Plant Growth and Development — NEET Notes

Definition of growth — the irreversible promise

Growth is one of the most fundamental and conspicuous characteristics of any living being. NCERT defines it with two words that decide a dozen NEET questions: growth is an irreversible permanent increase in size of an organ, its parts, or even of an individual cell. The word "irreversible" is the trap. A piece of wood placed in water swells — but it shrinks again when dried; that is not growth, only imbibition. A balloon inflated and deflated is not growing. Growth, in the biological sense, has direction: once added, the mass stays added.

At the cellular level, growth is principally a consequence of an increase in the amount of protoplasm. Because protoplasm itself is difficult to measure, botanists use proxies that track it closely — fresh weight, dry weight, length, area, volume, and cell number. One maize root apical meristem can produce more than 17,500 new cells per hour; a single watermelon cell can swell up to 3,50,000 times in volume. In one case growth means more cells, in the other it means bigger cells. A pollen tube is measured by length; a dorsiventral leaf by surface area. No single parameter fits every situation — which is precisely what NCERT Exercise 2 asks you to explain.

Plant growth carries another distinctive feature: it is generally indeterminate and open. Higher plants retain the capacity for unlimited growth throughout life, because cells of the meristem at the root apex, shoot apex, vascular cambium, and cork cambium can divide and self-perpetuate indefinitely. New cells are continually added; the plant body never stops getting larger in some dimension. This is why a banyan tree can live for centuries, adding girth and branches with every monsoon. Meristems of the root apex and shoot apex drive primary growth (elongation along the axis); the lateral meristems — vascular and cork cambium, appearing later in dicots and gymnosperms — drive secondary growth (increase in girth).

Growth is an irreversible permanent increase in size of an organ, its parts, or even of an individual cell.

NCERT definition — the wording is examinable

Phases of growth — meristematic, elongation, maturation

Look at any root tip under a microscope and you will see growth happening in three distinct zones, stacked along a few millimetres. NCERT divides the period of growth into three phases — and NEET tests both their names and their characteristics.

The three phases occur in the same root simultaneously but at different distances from the apex. The dividing cells at the very tip are meristematic; just behind them lies the elongation zone where cells stretch and the root visibly lengthens; further back, the cells stop expanding and acquire their final identity — xylem, phloem, cortex, epidermis. The classical experiment to demonstrate this is the parallel-line technique (NCERT Figure 13.3): mark equally spaced parallel lines across a young root with ink, and after a few hours, the lines closest to the tip will have spread the most — proving that the elongation zone, not the very apex, is where growth in length actually occurs.

Growth rates — arithmetic, geometric, and the sigmoid curve

Growth per unit time is the growth rate. An organism can add cells in two mathematically distinct ways, and NEET has tested both. In arithmetic growth, after a mitotic division, only one of the two daughter cells continues to divide while the other differentiates and matures. A root elongating at a constant rate is the textbook example. Plotted against time, the result is a straight line, captured by the formula L_t = L₀ + rt, where L₀ is the starting length, r the elongation per unit time, and t the time.

In geometric growth, both daughter cells retain the ability to divide and continue doing so. The number of dividing cells doubles every generation, and the rate of increase accelerates exponentially — at least until nutrients run out. Plotted against time, this gives a sigmoid or S-shaped curve with three distinguishable phases. The mathematical form of the exponential portion is W₁ = W₀ e^rt, where r is the relative growth rate (also called the efficiency index) and e is the base of natural logarithms.

Quantitative comparisons of growth use two distinct measures. Absolute growth rate is the total increase per unit time. Relative growth rate is the increase per unit time expressed on a common basis — typically per unit of starting material. Two leaves can show the same absolute increase in area (say, +5 cm² in a given time) but very different relative growth rates: the smaller leaf is "growing faster" relative to itself. NEET has tested this distinction through diagrams and direct definition questions.

Conditions for growth

A meristem can divide only if the environment supplies what cells need to enlarge and metabolise. The NCERT lists four non-negotiable requirements, and the NIOS supplement repeats them: water, oxygen, nutrients, and a suitable temperature. Each one acts through a specific mechanism — water provides turgor and the solvent for enzymes; oxygen releases the energy stored in respiratory substrates; mineral nutrients build protoplasm and serve as enzyme cofactors; temperature governs the rates of every metabolic step. Every plant species has an optimum temperature band (typically 28–30 °C for most crops, but a much wider tolerance of 4–45 °C in nature). Light and gravity are additional environmental signals that influence specific phases of growth — most visibly in phototropism and gravitropism.

Differentiation, dedifferentiation, redifferentiation

A meristematic cell is unspecialised. The journey from meristematic cell to xylem vessel, sieve tube, root hair, or guard cell involves three sequential processes that NEET tests almost every year through definition swaps. Differentiation is the maturation of cells derived from meristems into specialised cell types performing specific functions. To form a tracheary element, for example, a cell loses its protoplasm entirely and develops a strong, elastic, lignocellulosic secondary wall — a one-way structural commitment.

The remarkable feature of plant differentiation is that it is reversible. Dedifferentiation is the phenomenon where living differentiated cells, that have lost the capacity to divide, regain that capacity under certain conditions. Examples include the formation of interfascicular cambium and cork cambium from already differentiated parenchyma cells, and — most importantly for tissue culture — the formation of callus from leaf mesophyll cells placed in culture medium. NEET 2023, Q.109 asked this exact scenario, and the answer is dedifferentiation.

Once dedifferentiated cells have proliferated, they can again mature into specialised tissues — this third stage is redifferentiation. The vascular tissue of a woody stem produced by the interfascicular cambium is a redifferentiated tissue: a parenchyma cell dedifferentiated into a cambium cell, then redifferentiated into a xylem or phloem cell. Plant differentiation is therefore "open" — cells derived from the same meristem can take on different structures at maturity, and even after maturity, they can backtrack and start over.

Development & plasticity

Plants do not just grow; they develop. Development is the sum of growth and differentiation — the entire sequence of qualitative and quantitative changes an organism passes through, from the germination of a seed to senescence and death. Cell division → plasmatic growth → elongation → maturation → differentiation → senescence: every cell, tissue, and organ follows this trajectory. NIOS adds morphogenesis — the development of shape and external form — as a parallel process. Both intrinsic factors (genes, plant growth regulators) and extrinsic factors (light, temperature, water, oxygen, nutrition, gravity) control development jointly.

The most distinctive feature of plant development is plasticity: the ability to follow different developmental pathways in response to the environment, producing different structures from genetically identical cells. The classic NCERT example is heterophylly — different leaf shapes on the same plant. In cotton, coriander, and larkspur, juvenile leaves are markedly different in shape from mature leaves. In buttercup (Ranunculus), leaves produced in air differ from those produced under water — a developmental response to the surrounding medium. NEET 2021 (Q.106) and NEET 2022 (Q.112) both tested plasticity directly: maize, notably, does not show plasticity, while cotton, coriander, larkspur, and buttercup do.

Development = growth + differentiation.

NCERT — the equation behind every NEET match-the-following on this chapter

Plant growth regulators — the five families and their discovery

Plant growth regulators (PGRs) are small organic molecules — synthesised in one part of the plant, often transported to another — that promote, inhibit, or modify physiological processes. They are also called plant hormones or phytohormones. Despite their diverse chemistry, NCERT recognises exactly five families: auxins, gibberellins, cytokinins, ethylene, and abscisic acid. Three are growth promoters (auxins, gibberellins, cytokinins), one is a growth inhibitor (ABA), and one is largely inhibitory but gaseous (ethylene). Each was discovered by accident — and the discovery stories themselves are NEET material.

Auxins trace back to Charles Darwin and his son Francis (1880), who noticed that canary grass coleoptiles bent towards a unilateral light source — phototropism. After a sequence of decapitation and barrier experiments, they concluded the tip of the coleoptile was the site of a transmittable influence. F. W. Went later isolated the substance from oat coleoptile tips by collecting it onto agar blocks, naming it auxin. The first auxin was, somewhat unromantically, isolated from human urine.

Gibberellins were discovered through a rice disease called bakanae ("foolish seedling"), in which infected seedlings grew abnormally tall and weak. E. Kurosawa (1926) reported that sterile filtrates of the fungal pathogen Gibberella fujikuroi reproduced the symptoms in healthy seedlings. The active substance was later identified as gibberellic acid (GA₃); over 100 gibberellins, named GA₁, GA₂, GA₃… have since been identified across fungi and higher plants. All GAs are acidic.

Cytokinins emerged from F. Skoog's tobacco-stem callus experiments: callus tissue proliferated only when the medium contained an auxin plus one of several cell-division-promoting factors (vascular tissue extracts, yeast extract, coconut milk, or DNA). Miller and colleagues (1955) identified and crystallised the active compound from autoclaved herring sperm DNA and named it kinetin — a modified form of adenine. Kinetin does not occur naturally in plants; the search for natural cytokinins led to zeatin (from corn kernels and coconut milk).

Abscisic acid was discovered three times in parallel during the mid-1960s by three independent groups — as inhibitor-B, abscission II, and dormin. All three turned out to be chemically identical, and the molecule was renamed abscisic acid (ABA). It is a sesquiterpene derived from carotenoids.

Ethylene, the only gaseous PGR, was the first to be implicated as a plant hormone, though identified last. H. H. Cousins (1910) confirmed that ripe oranges released a volatile substance that hastened the ripening of stored unripe bananas. The volatile was later identified as ethylene (C₂H₄).

Auxins — apical dominance, cell elongation, weed killer

The word auxin comes from the Greek auxein, "to grow." It is now applied to indole-3-acetic acid (IAA) — the principal natural auxin — and to other compounds with similar growth-regulating properties. Natural auxins include IAA and IBA (indole-3-butyric acid); NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are synthetic. Auxins are produced in the growing apices of stems and roots and migrate from there to regions of action — typically downward in the shoot (polar transport).

The agricultural and horticultural reach of auxins is enormous. They initiate rooting in stem cuttings — the basis of vegetative propagation in horticulture (IBA is the standard for this). They induce flowering in pineapples and litchis (NEET 2019 tested this). They prevent fruit and leaf drop at early stages while paradoxically promoting abscission of older mature leaves and fruits (NEET 2017 tested the prevention side). They cause apical dominance — the apical bud inhibits axillary bud growth, which is why tea plantations and hedges are decapitated to encourage bushier lateral branching. Auxins induce parthenocarpy (development of seedless fruits) in tomatoes. And 2,4-D is the most widely used dicot weedicide in cereal fields — killing broad-leaved weeds while leaving monocot crops untouched (NEET 2021 tested this directly). Auxin also controls xylem differentiation and aids cell division.

Gibberellins — bolting, dormancy breaking, malting

Gibberellins (GAs) are the second class of growth promoters, all acidic, all named GA₁, GA₂, GA₃… in order of discovery. GA₃ is the most extensively studied. They produce a remarkably wide range of physiological effects, all converging on one theme: stem elongation. Spraying sugarcane crops with gibberellins increases stem length and yield by as much as 20 tonnes per acre (NEET 2020 Q.54 tested this). They increase the length of grape stalks, allowing better aeration and bunch quality. They cause apples to elongate and improve fruit shape. They delay senescence, letting fruits remain on the tree longer to extend market periods.

Two additional roles are NEET favourites. Gibberellins break seed and bud dormancy — and are essentially the antagonist of ABA in this regard. They speed up the malting process in the brewing industry by inducing α-amylase production in barley grains, allowing rapid starch hydrolysis to fermentable sugars (NEET 2022 hinted at this in Q.108). Gibberellins also promote bolting — the rapid internode elongation just before flowering in rosette plants like beet and cabbage. Spraying juvenile conifers with GAs hastens the maturity period, leading to early seed production (NEET 2023 Q.108 tested this directly). And they induce parthenocarpy in some species, complementing auxin's role in others.

Cytokinins — cell division, breaking apical dominance, delaying senescence

Cytokinins were discovered as kinetin, an adenine derivative obtained from autoclaved herring sperm DNA. Kinetin itself does not occur naturally in plants; the natural counterpart is zeatin, isolated from corn kernels and coconut milk. Cytokinins are synthesised wherever rapid cell division occurs — root apices, developing shoot buds, young fruits, endosperm of seeds. They are the most directly associated of all the hormones with cytokinesis (cell division), which is the etymological root of the name.

The physiological repertoire of cytokinins is short but distinctive. They stimulate cell division and lateral shoot growth. They help produce new leaves and chloroplasts, and promote adventitious shoot formation. They overcome apical dominance — the exact opposite of auxin's action (NEET 2022 Q.108 tested this distinction, eliminating cytokinin as a possible answer for an ethylene question by elimination). They promote nutrient mobilisation from older to younger tissues, which delays leaf senescence and helps leaves stay green longer. NCERT Exercise 8 asks which PGR you would use to delay leaf senescence — the answer is cytokinin.

The auxin-to-cytokinin ratio in tissue culture is the lever that decides what regenerates from callus: a higher auxin-to-cytokinin ratio favours root formation; a higher cytokinin-to-auxin ratio favours shoot formation; an intermediate ratio sustains callus proliferation. NCERT Exercise 10 asks what happens if you forget to add cytokinin to a culture medium — the callus stops proliferating into new tissues because cell division and shoot regeneration falter.

Ethylene — fruit ripening, climacteric rise, female flowers

Ethylene (C₂H₄) is the simplest organic molecule that functions as a hormone — a two-carbon gas that diffuses through the intercellular spaces of plants and even across plants stored together. It is synthesised in large amounts by tissues undergoing senescence and by ripening fruits. Ethylene's effects on seedlings include the famous "triple response": horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings emerging from soil.

Ethylene's celebrity status comes from fruit ripening. It enhances the respiration rate during ripening — a phenomenon called the respiratory climactic (or climacteric) rise. The hormone breaks seed and bud dormancy, initiates germination in peanut seeds, and triggers sprouting of potato tubers. In deep water rice, ethylene promotes rapid internode and petiole elongation, keeping leaves above floodwater (NEET 2023 Q.124 tested this exact application). Ethylene also promotes root growth and root hair formation, increasing the absorption surface area — NEET 2022 Q.108 tested this, identifying ethylene as the gaseous PGR that promotes root growth and root hair formation.

Commercially, ethylene initiates flowering and synchronises fruit-set in pineapples and induces flowering in mango. It increases the number of female flowers in cucumbers (NEET 2022 Q.117 tested this), boosting yield. The most widely used source of ethylene in agriculture is ethephon — an aqueous solution absorbed by the plant, releasing ethylene slowly inside the tissue. Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission for thinning cotton, cherry, and walnut crops.

Abscisic acid — stress hormone, dormancy, stomatal closure

Abscisic acid (ABA) is the principal growth inhibitor among the PGRs. Discovered for its role in regulating abscission and dormancy, it has emerged as the central hormone in plant stress responses. ABA acts as a general plant growth inhibitor and an inhibitor of metabolism. It inhibits seed germination. It stimulates the closure of stomata within minutes when leaves face water stress — the single most important quick action of any plant hormone. By raising the plant's tolerance to drought, salt, and cold, ABA earns its title: the stress hormone (NCERT Exercise 5 asks you to justify this name explicitly).

ABA plays a central role in seed development, maturation, and dormancy. By inducing dormancy, ABA helps seeds withstand desiccation and other unfavourable conditions. NEET 2020 Q.3 tested this directly — listing inhibitory substances governing seed dormancy and asking which one was NOT inhibitory; gibberellic acid was the answer because GA breaks dormancy while ABA induces it. In most physiological situations, ABA acts as an antagonist of gibberellins: where GA breaks dormancy, ABA imposes it; where GA promotes germination, ABA blocks it; where GA elongates stems, ABA inhibits growth.

Five hormones — a NEET-ready summary

The single most-tested matrix in this chapter is the five-hormone × physiological-function grid. NCERT Exercise 8 lists six tasks ("induce rooting in a twig", "quickly ripen a fruit", "delay leaf senescence", "induce growth in axillary buds", "'bolt' a rosette plant", "induce immediate stomatal closure") — every one of which maps to one of the five hormones. The factor grid below collects the essential function and a flagship biological role for each PGR.

Photoperiodism — flowering by the clock of day length

Beyond hormones, two environmental cues control the most dramatic developmental switch in a plant's life: the transition from vegetative growth to flowering. The first is light duration — photoperiodism. Plants measure not only the amount of light but the relative lengths of day and night. They flower (or don't) accordingly. Based on the critical day length required, plants fall into three categories.

The site of light perception is the leaf — not the shoot apex or axillary bud. Even a single leaf, exposed to the correct photoperiod while the rest of the plant sits in the wrong conditions, can induce flowering in the entire shoot. NEET 2019 (Q.53) and NEET 2021 (Q.123) both tested this — the answer to "site of perception of photoperiod" is always the leaf. A hypothetical hormone called florigen is thought to migrate from the leaf to the shoot apex, where it triggers flowering. NCERT Exercise 9 asks whether a defoliated plant would respond to photoperiodic cycles — the answer is no, because the perception site (the leaves) has been removed.

The photoreceptor for photoperiodism is the pigment phytochrome, which exists in two interconvertible forms. P_r absorbs red light (~660 nm) and converts to P_fr; P_fr absorbs far-red light (~730 nm) and converts back to P_r. Sunlight, rich in red, drives the equilibrium toward P_fr during the day; in darkness, P_fr slowly reverts to P_r. Long-day plants flower when P_fr levels are high (long day → little time for P_fr → P_r conversion); short-day plants flower when P_fr levels fall low (long night → ample P_fr → P_r conversion). The interconversion is the molecular clock that lets plants track the seasons.

P_r ⇌ P_fr — the molecular hourglass that tells a plant whether it is summer or winter.

Phytochrome interconversion

Vernalisation — flowering after a winter

The second environmental switch governing flowering is temperature. Some species, including many biennials and winter cereals, refuse to flower unless they first experience a prolonged period of cold. This requirement is called vernalisation — from the Latin vernus, "of spring." Treating seeds or seedlings of winter wheat with low temperatures (1–10 °C) for several weeks lets them flower in the same growing season as spring wheat; without the cold treatment, they remain vegetative.

Vernalisation has two major practical advantages. First, plants whose life cycle naturally spans two growing seasons (biennials such as cabbage, beet, and carrot) can be induced to flower in a single season if seeds are pre-treated at low temperature. Second, crops can be sown and harvested earlier — biennials effectively converted into annuals, doubling the cropping intensity over a fixed time. The site of perception of the low-temperature stimulus is the shoot apex and the embryo (in seeds) — a different anatomical site from the leaf-based perception of photoperiodism, which is a frequently tested distinction.