (ABA).
💡 ABA was discovered independently as three different substances (inhibitor-B, abscission II, dormin) that turned out to be the same compound.
Discovery of Ethylene (Cousins)
  1. Step 1: H.H. Cousins (1910) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas.
  2. Step 2: Later this volatile substance was identified as ethylene, a gaseous PGR.
💡 Ethylene is a gas and was discovered from the observation that ripened oranges affect unripened bananas stored nearby.
Mechanism of Apical Dominance
  1. Step 1: The growing apical bud produces auxins.
  2. Step 2: Auxins inhibit the growth of lateral (axillary) buds.
  3. Step 3: Removal of shoot tips (decapitation) removes the source of auxin, resulting in the growth of lateral buds.
  4. Step 4: This is widely applied in tea plantations and hedge-making.
💡 Apical dominance is due to auxin from the apical bud inhibiting lateral buds. Cytokinins help overcome apical dominance.
Role of Ethylene in Fruit Ripening
  1. Step 1: Ethylene is synthesised in large amounts by tissues undergoing senescence and ripening fruits.
  2. Step 2: Ethylene enhances the respiration rate during ripening, called respiratory climactic.
  3. Step 3: Ethylene is used to initiate flowering and synchronise fruit-set in pineapples and induces flowering in mango.
  4. Step 4: Ethephon, a source of ethylene, is used to hasten fruit ripening in tomatoes and apples.
💡 Ethylene is a gas; ethephon is a compound that releases ethylene slowly in aqueous solution. The respiratory climactic is a key feature of ethylene action.
Exam Traps — Don't Make These Mistakes
⚠ Growth vs Non-growth
✗ Swelling of a piece of wood when placed in water is growth.
→ Growth is an irreversible permanent increase. Swelling of wood in water is reversible (it shrinks when dried) and is not accompanied by metabolic processes.
✓ Swelling of wood in water is not growth because it is reversible and not accompanied by metabolic processes. Growth is an irreversible permanent increase accompanied by metabolism.
⚠ Arithmetic vs Geometric Growth
✗ In arithmetic growth, both daughter cells continue to divide.
→ In arithmetic growth, only one daughter cell continues to divide while the other differentiates. In geometric growth, both daughter cells retain the ability to divide.
✓ In arithmetic growth, after mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. In geometric growth, both progeny cells retain the ability to divide.
⚠ Absolute vs Relative Growth Rate
✗ If two leaves both increase by 5 cm², their growth rates are the same.
→ Absolute growth rate is the same (5 cm² per unit time), but relative growth rate depends on initial size. A smaller leaf has a higher relative growth rate.
✓ Absolute growth rate is the total growth per unit time (both leaves increased by 5 cm²). Relative growth rate is growth per unit initial parameter; the smaller leaf (A) has a much higher relative growth rate than the larger leaf (B).
⚠ Dedifferentiation vs Redifferentiation
✗ Dedifferentiation is when cells lose the capacity to divide and mature.
→ Dedifferentiation is the regain of division capacity by differentiated cells. Redifferentiation is when those cells lose division capacity again and mature.
✓ Dedifferentiation is the phenomenon where living differentiated cells that have lost the capacity to divide regain the capacity of division. Redifferentiation is when the cells produced by dedifferentiated meristems once again lose the capacity to divide but mature to perform specific functions.
⚠ Discovery of Auxin
✗ Charles Darwin and Francis Darwin isolated auxin from coleoptiles.
→ The Darwins only concluded that the tip of the coleoptile was the site of a transmittable influence that caused bending. F.W. Went later isolated auxin from tips of coleoptiles of oat seedlings.
✓ Charles Darwin and Francis Darwin observed phototropism in canary grass coleoptiles and concluded that the tip was the site of a transmittable influence. F.W. Went isolated auxin from coleoptile tips of oat seedlings.
⚠ Kinetin is Natural
✗ Kinetin is a naturally occurring cytokinin in plants.
→ Kinetin does not occur naturally in plants. It was discovered from autoclaved herring sperm DNA. Zeatin is a natural cytokinin isolated from corn-kernels and coconut milk.
✓ Kinetin is a synthetic cytokinin (a modified form of adenine) discovered from autoclaved herring sperm DNA. Natural cytokinins include zeatin, isolated from corn-kernels and coconut milk.
Compare & Contrast
Daughter cell fate after division
Arithmetic growth
Geometric growth
In arithmetic growth, only one daughter cell continues to divide while the other differentiates; in geometric growth, both daughter cells retain the ability to divide.
Basis of measurement
Absolute growth rate
Relative growth rate
Absolute growth rate is total growth per unit time; relative growth rate is growth per unit initial parameter (e.g., per unit initial size).
Sequence of events
Dedifferentiation
Redifferentiation
Dedifferentiation is the regain of division capacity by differentiated cells; redifferentiation is the subsequent maturation of cells produced by dedifferentiated meristems.
Effect on apical dominance
Auxin
Cytokinin
Auxin promotes apical dominance by inhibiting lateral bud growth; cytokinin helps overcome apical dominance and promotes lateral shoot growth.
Role in seed germination
Gibberellins
Abscisic acid (ABA)
Gibberellins promote seed germination and growth; ABA inhibits seed germination and induces dormancy. ABA acts as an antagonist to gibberellins.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1-2 SA (2-3 marks each), 1 LA (5 marks). Total approximately 10-12 marks.
Most asked: Phases of growth (meristematic, elongation, maturation) · Arithmetic vs geometric growth with formulas · Sigmoid growth curve · Absolute and relative growth rates · Differentiation, dedifferentiation, redifferentiation · Plant growth regulators: auxins, gibberellins, cytokinins, ethylene, abscisic acid — their discovery, physiological effects, and applications · Apical dominance · Heterophylly and plasticity
Don't skip: Compare and contrast arithmetic and geometric growth with formulas and graphs.; Explain the physiological effects and agricultural applications of any one plant growth regulator.; Describe the phases of growth in a root tip.; Differentiate between dedifferentiation and redifferentiation with examples.; Explain the role of ethylene in fruit ripening and its agricultural applications.
One-Line Revision (read aloud before the exam)
  1. Growth is an irreversible permanent increase in size accompanied by metabolic processes.
  2. Plants exhibit open form of growth due to the presence of meristems that retain the capacity to divide.
  3. Root apical meristem and shoot apical meristem are responsible for primary growth; vascular cambium and cork-cambium cause secondary growth.
  4. The three phases of growth are meristematic, elongation, and maturation.
  5. In arithmetic growth, only one daughter cell continues to divide; expressed as Lt = L0 + rt.
  6. In geometric growth, both daughter cells divide; expressed as W1 = W0 e^(rt), giving a sigmoid curve.
  7. Absolute growth rate is total growth per unit time; relative growth rate is growth per unit initial parameter.
  8. Differentiation is the maturation of cells to perform specific functions, e.g., tracheary elements lose protoplasm and develop lignocellulosic walls.
  9. Dedifferentiation is the regain of division capacity by differentiated cells; redifferentiation is the subsequent maturation of those cells.
  10. Development is the sum of growth and differentiation, including all changes from germination to senescence.
  11. Plasticity is the ability of plants to form different structures in response to environment, e.g., heterophylly.
  12. Auxins promote rooting, apical dominance, parthenocarpy, and are used as herbicides (2,4-D).
  13. Gibberellins increase stem length, delay senescence, promote bolting, and speed up malting.
  14. Cytokinins promote cell division, overcome apical dominance, and delay leaf senescence.
  15. Ethylene is a gaseous PGR that promotes fruit ripening, senescence, abscission, and breaks dormancy.
  16. Abscisic acid (ABA) is the stress hormone that inhibits growth, closes stomata, and induces seed dormancy.
  17. Apical dominance is the inhibition of lateral buds by auxin from the apical bud; removed by decapitation.
  18. Ethephon is a compound that releases ethylene slowly and is used to ripen fruits and thin flowers.
  19. The Darwins demonstrated the transmittable influence in coleoptiles; Went isolated auxin.
  20. Kinetin is a synthetic cytokinin; zeatin is a natural cytokinin from corn-kernels and coconut milk.

MCQs — 10 Questions with Answers

Q1. Which of the following best defines growth in a living organism?

  • A. Irreversible permanent increase in size requiring metabolic energy ✓
  • B. Temporary increase in volume due to water absorption
  • C. Increase in fresh weight only, without increase in cell number
  • D. Loss of meristematic capacity leading to fixed adult size

Answer: A — Growth is irreversible and permanent, requiring anabolic and catabolic metabolic processes; water swelling (B) is reversible, and growth includes multiple parameters, not fresh weight alone.

Q2. A piece of wood swells when placed in water. Why is this NOT considered growth in biological terms?

  • A. Because it increases in fresh weight
  • B. Because it is a reversible process requiring no metabolic energy ✓
  • C. Because wood cells cannot undergo mitosis
  • D. Because the wood is dead and dead cells cannot grow

Answer: B — Growth must be irreversible and permanent with metabolic activity; water swelling is reversible osmotic absorption with no energy expenditure or cell division.

Q3. In a root tip, cells of the meristematic phase are characterised by which of these features?

  • A. Large vacuoles, thick secondary walls, small nuclei
  • B. Large conspicuous nuclei, thin primary cellulosic walls, abundant plasmodesmata ✓
  • C. Maximum size, wall thickening, reduced protoplasm content
  • D. Active photosynthesis, high vacuole volume, matured cell types

Answer: B — Meristematic cells are actively dividing with large nuclei, thin walls for flexibility, and plasmodesmata for communication; features in A, C describe mature cells; D is incorrect for underground tissues.

Q4. The mathematical expression Lt = L₀ + rt represents which type of growth pattern, and what does the shape of its graph look like?

  • A. Geometric growth with an S-shaped sigmoid curve
  • B. Arithmetic growth with a linear straight-line curve ✓
  • C. Exponential growth with a J-shaped curve
  • D. Lag phase growth with a horizontal flat line initially

Answer: B — The linear equation Lt = L₀ + rt defines arithmetic growth where one daughter cell divides and one matures, producing a straight-line graph; geometric growth shows an S-curve with exponential phase.

Q5. Which statement correctly distinguishes plant growth from animal growth?

  • A. Plants show only primary growth, animals show only secondary growth
  • B. Plants retain unlimited growth capacity via persistent meristems (indeterminate), animals reach fixed adult size (determinate) ✓
  • C. Plant growth is always geometric, animal growth is always arithmetic
  • D. Plants measure growth in length only, animals measure growth in weight only

Answer: B — Plants maintain meristems throughout life enabling continuous indeterminate growth, while animals grow to a predetermined size then stop; both show primary and secondary growth in some tissues.

Q6. A maize root apical meristem produces 17,500 new cells per hour, while watermelon cells increase in size up to 3,50,000 times. What does this comparison reveal about growth measurement?

  • A. Watermelon growth is faster than maize growth because cells are larger
  • B. Growth parameters must match the organ type — cell number for maize, cell size for watermelon ✓
  • C. Both organs use the same growth mechanism of cell division
  • D. Maize shows geometric growth while watermelon shows arithmetic growth

Answer: B — Different organs grow by different mechanisms — maize adds numerous small cells (increase in number) while watermelon enlarges fewer cells dramatically (increase in size); growth measurement must reflect the dominant mechanism.

Q7. Which of the following is NOT a valid parameter for measuring plant growth?

  • A. Fresh weight and dry weight of the organ
  • B. Length of pollen tube and surface area of dorsiventral leaf
  • C. Cell volume and cell number in the meristem
  • D. Colour intensity and shade variation of the leaves ✓

Answer: D — Valid growth parameters (A, B, C) are proportional to protoplasm increase; colour intensity (D) reflects chlorophyll content and pigmentation, not structural growth or protoplasmic increase.

Q8. In geometric growth, if initial growth rate is slow (lag phase), what causes the rapid increase during the log phase, and what limits growth in the stationary phase?

  • A. Lag phase is limited by cell maturation; log phase increases due to cell size increase; stationary phase by nutrient depletion
  • B. Lag phase has reduced metabolic activity; log phase has maximum cell division when nutrients are abundant; stationary phase occurs due to limited nutrient supply ✓
  • C. Lag phase shows meristematic division; log phase shows elongation phase; stationary phase shows maturation phase
  • D. Lag phase occurs without meristems; log phase requires meristems; stationary phase occurs when meristems die

Answer: B — Lag phase shows initially slow growth with low metabolic activity; log phase shows exponential growth when abundant nutrients support maximum cell division; stationary phase occurs when nutrients become limited, slowing growth.

Q9. Which meristems are responsible for secondary growth in dicotyledonous plants, and in which direction do they add new tissues?

  • A. Root and shoot apical meristems add tissues towards the apex for elongation
  • B. Vascular cambium and cork-cambium add tissues radially outward for increase in girth ✓
  • C. Apical meristems add tissues radially, lateral meristems add tissues apically
  • D. Only vascular cambium functions in dicots; cork-cambium is found only in gymnosperms

Answer: B — Secondary growth in dicots is caused by vascular cambium (producing xylem and phloem) and cork-cambium (producing cork), adding tissues radially outward to increase organ girth; apical meristems cause primary growth along the axis.

Q10. ASSERTION: In arithmetic growth, both daughter cells produced by mitosis continue to divide indefinitely. REASON: In geometric growth, only one daughter cell divides while the other differentiates and matures.

  • A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion
  • B. Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion
  • C. Assertion is false but Reason is true ✓
  • D. Both Assertion and Reason are false

Answer: C — The Assertion is false — in arithmetic growth only one daughter cell divides while the other differentiates; the Reason is true and correctly describes arithmetic growth, making the statements reversed from what was claimed.

Flashcards

Define growth in biology with one characteristic feature that separates it from water absorption.

Growth is an irreversible permanent increase in size of an organ, cell, or individual that requires metabolic energy, distinguishing it from reversible physical changes like water absorption.

Why is plant growth called 'open form of growth'?

Plant growth is open form because meristems continuously divide and add new cells throughout the plant's life, allowing unlimited growth unlike animals with determinate final size.

Name the two types of meristems responsible for primary and secondary growth in dicots.

Root and shoot apical meristems cause primary growth (elongation along axis), while vascular cambium and cork-cambium cause secondary growth (increase in girth).

What are the three phases of growth at the root tip in order from apex to base?

Meristematic phase (dividing cells with large nuclei), elongation phase (vacuolation and cell expansion), maturation phase (wall thickening and specialisation).

Write the mathematical equation for arithmetic growth and identify what each term means.

Lt = L₀ + rt, where Lt is length at time t, L₀ is initial length at time zero, and r is growth rate per unit time.

In a watermelon, how is growth primarily expressed and why is this different from maize root?

Watermelon growth is expressed as increase in cell size (up to 3,50,000 times), while maize root growth is increase in cell number (17,500 cells per hour), because different organs grow by different mechanisms.

What shape curve is obtained when plotting growth parameter against time in geometric growth, and what does it represent?

An S-shaped sigmoid curve is obtained, showing lag phase (slow start), log/exponential phase (rapid growth), and stationary phase (limited nutrients slow growth).

Describe the characteristics of cells in the meristematic phase of root growth.

Meristematic cells are constantly dividing, rich in protoplasm, possess large conspicuous nuclei, have thin primary cellulosic walls, and abundant plasmodesmatal connections.

Distinguish between the capacity for growth in plants and animals.

Plants retain unlimited growth capacity throughout life due to persistent meristems (indeterminate), while animals reach a fixed final size after growth ceases (determinate).

What is the primary difference between arithmetic and geometric growth patterns in terms of cell division products?

In arithmetic growth only one daughter cell continues dividing while the other differentiates, whereas in geometric growth both daughter cells retain the ability to divide.

Important Board Questions

Define growth and explain why the swelling of wood in water is not considered growth in biological terms. [2 marks]

State that growth is irreversible permanent increase requiring metabolic energy; explain wood swelling is reversible osmotic water absorption without energy expenditure or protoplasm increase.

Describe the three phases of growth observed at the root tip, highlighting the structural and functional changes in cells during each phase. [5 marks]

Explain meristematic phase (dividing cells, large nuclei, thin primary walls, plasmodesmata); elongation phase (vacuolation, cell expansion, wall deposition); maturation phase (maximum size, wall thickening, specialisation). Mention location relative to apex for each.

Compare arithmetic and geometric growth patterns mathematically and graphically, explaining why a maize root exhibits arithmetic growth while seed/embryo growth is typically geometric. Which growth type is more common in nature and why? [6 marks]

Derive and explain Lt = L₀ + rt for arithmetic (linear graph, one daughter cell divides); explain geometric growth shows S-curve with lag-log-stationary phases (both daughters divide exponentially). Justify maize root as constant-rate elongation (arithmetic); seed as rapid exponential phase then plateau (geometric). Connect to nutrient availability and meristem capacity as factors determining growth type prevalence.

Assertion-Reason Questions for Plant Growth and Development

Q1. Assertion (A):

Plant growth is indeterminate and can continue throughout the life of the plant.

Reason (R):

This is because meristematic regions retain the capacity to divide and self-perpetuate, continuously adding new cells to the plant body.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true and the reason directly explains why plant growth is indeterminate. Meristems maintain their division capacity, enabling unlimited growth potential throughout the plant's life.

Q2. Assertion (A):

The meristematic phase of growth is characterized by cells with large, conspicuous nuclei and thin cellulosic primary cell walls.

Reason (R):

These characteristics allow meristematic cells to divide rapidly while maintaining thin walls for easy plasmodesmatal communication between adjacent cells.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true—meristematic cells do have these features. However, the reason doesn't fully explain why this is the case; the primary function is cell division, not just communication. The thin walls and abundant plasmodesmata are consequences of being undifferentiated, not the primary reason for these characteristics.

Q3. Assertion (A):

In arithmetic growth, the plot of length against time yields a linear curve.

Reason (R):

This is because only one daughter cell following mitotic division continues to divide while the other differentiates, resulting in a constant rate of cell addition per unit time.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true and the reason correctly explains the assertion. The arithmetic growth pattern (Lt = L0 + rt) produces a linear curve because the consistent division of one daughter cell lineage creates a constant growth rate.

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Case-Based Questions (CBQ) for Plant Growth and Development

Case Study:

A farmer in Punjab observed his wheat crop growing rapidly for the first three weeks after sowing, with consistent daily height increases. However, after the fourth week, growth slowed despite providing adequate water and nutrients. A botanist examined root samples using the parallel line technique and found maximum distance changes between marked lines in zones located 2-4 mm from the root apex, while zones beyond 5 mm showed thick cell walls and specialized cell types. The farmer's field measurements showed: Week 1: 5 cm height, Week 2: 10 cm height, Week 3: 15 cm height, Week 4: 18 cm height, Week 5: 19.5 cm height.

Q1. Based on the height measurements from weeks 1-3, what type of growth pattern does the wheat crop initially exhibit?

  • A. Geometric growth with exponential increase in height
  • B. Arithmetic growth with constant linear increase of 5 cm per week ✓
  • C. Indeterminate growth that continues at constant rate indefinitely
  • D. Logistic growth beginning its stationary phase

Answer: B

Q2. The parallel line technique showed maximum elongation in zones 2-4 mm from apex. Which growth phase do these zones represent?

  • A. Meristematic phase where active cell division occurs
  • B. Elongation phase where increased vacuolation and cell enlargement occur ✓
  • C. Maturation phase where cells differentiate into specialized tissues
  • D. Secondary growth phase initiated by vascular cambium

Answer: B

Q3. The zones beyond 5 mm showing thick cell walls and specialized cells indicate which growth phase?

  • A. The elongation phase continuing along the root
  • B. The dormancy phase where growth stops completely
  • C. The meristematic phase producing new cells
  • D. The maturation phase representing final cell development ✓

Answer: D

Q4. The slowing of growth from week 4 onwards (18→19.5 cm) despite adequate resources suggests the crop has entered which growth phase of the sigmoid curve?

  • A. Lag phase with slow initial growth
  • B. Log or exponential phase with rapid growth
  • C. Stationary phase where growth rate decreases significantly ✓
  • D. Death phase where growth becomes negative

Answer: C

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