Plant Growth and Development
How a plant builds its entire body from a single zygote through growth and differentiation, and how internal and external plant growth regulators steer every stage from seed germination to senescence.
Plant physiology is a reliable Prelims source — expect match-the-following on PGRs (hormone–function–discoverer–chemical class) plus questions on growth phases, growth curves and meristems. For Mains it maps to GS-III Science & Technology, especially the agriculture/biotechnology interface: PGRs in ripening and dormancy, and plant tissue culture made possible by dedifferentiation of parenchyma.
Understand the chapter
What Plant Growth Is — and Why It Never Stops
Growth is an irreversible, permanent increase in the size of an organ, its parts, or even a single cell, and is always accompanied by metabolism (both anabolic and catabolic) that consumes energy. Because plants retain meristems — pockets of cells that can divide and self-perpetuate — they show indeterminate or 'open' growth, adding new cells throughout life. Apical meristems (root and shoot) drive primary growth (elongation along the axis), while lateral meristems — vascular cambium and cork cambium — drive secondary growth (increase in girth) in dicots and gymnosperms.
- Irreversibility is the key test: reversible swelling of wood placed in water is NOT growth.
- Open form of growth: the meristem keeps adding cells; daughter cells later lose division capacity and mature.
- Primary growth = apical meristems (length); Secondary growth = lateral meristems/cambia (girth).
Growth Is Measurable — and Happens in Three Phases
At the cellular level growth reflects an increase in protoplasm, which is hard to measure directly, so it is gauged by proxies such as fresh weight, dry weight, length, area, volume and cell number. The same growth can be expressed differently: a single maize root apical meristem can produce over 17,500 new cells per hour (cell number), while a watermelon cell may enlarge up to 3,50,000 times (cell size). Along a root tip, growth passes through three successive zones — meristematic, elongation and maturation.
- Meristematic zone: constantly dividing cells, dense protoplasm, large nuclei, thin primary cellulosic walls, abundant plasmodesmata.
- Elongation zone: increased vacuolation, cell enlargement, new cell-wall deposition.
- Maturation zone: maximal size with wall thickening and protoplasmic modification — forms the mature tissues.
Growth Rates: Arithmetic vs Geometric (plus Conditions)
Growth rate (growth per unit time) can be arithmetic or geometric. In arithmetic growth only one daughter cell keeps dividing after mitosis while the other matures, giving a constant rate and a linear plot (Lt = L0 + rt). In geometric growth both daughter cells keep dividing, producing a slow lag phase, a rapid exponential (log) phase and finally a stationary phase as nutrients run low — the classic sigmoid (S-shaped) curve (W1 = W0 e^rt).
- Lt = L0 + rt (arithmetic, linear); W1 = W0 e^rt (geometric, exponential).
- r = relative growth rate = the plant's 'efficiency index'; absolute growth rate = total growth per unit time, relative = growth per unit time on a common/initial basis.
- Sigmoid curve (lag → log → stationary) is typical of cells, tissues and organs growing naturally.
- Conditions for growth: water (turgidity, cell enlargement, enzyme medium), oxygen (releases metabolic energy), nutrients (macro/micro for protoplasm and energy), optimum temperature, plus light and gravity signals.
Differentiation, Dedifferentiation and Redifferentiation
Cells produced by meristems mature and specialise to perform specific functions — this is differentiation, often involving major changes in cell wall and protoplasm (e.g., a tracheary element loses its protoplasm and develops strong, elastic, lignocellulosic secondary walls). Living differentiated cells can sometimes regain the ability to divide — dedifferentiation — as when parenchyma forms interfascicular cambium and cork cambium. When such meristems again produce cells that mature and lose division capacity, it is redifferentiation. Differentiation in plants is 'open' — a cell's final form depends on its position.
- Differentiation: meristem-derived cells mature into specialised structures.
- Dedifferentiation: mature living cells regain division capacity (→ interfascicular and cork cambium).
- Redifferentiation: dedifferentiated cells mature again, losing division capacity.
- Open differentiation by position: cells near root apex → root-cap; cells pushed to periphery → epidermis.
Development, Plasticity and Heterophylly
Development covers all changes an organism undergoes through its life cycle, from seed germination to senescence, and is broadly the sum of growth and differentiation. It is governed by intrinsic factors (intracellular/genetic plus intercellular chemicals such as PGRs) and extrinsic factors (light, temperature, water, oxygen, nutrition). Plants can follow different developmental pathways depending on environment or life phase — an ability called plasticity, best seen as heterophylly.
- Development = growth + differentiation; spans germination → senescence.
- Plasticity: different structures formed via different pathways (environment or life phase).
- Heterophylly: cotton, coriander, larkspur (juvenile vs mature leaves); buttercup (leaves in air vs water).
- Control = intrinsic (genetic + PGRs) + extrinsic (light, temperature, water, O2, nutrition).
Plant Growth Regulators: Five Groups, Two Roles
Plant growth regulators (PGRs) — also called phytohormones or plant hormones — are small, simple molecules of diverse chemistry. They split into two functional groups: growth promoters (auxins, gibberellins, cytokinins) that drive cell division, enlargement, pattern formation, flowering, fruiting and seed formation; and the stress/inhibitory group, chiefly abscisic acid (ABA), which controls dormancy and abscission. The gaseous PGR ethylene can act either way but is largely a growth inhibitor.
- Promoters: Auxins, Gibberellins, Cytokinins.
- Inhibitor: Abscisic acid (ABA); Ethylene fits either group but is mostly inhibitory.
- Chemistry: IAA = indole; GA3 = terpene; kinetin = adenine (purine) derivative; ABA = carotenoid derivative; ethylene (C2H4) = gas.
The Accidental Discoveries of the Five PGRs
Every major PGR was discovered by accident. Charles and Francis Darwin found that canary-grass coleoptiles bend toward unilateral light (phototropism) and that the coleoptile tip is the source of the transmittable influence; F.W. Went later isolated auxin from oat coleoptile tips. Gibberellin emerged from the 'bakanae' (foolish seedling) disease of rice caused by the fungus Gibberella fujikuroi, reported by E. Kurosawa (1926).
- Auxin (Greek 'auxein' = to grow): first isolated from human urine; Went obtained it from oat coleoptile tips.
- Gibberellin: from Gibberella fujikuroi 'bakanae' disease of rice; Kurosawa, 1926.
- Cytokinin/kinetin: Skoog's tobacco-callus work; Miller et al. crystallised kinetin, 1955.
- ABA: inhibitor-B + abscission II + dormin (mid-1960s) proved identical; Ethylene: Cousins (1910), oranges hastening banana ripening.
Key terms
- Growth
- Irreversible, permanent increase in size of an organ, its parts or a single cell, accompanied by energy-consuming metabolism.
- Meristem
- Localised region of dividing, self-perpetuating cells that enables a plant's open/indeterminate growth.
- Open (indeterminate) growth
- Continuous addition of new cells by meristem activity throughout a plant's life.
- Differentiation
- Maturation of meristem-derived cells into specialised structures and functions.
- Dedifferentiation
- Regaining of division capacity by mature living differentiated cells (e.g., interfascicular and cork cambium from parenchyma).
- Redifferentiation
- Re-maturation of dedifferentiated cells into specific functions, again losing the capacity to divide.
- Development
- Sum of growth and differentiation; all changes from seed germination to senescence.
- Plasticity
- Ability of a plant to follow different developmental pathways in response to environment or life phase (e.g., heterophylly).
- Plant Growth Regulators (PGRs)
- Small, simple, chemically diverse phytohormones regulating plant growth and development.
- Senescence
- Age-driven deterioration that ends development and leads to death.
Must-know facts exam-ready
- Growth = irreversible permanent increase in size; involves both anabolic and catabolic metabolism at the expense of energy.
- A single maize root apical meristem can produce more than 17,500 new cells per hour.
- A watermelon cell can increase in size by up to 3,50,000 times.
- Three phases of growth in order: meristematic → elongation → maturation.
- Arithmetic growth: Lt = L0 + rt → linear curve; only one daughter cell keeps dividing.
- Geometric/exponential growth: W1 = W0 e^rt → sigmoid (S) curve; both daughter cells divide.
- Sigmoid curve phases: lag → log (exponential) → stationary.
- r = relative growth rate = the plant's 'efficiency index'.
- Five PGRs: Auxins, Gibberellins, Cytokinins (promoters) and ABA, Ethylene (inhibitory; ethylene mostly).
- Chemical nature: IAA = indole; GA3 = terpene; kinetin = adenine derivative; ABA = carotenoid derivative; ethylene = gas.
- Auxin was first isolated from human urine; F.W. Went isolated it from oat coleoptile tips; Darwins studied phototropism in canary-grass coleoptiles.
- 'Bakanae' (foolish seedling) disease of rice (Gibberella fujikuroi) gave gibberellin — Kurosawa, 1926; kinetin crystallised by Miller et al. (1955); ethylene traced by Cousins (1910).
Timeline
- 1910H.H. Cousins notes ripened oranges hasten ripening of stored bananas — the volatile later identified as ethylene, a gaseous PGR.
- 1926E. Kurosawa links rice 'bakanae' disease to sterile filtrates of Gibberella fujikuroi — active substance later named gibberellic acid.
- 1955Miller et al. identify and crystallise the cytokinesis-promoting substance kinetin (a cytokinin) from Skoog's tobacco-callus studies.
- mid-1960sInhibitor-B, abscission II and dormin, reported independently, are shown to be identical and named abscisic acid (ABA).
Memory tricks remember it for good
Traps to avoid
- Growth is irreversible — the reversible swelling of dry wood placed in water is NOT growth (a classic trick).
- Arithmetic gives a LINEAR plot (one daughter divides, Lt=L0+rt); geometric gives a SIGMOID curve (both daughters divide, W1=W0 e^rt) — don't swap the curves or equations.
- Primary vs secondary growth: apical meristems add length (primary); lateral meristems/vascular & cork cambium add girth (secondary) — cambium is NOT apical.
- Dedifferentiation ≠ redifferentiation: 'de-' = a mature cell regains division; 're-' = that cell matures again and loses division.
- Auxin was first isolated from human urine (not a plant), though Went obtained it from oat coleoptile tips — both facts are tested.
- Ethylene is grouped as largely an INHIBITOR despite promoting fruit ripening; ABA drives dormancy and abscission.
Exam focus
🧠 Prelims angles
- PGR matching: hormone ↔ chemical class ↔ discoverer ↔ source organism (Darwin/canary grass, Went/oat, Kurosawa/Gibberella-rice, Miller/kinetin, Cousins/orange-banana).
- Growth equations and curves: arithmetic (linear, Lt=L0+rt) vs geometric (sigmoid, W1=W0 e^rt) and the lag–log–stationary phases.
- Meristems: apical vs lateral; primary vs secondary growth; the 'open' indeterminate nature of plant growth.
- The differentiation trio with examples (interfascicular and cork cambium arising from parenchyma).
- Plasticity/heterophylly examples: cotton, coriander, larkspur and buttercup.
- Classification of PGRs into growth promoters vs inhibitors, and ethylene's dual but mainly inhibitory role.
✍️ Mains angles GS-III
- Role of plant growth regulators in agriculture and horticulture.Classify into promoters (auxin, gibberellin, cytokinin) and inhibitors (ABA, ethylene); cite chapter-grounded effects — ethylene-led ripening, ABA-led dormancy/abscission — then link to crop and post-harvest management (GS-III).
- Significance of dedifferentiation and plant tissue culture for biotechnology.Use the parenchyma → callus → micropropagation logic; connect 'open' growth/differentiation and cellular plasticity to mass propagation and crop improvement.
- How do intrinsic and extrinsic factors jointly regulate plant development?Frame development = growth + differentiation; weigh genetic/PGR (intrinsic) against light, temperature, water, O2 and nutrition (extrinsic); cite plasticity/heterophylly as evidence.
Last-minute revision tick as you recall
- Growth = irreversible increase in size; reversible swelling (wood in water) ≠ growth.
- Indeterminate 'open' growth via meristems; primary = apical, secondary = lateral (vascular + cork cambium).
- Phases: meristematic → elongation → maturation.
- Arithmetic: Lt = L0 + rt (linear); Geometric: W1 = W0 e^rt (sigmoid: lag-log-stationary).
- r = relative growth rate = efficiency index; absolute = total growth/time.
- DDR: Differentiation → Dedifferentiation → Redifferentiation.
- 5 PGRs — AGC promote (Auxin, Gibberellin, Cytokinin); ABA + Ethylene inhibit.
- Chemistry: IAA=indole, GA3=terpene, kinetin=adenine, ABA=carotenoid, ethylene=gas.
- Discovery: auxin (urine/oat-Went), gibberellin (Gibberella-Kurosawa 1926), kinetin (Miller 1955), ethylene (Cousins 1910); plasticity = heterophylly.
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.