Plant Growth and Development

The chapter moves through four connected ideas. Section 13.1 defines growth, explains why plant growth is indeterminate and "open," describes its three phases, and introduces arithmetic versus geometric growth with absolute and relative growth rates. Section 13.2 takes up differentiation and its companions dedifferentiation and redifferentiation — the basis of grafting and tissue culture. Section 13.3 defines development as growth plus differentiation and introduces plasticity through heterophylly. Section 13.4, the longest section, surveys the five plant growth regulators (PGRs) — chemical nature, discovery, physiological effects and agricultural applications.

What is growth, and why is it measurable

Growth is defined as an irreversible, permanent increase in size of an organ, or its parts, or of a single cell. A leaf that wilts and then recovers turgor when watered has not "grown" and "un-grown" — that is a reversible change in water content, not growth. Growth is generally accompanied by anabolic and catabolic metabolic processes at the expense of energy, and at the cellular level is principally a consequence of increasing protoplasm. Since protoplasm is hard to measure directly, growth is instead measured through parameters roughly proportional to it: fresh weight, dry weight, length, area, volume and cell number. A single maize root apical meristem can give rise to more than 17,500 new cells per hour — growth as cell number. A watermelon cell can increase in size by up to 3,50,000 times — growth as cell size. A pollen tube's growth is measured by length; a dorsiventral leaf's, by surface area. No single parameter is adequate to describe growth throughout the life of a plant — the right yardstick depends on the organ and stage.

Indeterminate, open growth: the role of meristems

Animal growth is usually determinate: an organism grows to a characteristic adult size and stops. Plant growth is different — plants retain the capacity for unlimited growth throughout their life, a property called indeterminate growth, possible because of meristems, localised regions whose cells retain the capacity to divide and self-perpetuate. A meristematic cell divides to produce two daughter cells; typically one stays meristematic while the other is displaced and begins to differentiate. Because new cells are continually added by meristem activity, this pattern is called the open form of growth — in sharp contrast to the closed, determinate body plan of most animals. The root apical meristem and shoot apical meristem are responsible for primary growth — elongation of the plant body along its main axis. In dicotyledonous plants and gymnosperms, two additional lateral meristems — the vascular cambium and cork cambium — appear later in life and are responsible for secondary growth, the increase in girth of stems and roots (as in the annual rings of a tree trunk).

The three phases of growth: meristematic, elongation, maturation

Because growth in a plant is localised at meristems rather than spread uniformly across the body, a growing root or shoot tip shows a spatial sequence of phases from tip to base, illustrated in NCERT's Figure 13.3 (the parallel-line technique, where marked zones A, B, C, D immediately behind the apex elongate the most).

• Meristematic phase: at the root and shoot apex, constantly dividing cells rich in protoplasm, with large nuclei and thin, primary, cellulosic walls richly connected by plasmodesmata.
• Elongation phase: cells just behind the meristematic zone, marked by increased vacuolation, cell enlargement, and new cell wall deposition.
• Maturation phase: further back, cells attain maximal size, with wall thickening and protoplasmic modifications — most mature tissue types (xylem, phloem, epidermis) represent this phase.

Growth rate: arithmetic versus geometric growth

The increase in growth per unit time is the growth rate, and it can be expressed mathematically in two contrasting forms.

Arithmetic growth. Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The simplest example is a root elongating at a constant rate. Plotting the length of the organ against time gives a straight (linear) line:

where: $L_t$ is the length at time $t$, $L_0$ is the length at time zero, and $r$ is the growth rate (elongation per unit time). Because growth adds a fixed absolute amount per unit time, the curve is a straight line, not a curve that bends upward.

Geometric (exponential) growth. Here both progeny cells produced by mitotic division retain the ability to divide, and both continue to do so. Initial growth is slow — the lag phase — and then increases rapidly, at an exponential rate — the log (exponential) phase. With limited nutrient supply, the growth eventually slows to a stationary phase. Plotting the growth parameter against time under geometric growth gives a characteristic S-shaped (sigmoid) curve, typical of living organisms growing in a natural environment — seen in cells, tissues and organs of a plant. Exponential growth is expressed as:

where: $W_1$ is the final size (weight, height, number, etc.), $W_0$ is the initial size at the beginning of the period, $r$ is the growth rate, $t$ is the time of growth, and $e$ is the base of natural logarithms. Here $r$ is called the relative growth rate, also called the efficiency index — a measure of the plant's ability to produce new plant material. The final size $W_1$ therefore depends critically on the initial size $W_0$.

Absolute and relative growth rates

Quantitative comparisons between the growth of two systems can be made two ways. The absolute growth rate is total growth per unit time. The relative growth rate is growth per unit time expressed on a common basis — per unit initial leaf area, for instance. Two leaves of different starting sizes might show the same absolute increase in area (say both gain 5 cm²), yet the smaller leaf shows a much higher relative growth rate, since that gain is a larger fraction of its own starting size — same absolute gain, different relative gain, a favourite NEET reasoning question.

Conditions necessary for growth

Water, oxygen and nutrients are essential for growth. Plant cells grow chiefly by cell enlargement, which requires water: turgidity drives extension growth, and water is the medium for growth enzymes. Oxygen releases the metabolic energy for growth, and nutrients are needed for synthesising protoplasm and as an energy source. Every plant has an optimum temperature range for growth, and light and gravity affect specific growth phases.

Differentiation: cells becoming specialists

Cells derived from the root apical meristem, shoot apical meristem and cambium differentiate and mature to perform specific functions — this maturing process is differentiation. Cells undergo structural changes in their walls and protoplasm; the textbook example is the tracheary element (xylem cell), which loses its protoplasm entirely and develops a strong, elastic, lignocellulosic secondary wall able to conduct water under extreme tension.

Differentiation, like growth, is "open." Cells arising from the same meristem can end up with entirely different structures at maturity, dictated by location: cells away from the root apical meristem differentiate into root-cap cells, while cells at the periphery mature into epidermis — same origin, different fate.

Dedifferentiation and redifferentiation

Plants show a capability most animal tissues lack: a differentiated cell that has already lost its capacity to divide can regain it — a reversal called dedifferentiation. The classic examples are two secondary meristems formed from differentiated parenchyma: the interfascicular cambium and cork cambium. These divide and produce cells that again lose the ability to divide but mature for a specific function — this final maturation is redifferentiation. The cycle underlies wound healing, grafting, and, induced deliberately under sterile conditions, plant tissue culture — where parenchyma cells divide into an undifferentiated callus, later redifferentiated into whole new plantlets.

Development: growth plus differentiation, and plasticity

Development includes all the changes an organism goes through in its life cycle, from seed germination to senescence. At the cell level, NCERT's Figure 13.8 traces the sequence: a meristematic cell undergoes division and plasmatic growth, then differentiation and elongation, then maturation, then senescence and death — a sequence applying equally to tissues and organs. Development is the sum of growth and differentiation, controlled by intrinsic factors (genetic, PGRs) and extrinsic factors (light, temperature, water, oxygen, nutrition). Plants follow different developmental pathways by environment or life phase, producing different structures from the same genotype — plasticity. Its clearest illustration is heterophylly: in cotton, coriander and larkspur, juvenile leaves differ in shape from mature ones; in buttercup, leaves produced in air differ in shape from those produced underwater on the same plant — driven purely by environment.

Plant growth regulators: chemical nature and classification

Plant growth regulators (PGRs) are small molecules of diverse chemistry — also called plant hormones or phytohormones — falling into classes: indole compounds (IAA), adenine derivatives (kinetin), carotenoid derivatives (ABA), terpenes (GA3), and gases (ethylene). Functionally, PGRs fall into two groups. Growth promoters — cell division, enlargement, pattern formation, tropic growth, flowering, fruiting, seed formation — include auxins, gibberellins and cytokinins. The second group responds to wounds and stress and drives inhibiting activities like dormancy and abscission; abscisic acid belongs here. Ethylene could fit either group but is largely an inhibitor.

Physiological effects of the five PGRs

• Auxins (IAA, IBA natural; NAA, 2,4-D synthetic): initiate rooting in cuttings; promote flowering (pineapple); prevent early fruit/leaf drop but promote abscission of older leaves and fruits; cause apical dominance — decapitation releases lateral buds (tea plantations, hedge-making); induce parthenocarpy (tomato); 2,4-D kills dicot weeds without harming monocots; control xylem differentiation.
• Gibberellins (GA1, GA2, GA3...; over 100 known, all acidic): elongate grape stalks and axis length; elongate and improve apple shape; delay senescence; speed up malting (GA3); raise sugarcane yield; hasten conifer maturity; promote bolting in beet, cabbage and rosette plants.
• Cytokinins (kinetin, zeatin): specific effects on cytokinesis; synthesised where rapid cell division occurs (root apices, shoot buds, young fruits); produce new leaves and chloroplasts; promote lateral and adventitious shoot growth; overcome apical dominance; delay leaf senescence.
• Ethylene (gas): promotes horizontal seedling growth, axis swelling and apical hook formation; promotes senescence and abscission; drives fruit ripening via the respiratory climacteric; breaks seed/bud dormancy; elongates internodes in deep-water rice; promotes root and root hair growth; as ethephon, hastens ripening and promotes female flowers in cucumber.
• Abscisic acid: general growth and metabolic inhibitor; inhibits seed germination; closes stomata and raises stress tolerance (hence "stress hormone"); governs seed development, maturation, dormancy; usually antagonist to gibberellins.

Discovery of the plant growth regulators

Every one of the five major PGR groups was discovered by accident, while investigating something else entirely.

• Auxin: Charles and Francis Darwin found coleoptiles of canary grass bend towards unilateral light (phototropism), the tip being the site of a transmissible influence; F. W. Went isolated auxin from oat coleoptile tips.
• Gibberellin: the "bakanae" disease of rice, caused by Gibberella fujikuroi, was studied by E. Kurosawa (1926) — sterile fungal filtrates reproduced the disease, and the substance was identified as gibberellic acid.
• Cytokinin: F. Skoog found tobacco stem callus proliferated, beyond auxin alone, only with vascular tissue extract, yeast extract, coconut milk or DNA added; Miller et al. (1955) crystallised and named it kinetin.
• Abscisic acid: mid-1960s groups purified three inhibitors — inhibitor-B, abscission II, dormin — later shown identical, and named abscisic acid.
• Ethylene: H. H. Cousins (1910) found a volatile substance from ripened oranges hastened banana ripening; later identified as ethylene.

Board level - Distinguishing absolute and relative growth rate

Two leaves, A and B, are measured at the start of an experiment and again after one week. Leaf A grows from 10 cm² to 15 cm²; leaf B grows from 50 cm² to 55 cm². Which leaf shows the higher relative growth rate, even though both show the same absolute increase in area?

Step 1. Compute the absolute growth rate: both A and B increase in area by 5 cm² over the same week, so their absolute growth rates are equal.

Step 2. Compute the relative growth rate, expressing the same absolute increase as a fraction of the initial size. Leaf A: $5/10 = 0.5$, a 50 percent increase. Leaf B: $5/50 = 0.1$, a 10 percent increase.

Step 3. Compare: although both leaves added identical absolute area, leaf A's growth is much larger relative to its own starting size.

Leaf A shows the higher relative growth rate, because relative growth rate is measured on a common (initial-size) basis, whereas absolute growth rate ignores starting size.

Board level - Identifying the correct PGR for a horticultural task

A gardener wants to (a) induce rooting in a stem cutting, and (b) prevent harvested bananas from ripening too early during transport. Which PGR fits each task, and why?

Step 1. Auxins help initiate rooting in stem cuttings — a classic propagation application, since auxin promotes new root formation at the cut surface.

Step 2. Ethylene triggers fruit ripening, raising the respiration rate (the respiratory climacteric), and can be released by ripe fruit nearby (the classic orange-and-banana experiment); preventing early ripening means minimising ethylene exposure — ventilation, and not storing bananas with already-ripening fruit.

Step 3. State the reasoning: auxin (IAA, IBA or synthetic NAA) promotes adventitious root formation; minimising ethylene exposure delays ripening since ethylene triggers and accelerates it.

(a) Auxin, for example IBA. (b) Minimise ethylene exposure, since ethylene triggers and accelerates fruit ripening.

Competitive level (NEET) - Finding the relative growth rate from geometric growth data

A yeast culture grows geometrically. Its biomass increases from an initial weight $W_0 = 2$ g to a final weight $W_1 = 16$ g over a period of $t = 6$ hours. Using $W_1 = W_0\,e^{rt}$, find the relative growth rate $r$ (the efficiency index), and state which growth phase this calculation assumes the culture is in throughout.

Step 1. Substitute known values into the geometric growth equation: $16 = 2 \times e^{r \times 6}$.

Step 2. Isolate the exponential term: $16/2 = e^{6r}$, so $8 = e^{6r}$.

Step 3. Take the natural logarithm of both sides: $\ln(8) = 6r$. Since $\ln(8) = 3\ln(2) = 3 \times 0.693 = 2.079$.

Step 4. Solve for $r$: $r = 2.079/6 = 0.347$ per hour (approximately).

This calculation assumes the culture is throughout the log (exponential) phase of geometric growth — where both daughter cells keep dividing so growth compounds smoothly; it would not hold in the lag phase (too few dividing cells) or stationary phase (nutrients limiting), since $r$ is not constant there. Timing note: allocate about 2 minutes — a two-line log/exponential calculation once the equation is set up; the concept-identification part is worth equal marks and should not be skipped for time.

A frequent mix-up is treating growth, differentiation and development as synonyms rather than a nested hierarchy — growth is an increase in size; differentiation is structural/functional maturation of cells derived from that growth; development is their sum, plus senescence. Another error is assuming a differentiated plant cell is permanently "locked in" as in animal cells — plant cells uniquely retain the potential for dedifferentiation, which is why tissue culture and grafting work at all. Students also confuse arithmetic growth (constant increment, straight line) with geometric growth (compounding, sigmoid plot). On the PGR side, ethylene is often wrongly assumed purely inhibitory — it also promotes root and root hair growth, and internode elongation in deep-water rice. Similarly, auxin promotes abscission of older leaves and fruits even as it prevents abscission at early stages — concentration- and stage-dependent.

• Define growth as irreversible, and explain why no single parameter describes it across a plant's life.
• Explain why plant growth is indeterminate and open, and name the meristems for primary versus secondary growth.
• Describe the three phases of growth and the cellular changes of each.
• Distinguish arithmetic from geometric growth, write both equations, and explain the resulting curve shapes.
• Distinguish absolute from relative growth rate and solve simple numerical problems comparing them.
• Define differentiation, dedifferentiation and redifferentiation, with one plant example of each.
• Explain why development is the sum of growth and differentiation, tracing the cell-level sequence to senescence.
• Define plasticity via heterophylly, distinguishing stage-driven from environment-driven examples.
• Name the five PGRs, classify each as promoter or inhibitor, and recall how each was discovered.
• Recall at least three physiological effects and one agricultural application per PGR.

Ask the tutor: Want to work through a few more NEET-style numericals on arithmetic versus geometric growth rate calculations, or trace exactly how a coleoptile bends towards light using auxin's lateral redistribution?