NEET BiologyNCERT Class 11Chapter 13

Plant Growth and Development: NEET notes

A seed becomes a seedling, the seedling becomes a plant, and the plant keeps adding length and girth for as long as it lives. This chapter asks how that growth is measured and plotted, how new cells take on their final form, why the same plant can make differently shaped leaves, and how five small chemical messengers, the plant growth regulators, switch growth on and off.

What NEET asks

NEET tests the three phases of growth, the two growth equations (Lt = L0 + rt and W1 = W0 e^rt), the sigmoid curve, absolute against relative growth rate, the dedifferentiation examples, and above all the discoverers and uses of the five PGRs. Marks are lost by mixing up which regulator does what: auxin for rooting and apical dominance, gibberellin for bolting and stem length, cytokinin for overcoming apical dominance and delaying senescence, ethylene for ripening, and ABA for stomatal closure and dormancy.

1. Growth and why it never stops in plants

NCERT §13 (opening text) and §13.1–§13.1.1

Every plant cell descends from one zygote, yet a mature plant has roots, stems, leaves, flowers, fruits and seeds. Getting there takes two things working together, growth and differentiation, in a precise, ordered run of events; together they make up development.
The first step is seed germination. A seed germinates only when conditions in the environment favour growth; otherwise it stays in a period of suspended growth or rest, and resumes metabolism once conditions improve.
Growth is an irreversible, permanent increase in the size of an organ, its parts or even a single cell. It is usually accompanied by metabolic processes, both anabolic and catabolic, that consume energy. A leaf expanding is growth; wood swelling in water is not, because it is reversible and not driven by metabolism.
Plants keep the capacity for unlimited growth throughout their life because they have meristems at certain places. Meristem cells divide and perpetuate themselves; their products soon stop dividing and make up the plant body.
This form of growth, in which the meristem keeps adding new cells to the body, is called the open form of growth.
The root apical meristem and shoot apical meristem cause primary growth, lengthening the plant along its axis.
Later in life, dicots and gymnosperms add lateral meristems, the vascular cambium and the cork cambium. These thicken the organs where they work, so the plant widens as well as lengthens. This is secondary growth.

2. Measuring growth

NCERT §13.1.2

At the cellular level growth is mainly an increase in the amount of protoplasm. Protoplasm is hard to measure directly, so a quantity roughly proportional to it is measured instead.
Parameters used: increase in fresh weight, dry weight, length, area, volume and cell number.
A single maize root apical meristem can produce more than 17,500 new cells per hour. Here growth shows up as an increase in cell number.
Cells in a watermelon may increase in size up to 3,50,000 times. Here growth shows up as an increase in cell size.
The right parameter depends on the organ: a pollen tube's growth is measured as length, a dorsiventral leaf's as surface area.

3. Phases of growth

NCERT §13.1.3

Growth is divided into three phases: meristematic, elongation and maturation. A root tip shows all three in order from the apex backwards.
Meristematic phase: the constantly dividing cells at the root apex and shoot apex. They are rich in protoplasm, have large conspicuous nuclei, and thin primary cellulose walls with plenty of plasmodesmata.
Elongation phase: the cells just behind the meristematic zone. They show increased vacuolation, cell enlargement and deposition of new cell wall.
Maturation phase: further from the apex, behind the elongation zone. The cells reach their maximum size in wall thickening and protoplasmic change; most familiar tissue types belong here.
The zone of elongation can be found with the parallel line technique: marks drawn at equal spacing on a root tip spread apart most in the zones just behind the apex.

4. Arithmetic and geometric growth

NCERT §13.1.4

Growth rate is the increase in growth per unit time, so it can be expressed mathematically. The increase may be arithmetic or geometric.
Arithmetic growth: after mitosis only one daughter cell keeps dividing, while the other differentiates and matures. A root elongating at a constant rate is the simplest example.
Plotting length against time for arithmetic growth gives a straight line: Lt = L0 + rt, where Lt is length at time t, L0 is length at time zero and r is the growth rate, the elongation per unit time.
Geometric growth: both daughter cells keep the ability to divide and go on dividing. Growth starts slowly (lag phase), then rises rapidly at an exponential rate (log or exponential phase).
With limited nutrients, growth then slows and levels off (stationary phase). Growth plotted against time gives a sigmoid or S-shaped curve, typical of living organisms growing in a natural environment and of all cells, tissues and organs of a plant.
Exponential growth is written W1 = W0 e^rt, where W1 is final size (weight, height, number and so on), W0 is initial size, r is the growth rate, t is time and e is the base of natural logarithms.
Here r is the relative growth rate, a measure of the plant's ability to produce new plant material, called the efficiency index. The final size W1 therefore depends on the starting size W0.

5. Absolute and relative growth rate

NCERT §13.1.4

Growth of two living systems can be compared in two ways.
Absolute growth rate: the total growth per unit time, measured and compared as it is.
Relative growth rate: growth per unit time expressed on a common basis, for example per unit of the initial parameter.
Two leaves of different sizes, A and B, may each gain 5 cm² of area in the same time. Their absolute growth rates are equal, but the smaller leaf has the much higher relative growth rate, because 5 cm² is a larger fraction of its starting area.

6. Conditions for growth

NCERT §13.1.5

Water: plant cells grow by enlarging, which needs water. Turgidity of the cells helps extension growth, so growth and development are closely linked to the water status of the plant. Water is also the medium for the enzyme activity growth needs.
Oxygen: helps release the metabolic energy that growth activities require.
Nutrients: macro and micro essential elements are needed to synthesise protoplasm and also act as a source of energy.
Temperature: every plant has an optimum temperature range for growth; moving outside it can harm survival.
Signals from outside, such as light and gravity, also shape particular stages of growth.

7. Differentiation, dedifferentiation and redifferentiation

NCERT §13.2

Cells produced by the root and shoot apical meristems and the cambium mature to perform specific functions. This maturation is differentiation, and during it cells undergo minor to major changes in their walls and protoplasm.
Example: to become a tracheary element a cell loses its protoplasm and builds a strong, elastic, lignocellulosic secondary wall that can carry water over long distances even under extreme tension.
Dedifferentiation: living differentiated cells that had lost the ability to divide regain it under certain conditions. Examples: interfascicular cambium and cork cambium formed from fully differentiated parenchyma cells.
Redifferentiation: cells produced by such dedifferentiated tissue once again lose the ability to divide and mature to perform specific functions.
Plant growth is open, and so is differentiation: cells from the same meristem can have different structures at maturity.
Position helps decide the final form. A cell pushed out ahead of the root apical meristem becomes part of the root cap; a cell pushed sideways to the outer edge matures as epidermis.

8. Development and plasticity

NCERT §13.3

Development covers all the changes an organism goes through in its life cycle, from seed germination to senescence.
The sequence in a plant cell (also true of tissues and organs): a meristematic cell divides, grows (plasmatic growth), expands by elongation, differentiates and matures into a mature cell, which finally undergoes senescence and death.
Plasticity is the ability of plants to follow different pathways, in response to the environment or to phases of life, and so form different kinds of structures.
Heterophylly linked to phase of life: in cotton, coriander and larkspur the leaves of the juvenile plant differ in shape from those of the mature plant.
Heterophylly caused by the environment: buttercup makes differently shaped leaves in air and in water.
Development, the sum of growth and differentiation, is controlled by intrinsic factors (intracellular genetic factors and intercellular chemicals such as plant growth regulators) and extrinsic factors (light, temperature, water, oxygen, nutrition and so on).

9. Plant growth regulators and their discovery

NCERT §13.4.1–§13.4.2

Plant growth regulators (PGRs) are small, simple molecules of varied chemistry, also called plant growth substances, plant hormones or phytohormones.
Chemical families: IAA (indole-3-acetic acid) is an indole compound; kinetin (N⁶-furfurylamino purine) is an adenine derivative; ABA (abscisic acid) is derived from carotenoids; GA₃ (gibberellic acid) is a terpene; ethylene (C₂H₄) is a gas.
Promoters drive division, enlargement, pattern formation, tropic bending, flowering, fruiting and seed formation. Three groups belong here: auxins, gibberellins and cytokinins.
Growth inhibitors act in responses to wounds and to biotic and abiotic stresses, and in dormancy and abscission: abscisic acid. Ethylene could fit either group but is largely an inhibitor.
The discovery of each of the five major groups was accidental. Charles Darwin and his son Francis saw canary grass coleoptiles bend towards one-sided light (phototropism) and concluded that the tip was the source of a transmittable influence causing the bend. F.W. Went later isolated auxin from oat coleoptile tips.
Gibberellin: the bakanae (foolish seedling) disease of rice is caused by the fungus Gibberella fujikuroi. E. Kurosawa found that sterile filtrates of the fungus produced the symptoms in healthy seedlings; the active substance was later identified as gibberellic acid.
Cytokinin: F. Skoog and co-workers found that callus from tobacco internode segments proliferated only if, besides auxin, the medium had vascular tissue extract, yeast extract, coconut milk or DNA. Miller and co-workers later crystallised the active substance promoting cytokinesis and named it kinetin.
Abscisic acid: in the mid-1960s three independent groups purified inhibitor-B, abscission II and dormin, which proved to be chemically identical and were named abscisic acid.
Ethylene: H.H. Cousins confirmed that ripe oranges release a volatile substance that hastens the ripening of stored unripe bananas; it was later identified as ethylene.

10. Auxins

NCERT §13.4.3.1

The name auxin comes from the Greek auxein, to grow. The first auxin was isolated from human urine, and the name now covers IAA plus other natural and synthetic compounds that regulate growth the same way.
Auxins are generally made at the growing apices of stems and roots and move from there to where they act.
Natural auxins found in plants are IAA and IBA (indole butyric acid). The synthetic ones are 2,4-D (2,4-dichlorophenoxyacetic acid) and NAA, naphthalene acetic acid.
Uses: they start roots on stem cuttings, which is why they are widely used in plant propagation; they promote flowering, for example in pineapple; they stop young fruits and leaves from dropping early, yet speed the shedding of older, mature ones.
Apical dominance: in most higher plants the growing apical bud suppresses the growth of lateral (axillary) buds. Removing the shoot tip (decapitation) usually lets the lateral buds grow, which is why tea bushes are plucked and hedges are trimmed.
Auxins induce parthenocarpy, for example in tomato.
They are widely used as herbicides: 2,4-D kills dicotyledonous weeds but does not affect mature monocotyledonous plants, so gardeners use it for weed-free lawns.
Auxin also steers the differentiation of xylem and assists cell division.

11. Gibberellins and cytokinins

NCERT §13.4.3.2–§13.4.3.3

Gibberellins are growth promoters. More than 100 have been reported from organisms as different as fungi and higher plants, named GA₁, GA₂, GA₃ and so on. Gibberellic acid (GA₃) was one of the first found and is the most intensively studied. All GAs are acidic.
Because they lengthen the axis, gibberellins are used to lengthen grape stalks. They make fruits such as apple elongate and improve their shape.
They delay senescence, so fruit can stay on the tree longer and the market period is extended. GA₃ speeds up malting in the brewing industry.
Sugarcane stores its carbohydrate as sugar in the stem. A gibberellin spray lengthens the stem and can lift the harvest by up to 20 tonnes per acre.
Spraying juvenile conifers with GAs hastens maturity and so gives early seed production. GAs also promote bolting, the elongation of internodes just before flowering, in beet, cabbage and many plants with a rosette habit.
Cytokinins act specifically on cytokinesis. The first one found, kinetin, is a modified adenine (a purine) obtained from autoclaved herring sperm DNA. Plants themselves do not make kinetin.
Zeatin, a natural cytokinin, was isolated from corn kernels and coconut milk; several natural and some synthetic cytokinins are now known.
Natural cytokinins are made where cells divide rapidly: root apices, developing shoot buds, young fruits.
Cytokinins help form new leaves, chloroplasts in leaves, lateral shoots and adventitious shoots. They help overcome apical dominance, and by drawing nutrients into a leaf they delay its senescence.

12. Ethylene and abscisic acid

NCERT §13.4.3.4–§13.4.3.5

Ethylene is a simple gaseous PGR, made in large amounts by tissues undergoing senescence and by ripening fruits.
On seedlings it causes horizontal growth, a swollen axis and, in dicots, the apical hook. It speeds senescence and abscission, especially of leaves and flowers.
Ethylene is highly effective in fruit ripening and raises the rate of respiration during ripening; this rise is called the respiratory climacteric.
It breaks seed and bud dormancy, starts germination in peanut seeds and sprouting of potato tubers. It promotes rapid internode and petiole elongation in deep-water rice, keeping the upper shoot above water. It promotes root growth and root hair formation, increasing the absorbing surface.
Ethylene is used to start flowering and synchronise fruit-set in pineapple, and induces flowering in mango. Its most widely used source is ethephon, which is absorbed from aqueous solution, moves through the plant and releases ethylene slowly.
Ethephon hastens ripening in tomato and apple, speeds abscission of flowers and fruits (thinning of cotton, cherry, walnut) and promotes female flowers in cucumber, raising the yield.
Abscisic acid (ABA) was found for its role in abscission and dormancy. It acts as a general inhibitor of plant growth and metabolism and inhibits seed germination.
ABA stimulates stomatal closure and raises tolerance to many kinds of stress, so it is called the stress hormone. It is important in seed development, maturation and dormancy; by inducing dormancy it helps seeds withstand desiccation. In most situations ABA is an antagonist of GAs.
PGR roles can be complementary or antagonistic, individual or synergistic. Events such as seed and bud dormancy, abscission, senescence and apical dominance involve more than one PGR.
PGRs are just one layer of internal control. Genes and outside factors act too, and many outside factors, such as light and temperature, work by changing PGR levels.

Must-know facts

Growth: irreversible, permanent increase in size, needing energy. Development = growth + differentiation.
Open form of growth: meristems keep adding cells throughout life.
Primary growth: root and shoot apical meristems (length). Secondary growth: vascular cambium and cork cambium (girth), in dicots and gymnosperms.
Parameters: fresh weight, dry weight, length, area, volume, cell number. Maize root apical meristem: over 17,500 new cells per hour; watermelon cells: up to 3,50,000 times larger.
Three phases of growth: meristematic, elongation, maturation.
Arithmetic growth: one daughter divides; Lt = L0 + rt; straight line.
Geometric growth: both daughters divide; lag, log (exponential), stationary; sigmoid curve; W1 = W0 e^rt; r = relative growth rate = efficiency index.
Absolute growth rate: total growth per unit time. Relative growth rate: growth per unit time per unit initial size.
Dedifferentiation examples: interfascicular cambium and cork cambium from parenchyma.
Heterophylly (plasticity): cotton, coriander, larkspur (phase of life); buttercup (air against water).
PGR chemistry: IAA indole; kinetin adenine derivative; ABA carotenoid derivative; GA₃ terpene; ethylene gas.
Discoverers: Darwins (coleoptile tip, canary grass), Went (auxin, oat), Kurosawa (bakanae, Gibberella fujikuroi), Skoog and Miller (kinetin), three groups (inhibitor-B, abscission II, dormin = ABA), Cousins (ethylene, oranges and bananas).
Auxin: rooting of cuttings, apical dominance, parthenocarpy in tomato, 2,4-D herbicide for dicot weeds, xylem differentiation.
Gibberellin: grape stalk length, apple shape, delays senescence, malting, sugarcane stem (up to 20 tonnes per acre), bolting in rosette plants.
Cytokinin: cytokinesis, overcomes apical dominance, delays leaf senescence; kinetin from herring sperm DNA, zeatin from corn kernels and coconut milk.
Ethylene: fruit ripening, respiratory climacteric, apical hook, deep-water rice elongation, ethephon, female flowers in cucumber.
ABA: stomatal closure, stress hormone, seed dormancy, inhibits germination, antagonist of GAs.

Common traps

Calling a piece of wood swelling in water an example of growth.

Growth is irreversible and driven by metabolism; swelling by soaking is reversible and involves no metabolism.

Thinking the zone of elongation is the meristem at the very tip.

The tip is the meristematic zone; elongation happens in the cells just behind it, which spread apart most in the parallel line test.

Using W1 = W0 e^rt for a root elongating at a constant rate.

Constant elongation is arithmetic growth: Lt = L0 + rt, a straight line. The exponential form applies to geometric growth.

Assuming the larger leaf has the higher relative growth rate when both gain the same area.

Same absolute gain means the smaller leaf has the higher relative rate, since the gain is a bigger fraction of its starting size.

Naming vascular cambium in the stele as a product of dedifferentiation.

NCERT's dedifferentiation examples are interfascicular cambium and cork cambium, formed from fully differentiated parenchyma.

Saying cytokinin maintains apical dominance.

Auxin from the apical bud maintains apical dominance; cytokinins help overcome it and promote lateral shoot growth.

Saying kinetin is a natural plant cytokinin.

Kinetin came from autoclaved herring sperm DNA and does not occur naturally in plants; zeatin is a natural one.

Believing 2,4-D kills all plants in a lawn.

2,4-D kills dicot weeds but does not affect mature monocots such as lawn grass.

Key terms

Growth: An irreversible, permanent increase in size of a cell, organ or organism, usually needing energy.
Meristem: A group of cells that keep dividing and perpetuating themselves, adding new cells to the plant body.
Open form of growth: Growth in which meristems keep adding new cells to the body throughout life.
Arithmetic growth: Growth in which only one daughter cell of each division keeps dividing; length rises in a straight line with time.
Geometric growth: Growth in which both daughter cells keep dividing; gives lag, log and stationary phases.
Sigmoid curve: The S-shaped curve of growth against time, with lag, exponential and stationary phases.
Efficiency index: The relative growth rate r in W1 = W0 e^rt, measuring a plant's ability to produce new material.
Absolute growth rate: Total growth per unit time.
Relative growth rate: Growth per unit time expressed per unit of initial size.
Differentiation: Maturation of cells from meristems to perform specific functions, with changes to walls and protoplasm.
Dedifferentiation: A differentiated cell regaining the ability to divide, as when parenchyma forms cork cambium.
Redifferentiation: Cells from dedifferentiated tissue losing the ability to divide again and maturing for a function.
Plasticity: The ability of a plant to form different structures in response to environment or phase of life.
Heterophylly: Different leaf shapes on the same plant, by life phase (larkspur) or environment (buttercup).
Apical dominance: Suppression of lateral bud growth by the growing apical bud.
Parthenocarpy: Development of fruit without fertilisation; induced by auxins in tomato.
Bolting: Internode elongation just before flowering in rosette plants, promoted by gibberellins.
Respiratory climacteric: The rise in respiration rate during fruit ripening, enhanced by ethylene.
Ethephon: A compound that releases ethylene slowly inside the plant; the usual commercial source of ethylene.
Stress hormone: Abscisic acid, which closes stomata and raises tolerance to many stresses.

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