Plant Growth and Development | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 13, Plant Growth and Development: what growth is and how it is measured, the phases and rates of growth, differentiation, dedifferentiation and redifferentiation, development and plasticity, and the five groups of plant growth regulators with their discovery, effects and uses. It follows the current NCERT text section by section, with every name, year and number as NCERT states it. The current text names photoperiodism and vernalisation but no longer explains them, so they are not covered here.

What is growth, and what makes plant growth unique?

All cells of a plant are descendants of the zygote. The development of a mature plant from a zygote (fertilised egg) follows a precise and highly ordered succession of events. The first step in the process of plant growth is seed germination. A seed germinates when favourable conditions for growth exist in the environment. In the absence of such conditions the seed does not germinate and goes into a period of suspended growth or rest. Once favourable conditions return, the seed resumes metabolic activities and growth takes place.

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

Growth is generally accompanied by metabolic processes, both anabolic and catabolic, that occur at the expense of energy. The expansion of a leaf is therefore growth.

Note: NCERT asks how you would describe the swelling of a piece of wood placed in water. It is not growth. The wood takes up water and swells, but the increase is not an irreversible increase brought about by metabolic processes.

Why is plant growth called indeterminate?

Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability is due to the presence of meristems at certain locations in the plant body. The cells of such meristems have the capacity to divide and self-perpetuate. The product, however, soon loses the capacity to divide, and such cells make up the plant body.

Definition: The open form of growth is the form of growth in which new cells are always being added to the plant body by the activity of the meristem.

A shoot and a root are drawn as two cylinders, one above the other. The shoot apical meristem is labelled at the top of the shoot and the root apical meristem at the bottom of the root. The vascular cambium is labelled along the length of both. Arrows show the direction of growth of cells and organs: along the axis at the two apical meristems, and sideways at the vascular cambium.

See Fig. 13.2 in your NCERT textbook

How is growth measured?

At the cellular level, growth is principally a consequence of an increase in the amount of protoplasm. Since an increase in protoplasm is difficult to measure directly, one generally measures some quantity that is more or less proportional to it. Growth is therefore measured by a variety of parameters, some of which are increase in:

• fresh weight,
• dry weight,
• length,
• area,
• volume, and
• cell number.

Note: This table is the answer to the exercise question on why no single parameter is good enough to demonstrate growth throughout the life of a flowering plant. Different organs and stages grow in different ways, so cell number suits a root meristem, cell size suits a watermelon, length suits a pollen tube and surface area suits a leaf.

What are the three phases of growth?

The period of growth is generally divided into three phases: meristematic, elongation and maturation. They are best understood by looking at a root tip, moving away from the tip.

Two seedlings are shown, one younger and one older, with lines marked across the root and lettered A to G, with A nearest the root tip. Lines join the same marks on the two seedlings. In the older seedling the marks near the tip have moved far apart. Zones A, B, C and D, immediately behind the apex, have elongated most.

See Fig. 13.3 in your NCERT textbook

What is growth rate, and how do arithmetic and geometric growth differ?

Definition: Growth rate is the increased growth per unit time.

The growth rate shows an increase that may be arithmetic or geometrical.

Arithmetic growth

Mathematically, arithmetic growth is expressed as Lₜ = L₀ + rt, where Lₜ is the length at time t, L₀ is the length at time zero, and r is the growth rate, or elongation per unit time.

Time is on the x-axis and the height of the plant on the y-axis. The points lie on a straight line that rises steadily from left to right. The line starts above zero on the y-axis, because the organ already has a length L₀ at time zero.

See Fig. 13.5 in your NCERT textbook

Worked example: arithmetic growth

1. Suppose a root is 2 cm long at the start and elongates at a constant rate of 0.5 cm per day. Find its length after 6 days.
2. Write the expression: Lₜ = L₀ + rt.
3. Substitute: L₀ = 2 cm, r = 0.5 cm per day, t = 6 days.
4. Lₜ = 2 + (0.5 × 6) = 2 + 3 = 5 cm.

Geometrical growth and the sigmoid curve

In most systems growth passes through three phases.

1. Lag phase: the initial growth is slow.
2. Log or exponential phase: growth increases rapidly, at an exponential rate. Both the progeny cells following mitotic cell division retain the ability to divide and continue to do so.
3. Stationary phase: with limited nutrient supply, the growth slows down.

If the parameter of growth is plotted against time, a typical sigmoid or S-curve is obtained. A sigmoid curve is a characteristic of a living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.

Time is on the x-axis and the size or weight of the organ on the y-axis. The curve starts almost flat (lag phase), then climbs steeply (exponential phase), and then levels off at the top (stationary phase), giving an S shape. NCERT describes it as typical of cells in culture, and of many higher plants and plant organs.

See Fig. 13.6 in your NCERT textbook

Exponential growth is expressed as W₁ = W₀eʳᵗ (W₀ multiplied by e raised to the power rt), where:

• W₁ is the final size (weight, height, number and so on),
• W₀ 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 the relative growth rate. It is also the measure of the ability of the plant to produce new plant material, referred to as the efficiency index. Hence the final size W₁ depends on the initial size W₀.

Dark cells are cells capable of division and light cells are cells that lose the capacity to divide. In (a), arithmetic growth, each row has only one dark cell, and the number of light cells increases by one in each row. In (b), geometric growth, every cell is dark and the number of cells doubles from row to row. Part (c) shows stages during embryo development: the zygote divides, then comes a geometric phase in which all cells divide, and finally an arithmetic phase in which some cells have lost the capacity to divide.

See Fig. 13.4 in your NCERT textbook

Note: The body of the chapter names the three phases of the sigmoid curve as lag, log (exponential) and stationary. The NCERT summary calls them the lag, the log and the senescent phase. Use the names that the question uses.

What are absolute and relative growth rates?

Quantitative comparisons between the growth of living systems can be made in two ways.

Two leaves of different sizes are drawn, each with a dashed outline showing its size after a given time. Leaf A has an area of 5 cm² and grows to A¹ with an area of 10 cm². Leaf B has an area of 50 cm² and grows to B¹ with an area of 55 cm². Both leaves have increased their area by 5 cm² in the given time.

See Fig. 13.7 in your NCERT textbook

Worked example: which leaf has the higher relative growth rate?

1. Absolute growth of leaf A = 10 − 5 = 5 cm². Absolute growth of leaf B = 55 − 50 = 5 cm². The absolute growth is the same.
2. Relative growth of leaf A = increase ÷ initial area = 5 ÷ 5 = 1, that is 100 per cent.
3. Relative growth of leaf B = 5 ÷ 50 = 0.1, that is 10 per cent.
4. Leaf A shows the much higher relative growth rate, because the same increase is measured against a smaller initial area.

What conditions are necessary for growth?

NCERT names water, oxygen and nutrients as very essential elements for growth. In addition, every plant needs a suitable temperature, and environmental signals such as light and gravity affect certain phases of growth. The table gives the reason for each.

Plant growth and further development are thus intimately linked to the water status of the plant.

What are differentiation, dedifferentiation and redifferentiation?

During differentiation, cells undergo few to major structural changes, both in their cell walls and in their protoplasm.

Why is differentiation in plants called open?

Growth in plants is open. NCERT says that differentiation in plants is open too, because cells or tissues arising out of the same meristem have different structures at maturity. The final structure at maturity of a cell or tissue is also determined by the location of the cell within. For example, cells positioned away from the root apical meristem differentiate as root-cap cells, while those pushed to the periphery mature as epidermis.

Note: The order is fixed: differentiation first, then dedifferentiation, then redifferentiation. A cell cannot dedifferentiate unless it was differentiated, and redifferentiated cells are the products of a dedifferentiated tissue.

What is development, and what is plasticity?

Definition: Development is a term that includes all the changes that an organism goes through during its life cycle, from germination of the seed to senescence.

Broadly, development is considered as the sum of growth and differentiation. The sequence of processes that constitute the development of a cell of a higher plant also applies to tissues and organs.

1. A meristematic cell undergoes cell division and plasmatic growth.
2. The cell then undergoes differentiation.
3. It undergoes expansion (elongation).
4. It undergoes maturation and becomes a mature cell.
5. The mature cell passes into senescence.
6. Senescence ends in death.

A flow chart reading from left to right. It begins with the meristematic cell, with a loop marked cell division returning to it. Arrows labelled plasmatic growth, differentiation, expansion (elongation) and maturation lead to the mature cell on the right. From the mature cell an arrow rises to senescence, and from senescence to death.

See Fig. 13.8 in your NCERT textbook

Definition: Plasticity is the ability of plants to follow different pathways in response to the environment or phases of life to form different kinds of structures.

The example of plasticity is heterophylly.

In (a), larkspur, a juvenile leaf and an adult leaf are drawn side by side, and the adult leaf is more deeply divided than the juvenile one. In (b), buttercup, the leaf from the terrestrial habitat has broad lobes, while the leaf from the water habitat is cut into many fine, narrow segments.

See Fig. 13.9 in your NCERT textbook

What controls development?

Development in plants, that is both growth and differentiation, is under the control of intrinsic and extrinsic factors.

What are plant growth regulators, and how are they grouped?

Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. They are variously described in the literature as plant growth substances, plant hormones or phytohormones.

Based on their functions in a living plant body, the PGRs can be broadly divided into two groups.

The gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.

How were the five groups of plant growth regulators discovered?

NCERT notes that the discovery of each of the five major groups of PGRs was accidental.

Four coleoptiles, a to d, with arrows showing the direction of light from one side. In (a) the intact coleoptile bends towards the light. In (b) the tip has been cut off and the coleoptile stays straight. In (c) the tip is covered with a cap and the coleoptile stays straight. In (d) the lower part is shielded, the tip is left exposed, and the coleoptile bends towards the light. Bending happens only when the tip is present and receives the light.

See Fig. 13.10 in your NCERT textbook

Note: Keep two NCERT statements about auxin apart. Auxin was isolated by F.W. Went from the tips of coleoptiles of oat seedlings. Auxin was first isolated from human urine. Both statements are in the chapter, and either can be asked.

What are the physiological effects of auxins, gibberellins and cytokinins?

Auxins

The word auxin comes from the Greek "auxein", to grow. The term is applied to indole-3-acetic acid (IAA) and to other natural and synthetic compounds having certain growth-regulating properties. Auxins are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.

Effects and uses of auxins:

• They help to initiate rooting in stem cuttings, an application widely used for plant propagation.
• They promote flowering, for example in pineapples.
• They help to prevent fruit and leaf drop at early stages, but promote the abscission of older mature leaves and fruits.
• They induce parthenocarpy, for example in tomatoes.
• They are widely used as herbicides. 2,4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants. Gardeners use it to prepare weed-free lawns.
• Auxin controls xylem differentiation and helps in cell division.

Definition: Apical dominance is the phenomenon, seen in most higher plants, in which the growing apical bud inhibits the growth of the lateral (axillary) buds.

Removal of the shoot tips (decapitation) usually results in the growth of lateral buds. This is widely applied in tea plantations and hedge-making. With the apical bud gone, the lateral buds grow into branches, so a tea bush gives more leafy shoots and a hedge becomes dense.

Two plants. Plant (a) has its apical bud intact and its lateral buds have not grown out. Plant (b) has had its apical bud removed, and its lateral buds have grown into branches after decapitation.

See Fig. 13.11 in your NCERT textbook

Gibberellins

Gibberellins are another kind of promotory PGR. More than 100 gibberellins have been reported from widely different organisms, such as fungi and higher plants. They are denoted GA₁, GA₂, GA₃ and so on. Gibberellic acid (GA₃) was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic.

• Their ability to cause an increase in the length of the axis is used to increase the length of grape stalks.
• They cause fruits like apple to elongate and improve in shape.
• They delay senescence, so fruits can be left on the tree longer to extend the market period.
• GA₃ is used to speed up the malting process in the brewing industry.
• Spraying a sugarcane crop with gibberellins increases the length of the stem, increasing the yield by as much as 20 tonnes per acre. Sugarcane stores carbohydrate as sugar in its stem.
• Spraying juvenile conifers with GAs hastens the maturity period, leading to early seed production.
• They promote bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

Cytokinins

Cytokinins have specific effects on cytokinesis. They were discovered as kinetin (a modified form of adenine, a purine) from autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. The search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk. Since then several naturally occurring cytokinins, and some synthetic compounds with cell division promoting activity, have been identified.

Natural cytokinins are synthesised in regions where rapid cell division occurs, for example root apices, developing shoot buds and young fruits.

• They help to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
• They help to overcome apical dominance.
• They promote nutrient mobilisation, which helps in the delay of leaf senescence.

What are the physiological effects of ethylene and abscisic acid?

Ethylene

Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and by ripening fruits.

• Its influences on plants include horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings.
• It promotes senescence and abscission of plant organs, especially of leaves and flowers.
• It is highly effective in fruit ripening. It enhances the respiration rate during the ripening of fruits. This rise in the rate of respiration is called the respiratory climactic.
• It breaks seed and bud dormancy, and initiates germination in peanut seeds and sprouting of potato tubers.
• It promotes rapid internode or petiole elongation in deep water rice plants, which helps the leaves or upper parts of the shoot to remain above water.
• It promotes root growth and root hair formation, helping plants to increase their absorption surface.
• It is used to initiate flowering and to synchronise fruit-set in pineapples. It also induces flowering in mango.

Since ethylene regulates so many physiological processes, it is one of the most widely used PGRs in agriculture. The most widely used compound as a source of ethylene is ethephon. Ethephon in an aqueous solution is readily absorbed and transported within the plant, and releases ethylene slowly.

• Ethephon hastens fruit ripening in tomatoes and apples.
• It accelerates abscission in flowers and fruits (thinning of cotton, cherry and walnut).
• It promotes female flowers in cucumbers, thereby increasing the yield.

Abscisic acid

Abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy, but it has other wide-ranging effects. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism.

• ABA inhibits seed germination.
• ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. It is therefore also called the stress hormone.
• ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, it helps seeds to withstand desiccation and other factors unfavourable for growth.
• In most situations, ABA acts as an antagonist to GAs.

How do the PGRs work together?

For any and every phase of growth, differentiation and development of plants, one or the other PGR has some role to play. Such roles could be complementary or antagonistic, individualistic or synergistic. There are also events in the life of a plant where more than one PGR interacts to affect that event, for example dormancy in seeds and buds, abscission, senescence and apical dominance.

The role of PGRs is only one kind of intrinsic control. Along with genomic control and extrinsic factors, they play an important role in plant growth and development. Many extrinsic factors, such as temperature and light, control plant growth and development via PGRs. NCERT lists some such events: vernalisation, flowering, dormancy, seed germination and plant movements.

Which plant growth regulator would you use? The NCERT exercise answers

What would be expected to happen if...

"Both growth and differentiation in higher plants are open." Comment.

Growth is open because meristems keep adding new cells to the plant body throughout life. Differentiation is open because cells arising from the same meristem have different structures at maturity, and the final structure of a cell is also determined by its location: cells positioned away from the root apical meristem become root-cap cells, while those pushed to the periphery mature as epidermis.

Glossary

• Growth — An irreversible permanent increase in size of an organ or its parts, or even of an individual cell.
• Meristem — A region of cells that have the capacity to divide and self-perpetuate, responsible for the unlimited growth of plants.
• Open form of growth — Growth in which new cells are always being added to the plant body by the activity of the meristem.
• Growth rate — The increased growth per unit time, which can be expressed mathematically.
• Efficiency index — The relative growth rate r, a measure of the ability of the plant to produce new plant material.
• Differentiation — The act by which cells derived from meristems and cambium mature to perform specific functions.
• Dedifferentiation — The phenomenon in which living differentiated cells regain the capacity of division under certain conditions.
• Redifferentiation — The maturation of cells produced by dedifferentiated tissue, which once again lose the capacity to divide but mature to perform specific functions.
• Development — All the changes an organism goes through during its life cycle, from germination of the seed to senescence.
• Plasticity — The ability of plants to follow different pathways in response to environment or phases of life to form different structures.
• Heterophylly — The occurrence of leaves of different shapes on the same plant at different phases of life or in different environments.
• Apical dominance — The inhibition of the growth of lateral (axillary) buds by the growing apical bud.
• Parthenocarpy — The development of fruit without fertilisation, giving seedless fruit; auxins induce it, for example in tomatoes.
• Bolting — Internode elongation just prior to flowering, promoted by gibberellins in beet, cabbages and rosette plants.
• Respiratory climactic — The rise in the rate of respiration during the ripening of fruits, brought about by ethylene.
• Ethephon — The most widely used compound as a source of ethylene; it is absorbed in aqueous solution and releases ethylene slowly.

Common errors and misconceptions

• Misconception: Any increase in size is growth. Correct: Growth is an irreversible permanent increase in size. The swelling of wood in water is not growth.
• Misconception: In arithmetic growth both daughter cells keep dividing. Correct: In arithmetic growth only one daughter cell continues to divide. Both divide in geometrical growth.
• Misconception: The sigmoid curve is a straight line on a graph of size against time. Correct: The straight line belongs to arithmetic growth. Geometrical growth gives the S-shaped sigmoid curve.
• Misconception: The leaf with the larger increase in area has the higher relative growth rate. Correct: Relative growth rate is measured per unit initial size. In Figure 13.7 both leaves gain 5 cm², but the smaller leaf A has the higher relative growth rate.
• Misconception: The formation of cork cambium from parenchyma is redifferentiation. Correct: It is dedifferentiation. The cells that the cork cambium then produces are redifferentiated.
• Misconception: Kinetin is a natural cytokinin of plants. Correct: Kinetin does not occur naturally in plants. Zeatin, from corn-kernels and coconut milk, is a natural cytokinin.
• Misconception: Ethylene is purely a growth inhibitor. Correct: Ethylene could fit either group, though it is largely an inhibitor. It also breaks dormancy and promotes root growth and root hair formation.
• Misconception: 2,4-D kills all plants. Correct: 2,4-D kills dicotyledonous weeds and does not affect mature monocotyledonous plants.
• Misconception: Auxins and cytokinins have the same effect on lateral buds. Correct: The apical bud, a site of auxin production, keeps lateral buds suppressed. Cytokinins help to overcome apical dominance.

Exam-style questions with model answers

Q1. Define plasticity and give one example. [1 mark]

1. Plasticity is the ability of plants to follow different pathways in response to environment or phases of life to form different kinds of structures, for example heterophylly in cotton, coriander, larkspur or buttercup.

Q2. Why is abscisic acid also known as the stress hormone? [2 marks]

1. ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses.
2. By inducing dormancy it also helps seeds to withstand desiccation and other factors unfavourable for growth.

Q3. What is apical dominance? Name one agricultural practice based on it. [2 marks]

1. Apical dominance is the phenomenon in which the growing apical bud inhibits the growth of the lateral (axillary) buds.
2. Removal of shoot tips (decapitation) results in the growth of lateral buds. It is applied in tea plantations and hedge-making.

Q4. Distinguish between arithmetic and geometrical growth, with their mathematical expressions. [3 marks]

1. In arithmetic growth only one daughter cell continues to divide after mitosis, while the other differentiates and matures. In geometrical growth both progeny cells retain the ability to divide.
2. Arithmetic growth gives a linear curve and is expressed as Lₜ = L₀ + rt, where r is the growth rate or elongation per unit time.
3. Geometrical growth gives a sigmoid curve and is expressed as W₁ = W₀eʳᵗ, where r is the relative growth rate, also called the efficiency index.

Q5. Explain differentiation, dedifferentiation and redifferentiation with one example each. [3 marks]

1. Differentiation: cells derived from meristems mature to perform specific functions. For example, cells lose their protoplasm and develop lignocellulosic secondary walls to form a tracheary element.
2. Dedifferentiation: living differentiated cells regain the capacity of division. For example, interfascicular cambium and cork cambium form from fully differentiated parenchyma cells.
3. Redifferentiation: the cells produced by such meristems lose the capacity to divide again and mature to perform specific functions, as the cells produced by the cork cambium do.

Q6. Two leaves, A of 5 cm² and B of 50 cm², each increase in area by 5 cm² in the same time. Compare their absolute and relative growth rates. [3 marks]

1. The absolute growth is the same for both: 5 cm² in the given time.
2. Relative growth of A = 5 ÷ 5 = 1, or 100 per cent. Relative growth of B = 5 ÷ 50 = 0.1, or 10 per cent.
3. Leaf A has the much higher relative growth rate, because relative growth rate is expressed per unit initial parameter.

Q7. Give three uses of gibberellins in agriculture or industry. [3 marks]

1. Spraying sugarcane with gibberellins increases the length of the stem, increasing the yield by as much as 20 tonnes per acre.
2. GA₃ is used to speed up the malting process in the brewing industry.
3. Gibberellins increase the length of grape stalks, and make fruits like apple elongate and improve in shape. (Delaying senescence, early seed production in conifers and bolting are also acceptable.)

Q8. Describe the sigmoid growth curve and explain its phases. Write the expression for exponential growth. [5 marks]

1. When a parameter of growth is plotted against time for geometrical growth, a typical sigmoid or S-curve is obtained. It is characteristic of a living organism growing in a natural environment, and is typical for all cells, tissues and organs of a plant.
2. Lag phase: the initial growth is slow.
3. Log or exponential phase: growth increases rapidly at an exponential rate, because both progeny cells following mitotic division retain the ability to divide.
4. Stationary phase: with limited nutrient supply the growth slows down.
5. Exponential growth is expressed as W₁ = W₀eʳᵗ, where W₁ is the final size, W₀ the initial size, r the growth rate (relative growth rate, or efficiency index), t the time of growth and e the base of natural logarithms.

Q9. List the five main groups of plant growth regulators. Write a note on the discovery, physiological functions and agricultural applications of auxins. [5 marks]

1. The five groups are auxins, gibberellins, cytokinins, abscisic acid and ethylene.
2. Discovery: Charles Darwin and Francis Darwin observed that coleoptiles of canary grass bend towards unilateral light, and that the tip was the site of the transmittable influence. F.W. Went isolated auxin from the tips of coleoptiles of oat seedlings.
3. Auxins are produced by the growing apices of stems and roots. IAA and IBA have been isolated from plants; NAA and 2,4-D are synthetic.
4. Functions: they initiate rooting in stem cuttings, promote flowering in pineapples, prevent early fruit and leaf drop, promote abscission of older leaves and fruits, cause apical dominance, induce parthenocarpy in tomatoes, control xylem differentiation and help in cell division.
5. Applications: rooting of cuttings for plant propagation; decapitation in tea plantations and hedge-making; 2,4-D as a herbicide that kills dicotyledonous weeds to give weed-free lawns.

Key takeaways

• Growth is an irreversible permanent increase in size; plant growth is indeterminate because meristems keep adding new cells, which is the open form of growth.
• Growth is measured by fresh weight, dry weight, length, area, volume or cell number, since the increase in protoplasm is difficult to measure directly.
• The three phases of growth seen from the root tip are meristematic, elongation and maturation, each with its own cell characteristics.
• Arithmetic growth follows Lₜ = L₀ + rt and gives a straight line; geometrical growth follows W₁ = W₀eʳᵗ and gives a sigmoid curve.
• Absolute growth rate is total growth per unit time; relative growth rate is growth per unit time per unit initial parameter.
• Differentiated cells can regain the capacity to divide (dedifferentiation), and the cells they produce mature again (redifferentiation).
• Development is the sum of growth and differentiation; plasticity is shown by heterophylly in cotton, coriander, larkspur and buttercup.
• Auxins, gibberellins and cytokinins are growth promoters; abscisic acid is an inhibitor; ethylene could fit either group but is largely an inhibitor.
• Auxins cause apical dominance and rooting, gibberellins cause bolting and stem elongation, and cytokinins promote cell division and delay leaf senescence.
• Ethylene ripens fruit and is supplied as ethephon; abscisic acid closes stomata, induces dormancy and is called the stress hormone.

Test yourself

How many new cells per hour can a single maize root apical meristem give rise to?

One single maize root apical meristem can give rise to more than 17,500 new cells per hour.

Which meristems cause the increase in girth of a plant organ?

The lateral meristems, the vascular cambium and the cork-cambium, cause the increase in girth, which is known as secondary growth.

What does r stand for in the expression W₁ = W₀eʳᵗ?

In this expression r is the relative growth rate, which is also called the efficiency index of the plant.

Which fungus causes the bakanae disease of rice seedlings?

The bakanae or foolish seedling disease of rice seedlings is caused by the fungal pathogen Gibberella fujikuroi.

From what was kinetin discovered?

Kinetin, a modified form of adenine, was discovered from autoclaved herring sperm DNA, and it does not occur naturally in plants.

Which three inhibitors were later found to be the same substance, abscisic acid?

Inhibitor-B, abscission II and dormin were proved to be chemically identical, and the substance was named abscisic acid.

What did H.H. Cousins confirm in 1910?

Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas; it was later identified as ethylene.

Which synthetic auxin is used to kill dicotyledonous weeds?

The synthetic auxin 2,4-D is widely used to kill dicotyledonous weeds, and it does not affect mature monocotyledonous plants.

Which PGR promotes bolting in beet and cabbages?

Gibberellins promote bolting, which is internode elongation just prior to flowering, in beet, cabbages and many plants with rosette habit.

What is the most widely used source of ethylene in agriculture?

Ethephon is the most widely used compound as a source of ethylene; in aqueous solution it is absorbed by the plant and releases ethylene slowly.