Plant Growth and Development

2.1 Core concepts

• Development = growth + differentiation, a precise and highly ordered succession of events that produces roots, leaves, branches, flowers, fruits and seeds, and ends in death; seed germination is the first step and needs favourable conditions, failing which the seed remains in suspended growth or rest (NCERT §13 intro, p. 166).
• Growth is defined as an irreversible permanent increase in size of an organ, its parts, or even a single cell, generally accompanied by anabolic and catabolic metabolism at the expense of energy (NCERT §13.1, p. 167).
• Plant growth is indeterminate — plants retain unlimited growth capacity throughout life due to meristems whose cells divide and self-perpetuate; this is called the open form of growth (NCERT §13.1.1, p. 167).
• Root apical meristem and shoot apical meristem drive primary growth (elongation along the axis); in dicots and gymnosperms, lateral meristems — vascular cambium and cork cambium — appear later and cause secondary growth (increase in girth) (NCERT §13.1.1, p. 168).
• Growth at the cellular level is principally an increase in protoplasm, measured indirectly by fresh weight, dry weight, length, area, volume or cell number; one maize root apical meristem can give rise to more than 17,500 new cells per hour, whereas a watermelon cell can increase in size up to 3,50,000 times (NCERT §13.1.2, p. 168).
• Three phases of growth in a root tip: meristematic (cells with dense protoplasm, large nuclei, thin primary cellulosic walls, abundant plasmodesmata), elongation (increased vacuolation, cell enlargement, new wall deposition), and maturation (maximal wall thickening and protoplasmic modifications) (NCERT §13.1.3, pp. 168–169).
• Arithmetic growth: after mitosis only one daughter cell continues to divide while the other differentiates; expressed as Lₜ = L₀ + rt, giving a linear curve (e.g., root elongating at a constant rate) (NCERT §13.1.4, p. 170).
• Geometric growth: both daughter cells retain divisional ability — initial slow lag phase, then exponential/log phase, then a stationary phase due to limited nutrients; plotted against time it gives a sigmoid (S-shaped) curve, characteristic of all cells, tissues and organs in nature; expressed as W₁ = W₀ eʳᵗ, where r is the relative growth rate and the efficiency index (NCERT §13.1.4, pp. 170–171).
• Absolute growth rate = total growth per unit time; relative growth rate = growth per unit time expressed on a common basis (e.g., per unit initial parameter); two leaves with the same absolute increase (5 cm²) can have very different relative growth rates (NCERT §13.1.4, p. 171, Fig. 13.7).
• Conditions for growth: water (turgidity drives cell enlargement, medium for enzymes), oxygen (releases metabolic energy), nutrients (macro + micro elements for protoplasm and energy), optimum temperature range, plus environmental signals such as light and gravity (NCERT §13.1.5, p. 171).
• Differentiation = act leading to maturation of cells from meristems/cambium, involving structural changes in walls and protoplasm — e.g., a tracheary element loses its protoplasm and develops strong, elastic, lignocellulosic secondary walls to transport water under tension (NCERT §13.2, p. 172).
• Dedifferentiation = living differentiated cells that had lost divisional capacity regain it — e.g., interfascicular cambium and cork cambium form from fully differentiated parenchyma; these meristems then redifferentiate to give cells that again lose divisional capacity but mature for specific functions (NCERT §13.2, p. 172).
• Differentiation in plants is open: cells from the same meristem develop different structures depending on location — e.g., cells away from the root apical meristem become root-cap cells, while peripheral cells mature into epidermis (NCERT §13.2, p. 172).
• Plasticity — plants follow different developmental pathways in response to environment or life phase, e.g., heterophylly in cotton, coriander and larkspur (juvenile vs adult leaves) and in buttercup (leaves in air vs in water) (NCERT §13.3, p. 173, Fig. 13.9).
• PGRs are small simple molecules of diverse chemistry: indole compounds (IAA), adenine derivatives (kinetin = N⁶-furfurylamino purine), carotenoid derivatives (ABA), terpenes (GA₃), or gases (ethylene, C₂H₄); also called plant growth substances, plant hormones or phytohormones (NCERT §13.4.1, p. 174).
• Two functional groups of PGRs: promoters (auxins, gibberellins, cytokinins) and inhibitors/stress responders (ABA); ethylene fits either group but is largely a growth inhibitor (NCERT §13.4.1, p. 174).
• Discovery — Auxin: Charles Darwin and Francis Darwin observed that canary grass coleoptiles bent towards unilateral light (phototropism); the tip was the site of the transmittable influence. F. W. Went later isolated auxin from oat coleoptile tips (NCERT §13.4.2, p. 174, Fig. 13.10).
• Discovery — Gibberellin: 'bakanae' (foolish seedling) disease of rice caused by Gibberella fujikuroi; E. Kurosawa (1926) reproduced symptoms using sterile fungal filtrates; the active substance was later identified as gibberellic acid (NCERT §13.4.2, p. 175).
• Discovery — Cytokinin: F. Skoog and co-workers found that tobacco internode callus proliferated only when auxin was supplemented with vascular tissue extract, yeast extract, coconut milk or DNA; Miller et al. (1955) isolated and crystallised the cytokinesis-promoting substance and named it kinetin (from autoclaved herring sperm DNA) (NCERT §13.4.2, p. 175).
• Discovery — ABA: in the mid-1960s, three independent groups characterised inhibitor-B, abscission II and dormin, which were later shown to be chemically identical and named abscisic acid (ABA) (NCERT §13.4.2, p. 175).
• Discovery — Ethylene: H. H. Cousins (1910) confirmed that ripened oranges release a volatile substance that hastens ripening of stored unripened bananas; this was later identified as ethylene (NCERT §13.4.2, p. 175).
• Auxins (Greek auxein, "to grow") — first isolated from human urine; natural: IAA, IBA; synthetic: NAA, 2,4-D; produced at growing apices of stems and roots and migrate to sites of action; promote rooting in stem cuttings, flowering in pineapples, prevent early fruit/leaf drop, promote abscission of older mature leaves/fruits, induce apical dominance (apical bud suppresses axillary buds — decapitation releases laterals; used in tea plantations and hedge-making), induce parthenocarpy (e.g., tomatoes), and 2,4-D acts as a herbicide killing dicot weeds while sparing mature monocots; auxin also controls xylem differentiation and helps in cell division (NCERT §13.4.3.1, pp. 175–176).
• Gibberellins — more than 100 known (GA₁, GA₂, GA₃...); GA₃ was the first discovered and most studied; all GAs are acidic; increase length (lengthens grape stalks; elongates and improves apple shape; delays senescence so fruit can stay on the tree longer), speed up malting in the brewing industry, increase sugarcane stem length and yield (up to 20 tonnes per acre), hasten maturity of juvenile conifers for early seed production, and promote bolting (internode elongation just before flowering in rosette plants like beet and cabbage) (NCERT §13.4.3.2, p. 176).
• Cytokinins — discovered as kinetin (modified adenine, a purine) from autoclaved herring sperm DNA; kinetin does not occur naturally; zeatin isolated from corn-kernels and coconut milk; synthesised where rapid cell division occurs (root apices, developing shoot buds, young fruits); help produce new leaves, chloroplasts, lateral shoots, adventitious shoots; overcome apical dominance; promote nutrient mobilisation thereby delaying leaf senescence (NCERT §13.4.3.3, pp. 176–177).
• Ethylene — simple gaseous PGR synthesised in large amounts by senescing tissues and ripening fruits; causes horizontal seedling growth, axis swelling and apical hook in dicot seedlings; promotes senescence and abscission of leaves/flowers; highly effective in fruit ripening and raises the respiration rate ("respiratory climacteric"); breaks seed and bud dormancy, initiates germination in peanut, sprouting of potato tubers; promotes rapid internode/petiole elongation in deep-water rice (keeps leaves above water); promotes root growth and root hair formation; initiates and synchronises flowering/fruit-set in pineapple and induces flowering in mango; ethephon is the most widely used ethylene source — hastens ripening in tomato/apple, accelerates abscission (thinning cotton, cherry, walnut), promotes female flowers in cucumber (NCERT §13.4.3.4, p. 177).
• Abscisic acid (ABA) — discovered via regulation of abscission and dormancy; acts as a general plant growth inhibitor and inhibitor of metabolism; inhibits seed germination; stimulates stomatal closure and increases tolerance to stresses — hence the "stress hormone"; important in seed development, maturation and dormancy (helps seeds withstand desiccation); antagonistic to GAs in most situations (NCERT §13.4.3.5, pp. 177–178).
• PGR roles can be complementary or antagonistic, individualistic or synergistic; events like seed/bud dormancy, abscission, senescence and apical dominance involve more than one PGR; PGRs are only one kind of intrinsic control, working alongside genomic control and extrinsic factors (temperature, light) which often act via PGRs to control events such as vernalisation, flowering, dormancy, seed germination and plant movements (NCERT §13.4.3.5, p. 178).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 13.1, p. 167 — germination and seedling development in bean (seed coat, epicotyl hook, cotyledons, hypocotyl, soil line).
• Figure 13.2, p. 168 — locations of root apical meristem, shoot apical meristem and vascular cambium with arrows showing direction of growth.
• Figure 13.3, p. 168 — detection of zones of elongation in a root by the parallel-line technique (zones A, B, C, D immediately behind the apex elongate most).
• Figure 13.4, p. 169 — arithmetic vs geometric growth; embryo stages showing geometric phase (all cells divide) and arithmetic phase (some daughters lose divisional capacity).
• Figure 13.5, p. 170 — constant linear growth curve (height vs time).
• Figure 13.6, p. 170 — idealised sigmoid growth curve with lag, exponential and stationary phases.
• Figure 13.7, p. 171 — comparison of absolute vs relative growth rates using two leaves A (5 → 10 cm²) and B (50 → 55 cm²) that show the same absolute increase but very different relative increases.
• Figure 13.8, p. 173 — developmental sequence in a plant cell: Meristematic cell → (cell division, plasmatic growth, expansion/elongation, differentiation, maturation) → Mature cell → senescence → death.
• Figure 13.9, p. 173 — heterophylly in larkspur (juvenile vs adult) and buttercup (terrestrial vs aquatic leaves).
• Figure 13.10, p. 174 — Darwin/Went-style coleoptile tip experiment showing the tip is the source of auxin and phototropic response.
• Figure 13.11, p. 176 — apical dominance: plant with intact apical bud vs decapitated plant showing growth of lateral buds.

2.4 Common confusions / NTA trap points

• Arithmetic vs geometric growth — students often swap the formulae. Lₜ = L₀ + rt is arithmetic (linear); W₁ = W₀ eʳᵗ is geometric (exponential). Sigmoid curve belongs to geometric growth in a natural environment.
• Absolute vs relative growth rate — two leaves can have the same absolute increase yet very different relative rates; in Fig. 13.7 the smaller leaf A has the higher relative growth rate.
• Kinetin source — kinetin was crystallised from autoclaved herring sperm DNA, not from a plant; zeatin is the natural cytokinin from corn-kernels and coconut milk.
• PGR chemistry trap — IAA is an indole compound, kinetin is an adenine (purine) derivative, ABA is a carotenoid derivative, GA₃ is a terpene, ethylene is a gas; NTA loves to scramble these matches.
• Auxin discovery — Darwin only inferred a transmittable influence in the coleoptile tip; F. W. Went actually isolated auxin from oat coleoptile tips.
• 2,4-D selectivity — kills dicot weeds, does not affect mature monocots; used for weed-free lawns.
• Bolting vs apical dominance — bolting is gibberellin-induced internode elongation before flowering in rosette plants; apical dominance is auxin-induced suppression of laterals. Cytokinin overcomes apical dominance.
• ABA vs ethylene — both inhibit, but ABA is the stress hormone (closes stomata, inhibits germination) while ethylene is the ripening hormone (climacteric, abscission, dormancy-breaking in peanut/potato).