NCERT Solutions for Class 11 Biology Chapter 13: Plant Growth and Development

Concept. A flowering plant develops from a single-celled zygote into a complex body of roots, stems, leaves, flowers, fruits and seeds in a precise and highly ordered sequence. All the cells of the mature plant are descendants of the zygote, yet they have very different structures and functions. The eight terms in this question describe the successive and alternative events through which this happens: cells are first produced (meristem, growth), then specialised (differentiation), may later revert and specialise again (dedifferentiation, redifferentiation), and the whole life-long process is called development. Learning them as one connected story, rather than eight isolated lines, is what makes the definitions easy to recall and apply.

1. Growth is an irreversible permanent increase in size of an organ or its parts or even of an individual cell. It is generally accompanied by anabolic and catabolic metabolism at the expense of energy. At the cellular level it is mainly an increase in the amount of protoplasm, and it is measured as an increase in fresh weight, dry weight, length, area, volume or cell number. Expansion of a leaf is growth, whereas the reversible swelling of wood in water is not.
2. Differentiation is the process by which cells derived from the root apical meristem, shoot apical meristem and cambium mature to perform specific functions. During it the cell wall and protoplasm undergo few to major structural changes; a cell becoming a tracheary element, for example, loses its protoplasm and builds a strong, elastic, lignocellulosic secondary wall. Differentiation ends the capacity for division.
3. Development is the term for all the changes that an organism goes through during its life cycle from germination of the seed to senescence. In plants it is considered the sum of growth and differentiation and is controlled by intrinsic (genetic and hormonal) and extrinsic (light, temperature, water, oxygen, nutrition) factors.
4. Dedifferentiation is the phenomenon in which living differentiated cells that have lost the capacity to divide regain it under certain conditions. Fully differentiated parenchyma cells giving rise to the interfascicular cambium and the cork cambium are NCERT’s examples. It also explains why a callus can be raised from a mature explant in tissue culture.
5. Redifferentiation is the process in which the cells produced by a dedifferentiated meristem once again lose the capacity to divide and mature to perform specific functions. The secondary xylem, secondary phloem, secondary cortex and cork formed by the vascular cambium and cork cambium of a woody dicot are products of redifferentiation.
6. Determinate growth is growth that is limited and stops after a definite size or stage is reached. Most plant organs such as leaves, flowers and fruits show it: they appear, grow to a characteristic size, and then stop growing, even though the plant as a whole keeps growing. It is contrasted with the open or indeterminate growth of the plant axis, which goes on throughout life.
7. Meristem is a region of the plant body made of cells that retain the capacity to divide and self-perpetuate. The product cells soon lose this capacity and form the permanent tissues. Root and shoot apical meristems (and sometimes intercalary meristems) cause primary growth, while vascular cambium and cork cambium (lateral meristems) cause secondary growth. Meristem cells are rich in protoplasm, have large conspicuous nuclei and thin cellulosic primary walls with abundant plasmodesmata.
8. Growth rate is the increased growth per unit time. It can be expressed mathematically and is either arithmetic (L_t = L₀ + rt) or geometrical (W₁ = W₀e^rt). Rates may be compared as absolute growth rate (total growth per unit time) or relative growth rate (growth per unit time expressed per unit initial parameter).

Extra depth. Notice how the terms link: the meristem supplies cells, growth increases their number or size at a measurable rate, differentiation gives them a function, and development is the total of both across the life cycle. Dedifferentiation followed by redifferentiation is a characteristic flexibility of plants (animals rarely do this), and it is the principle behind wound healing, secondary growth and plant tissue culture. Differentiation in plants is also open, meaning cells derived from the same meristem may mature into different structures depending on their position.

Conclusion: Growth, differentiation and development are linked stages of a plant’s life: meristems create cells, growth enlarges them at a measurable rate, differentiation assigns each a function, and development is their combined result, with dedifferentiation and redifferentiation giving plants their characteristic flexibility.

Concept. Growth, at the cellular level, is basically an increase in the amount of protoplasm. Since protoplasm cannot be measured directly, we measure some quantity that is more or less proportional to it — fresh weight, dry weight, length, area, volume or cell number. The question asks why we cannot rely on just one of these for the whole life of a flowering plant. The answer is that a flowering plant is not a uniform structure growing in one way: different organs, in different phases of life, grow by different mechanisms (more cells, bigger cells, longer cells, bigger surface), and each parameter captures only one of these.

1. Different organs grow in different ways. The growth of a pollen tube is naturally measured as increase in length; in a dorsiventral leaf it is shown by an increase in surface area; in a fruit or a swelling storage organ by volume; and in the stem and root by girth as well as length. A single parameter such as length would be meaningless for a leaf or a fruit.
2. Growth may be due to cell number or cell size. NCERT gives the contrast: one maize root apical meristem produces more than 17,500 new cells per hour (growth expressed as increase in cell number), whereas cells of a watermelon may enlarge up to 3,50,000 times (growth expressed as increase in cell size). Counting cells would capture the first but miss the second completely.
3. Growth changes its mode with the phase of life. In the meristematic phase growth is mainly cell division; in the elongation phase it is cell enlargement with vacuolation; in the maturation phase size is almost unchanged while walls thicken. A parameter useful in one phase fails in another.
4. Each parameter has its own pitfalls. Fresh weight changes with water content, so a seed taking up water in imbibition increases in fresh weight without real growth, and a wilted plant loses fresh weight without losing protoplasm. Dry weight excludes water but cannot be followed in the same living plant, and it can even fall during germination when reserves are respired. Length ignores girth (secondary growth), and volume is hard to measure for irregular organs.
5. Some organs have determinate growth and others indeterminate. Leaves, flowers and fruits reach a fixed size and stop, while the axis continues to grow through meristems. One number cannot describe both.

Extra depth. Because of these limitations, plant physiologists choose the parameter to match the organ and the phase, or use several together. This also shows why the earlier definition of growth uses the broad phrase “increase in size of an organ or its parts or even of an individual cell”: the size may be length, area, volume, weight or number, depending on what is growing.

Conclusion: No single parameter can describe the growth of a flowering plant throughout its life because different organs grow by cell division, cell enlargement or both, in different phases, and each measure (weight, length, area, volume or cell number) captures only one aspect; hence the parameter must be chosen to suit the organ and the phase.

Concept. The “increased growth per unit time” is called the growth rate. An organism or part of it can produce more cells in a variety of ways, and the growth rate shows an increase that may be arithmetic or geometrical. The four parts of this question build on each other: (a) and (b) are the two ways cells multiply, (c) is the curve produced when growth goes on in a real environment, and (d) is how two growing systems are compared quantitatively.

(a) Arithmetic growth. Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The number of dividing cells therefore stays constant, and new cells are added at a constant rate. The simplest example is a root elongating at a constant rate. When length is plotted against time, a straight line (linear curve) is obtained (NCERT Fig. 13.5), and it is expressed mathematically as:

L_t = L₀ + rt

• L_t = length at time ‘t’; L₀ = length at time ‘zero’; r = growth rate (elongation per unit time).
• Because r is constant, equal increments of length are added in equal intervals of time, e.g. a root with L₀ = 2 cm growing at r = 0.5 cm per day is 2 + 0.5 × 6 = 5 cm after 6 days (illustrative figures).

(b) Geometric growth. In most systems initial growth is slow (lag phase) and then increases rapidly at an exponential rate (log or exponential phase). Here both progeny cells following mitosis retain the ability to divide and continue to do so, so the number of dividing cells doubles each cycle. With limited nutrient supply, growth slows down and enters a stationary phase. It is expressed as:

W₁ = W₀ e^rt

• W₁ = final size (weight, height, number etc.); W₀ = initial size at the beginning of the period; r = growth rate; t = time of growth; e = base of natural logarithms.
• r is the relative growth rate and also a measure of the ability of the plant to produce new plant material, called the efficiency index. The final size W₁ therefore depends on the initial size W₀: a larger starting size gives a larger final size.
• Embryo development (NCERT Fig. 13.4c) shows both: after the zygote divides, all cells divide in the geometric phase, and later only some cells keep dividing in the arithmetic phase.

(c) Sigmoid growth curve. If any parameter of growth is plotted against time for geometric growth, a typical sigmoid or S-shaped curve is obtained. It has three parts: the lag phase (slow initial growth as cells prepare), the log (exponential) phase (rapid growth at the maximum rate) and the stationary phase (growth slows and stops as nutrients and space become limiting). A sigmoid curve is characteristic of living organisms growing in a natural environment and is typical for all cells, tissues and organs of a plant. For a tree showing seasonal activity one expects a series of repeated S-curves, one for each growing season.

(d) Absolute and relative growth rates. Absolute growth rate is the measurement and comparison of total growth per unit time. Relative growth rate is the growth of the given system per unit time expressed on a common basis, e.g. per unit initial parameter. In NCERT Fig. 13.7, leaves A and B are of different sizes but both gain 5 cm² of area in the given time (A: 5 to 10 cm², B: 50 to 55 cm²). Their absolute rates are equal, but the relative rate of A is 5/5 = 1 (100%) while that of B is 5/50 = 0.1 (10%). Thus leaf A shows the much higher relative growth rate, because the same absolute gain is added to a much smaller initial size.

Extra depth. Arithmetic growth keeps a fixed number of dividing cells, so it can be sustained only for a limited length; geometric growth is explosive but cannot continue indefinitely, which is why it ends in the stationary phase. Real organs usually show a bit of each: geometric early, arithmetic later (as in the embryo), and finally a plateau.

Conclusion: Arithmetic growth adds cells at a constant rate (linear curve, L_t = L₀ + rt), geometric growth doubles dividing cells each cycle (W₁ = W₀e^rt) and under natural limits yields the sigmoid curve of lag, log and stationary phases, while absolute growth rate gives total increase per unit time and relative growth rate gives increase per unit initial size.

Concept. Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition that are variously called plant growth substances, plant hormones or phytohormones. They are the intercellular intrinsic factors that control growth and development: produced in one part of the plant, they act in the same or another part in very small amounts. According to NCERT the discovery of each of the five major groups was accidental, and any PGR has diverse physiological effects while diverse PGRs may also show similar effects.

Five main groups of natural PGRs

Note on AUXINS

1. Discovery. The story began with Charles Darwin and his son Francis Darwin, who observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments they concluded that the tip of the coleoptile was the site of a transmittable influence that caused the bending of the entire coleoptile. Auxin was later isolated by F. W. Went from tips of coleoptiles of oat seedlings. The name comes from the Greek auxein, “to grow”, and auxin was first isolated from human urine.

2. Nature and source. The term auxin is applied to indole-3-acetic acid (IAA) and to other natural and synthetic compounds with certain growth-regulating properties. Natural auxins are IAA and indole butyric acid (IBA); NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are synthetic. They are generally produced by the growing apices of stems and roots, from where they migrate to the regions of action.

3. Physiological functions and agricultural/horticultural applications

1. Rooting of cuttings: auxins initiate rooting in stem cuttings, which is widely used in plant propagation by nurseries.
2. Flowering: auxins promote flowering, e.g. in pineapple, so that fruiting can be synchronised.
3. Fruit and leaf drop: they prevent fruit and leaf drop at early stages, saving the crop, but promote the abscission of older mature leaves and fruits.
4. Apical dominance: in most higher plants the growing apical bud inhibits the growth of lateral (axillary) buds. Removal of the shoot tip (decapitation) usually results in growth of lateral buds. It is exploited in tea plantations and hedge-making: pruning the tip makes the plant bushy with more branches, more young leaves for plucking, and a dense hedge.
5. Parthenocarpy: auxins induce development of fruit without fertilisation, e.g. in tomatoes, giving seedless fruits.
6. Herbicide: 2,4-D is widely used to kill dicotyledonous weeds and does not affect mature monocots, so gardeners use it to make weed-free lawns.
7. Differentiation and division: auxin controls xylem differentiation and helps in cell division.

Extra depth — the other four groups at a glance. Gibberellins (from bakanae disease of rice, Kurosawa 1926) lengthen stems, cause bolting and increase sugarcane yield; cytokinins (kinetin, Miller et al. 1955; zeatin from corn and coconut milk) promote cytokinesis and delay senescence; ethylene (Cousins, 1910) ripens fruit; ABA (inhibitor-B, abscission II, dormin) closes stomata and induces dormancy. In a growing plant they act together — synergistically or antagonistically — and not in isolation.

Conclusion: The five main natural PGRs are auxins, gibberellins, cytokinins, ethylene and abscisic acid; auxin, discovered through the Darwins’ coleoptile experiments and isolated by Went, regulates rooting, flowering, apical dominance, parthenocarpy and weed control, making it one of the most useful tools of agriculture and horticulture.

Concept. Abscisic acid (ABA) is a plant growth regulator derived from carotenoids. It was discovered independently in the mid-1960s as three different inhibitors — inhibitor-B, abscission II and dormin — which were later proved chemically identical and named abscisic acid. It was first noticed for its role in abscission and dormancy, but like other PGRs it has wide-ranging effects on growth and development. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism, which is exactly what a plant needs when conditions turn unfavourable.

The PGRs of the growth-inhibiting group, to which ABA belongs, play an important role in plant responses to wounds and stresses of biotic and abiotic origin. ABA is called the stress hormone for the following reasons:

1. It closes stomata. ABA stimulates the closure of stomata. Under water deficit, closing stomata reduces transpirational loss of water, so the plant conserves water and avoids wilting. (In guard cells this is achieved by loss of solutes and hence turgor; this mechanism is general plant-physiology knowledge beyond the NCERT text.)
2. It increases tolerance to various stresses. ABA raises the tolerance of plants to stresses such as drought, salinity, cold and other unfavourable conditions, helping the plant survive until conditions improve.
3. It induces dormancy. By inducing seed and bud dormancy, ABA helps seeds withstand desiccation and other factors unfavourable for growth, so they germinate only when conditions are suitable.
4. It inhibits seed germination and metabolism, keeping growth suspended in adverse environments.
5. It plays a role in seed development and maturation, preparing the seed for survival through the unfavourable period.
6. It is an antagonist of gibberellins. In most situations ABA acts opposite to GAs: GAs break dormancy and promote germination whereas ABA maintains dormancy. The balance between them lets the plant switch between growth and rest.

Extra depth. ABA contrasts with auxins, gibberellins and cytokinins, which are growth promoters. Together, promoters and inhibitors form a balance — for any phase of growth, differentiation and development one or the other PGR has a role, complementary or antagonistic. When the environment becomes harsh the balance tips towards ABA and the plant shifts from growth to survival mode.

Conclusion: Abscisic acid is called the stress hormone because it closes stomata, raises tolerance to drought and other stresses, and induces seed and bud dormancy and desiccation tolerance, acting as the antagonist of gibberellins so that the plant conserves resources when conditions are unfavourable.

Concept. In animals, growth and structure are largely closed: the body plan is fixed in the embryo and growth stops at maturity. Plants are different. NCERT describes plant growth as open — new cells are always being added to the plant body by meristematic activity — and the same idea is extended to differentiation. The statement means that in higher plants neither the continuation of growth nor the final fate of a cell is fixed rigidly in advance.

1. Growth is open. Plants retain the capacity for unlimited growth throughout life because of meristems at certain locations (root apical meristem, shoot apical meristem and, in dicots and gymnosperms, vascular cambium and cork cambium). Meristem cells divide and self-perpetuate; the cells produced soon lose the capacity to divide and make up the plant body. Since new cells are continuously added, growth is indeterminate — the “open form of growth”. The apical meristems cause primary growth (elongation along the axis) and the lateral meristems cause secondary growth (increase in girth). Organs such as leaves, flowers and fruits show determinate growth, but the axis as a whole stays open.

2. Differentiation is open. NCERT explains that even differentiation in plants is open because cells and tissues arising out of the same meristem have different structures at maturity, and the final structure of a cell or tissue is determined by the location of the cell within the organ. For example, in a root, cells positioned away from the root apical meristem differentiate as root-cap cells, while those pushed to the periphery mature as epidermis. Similarly the cambium produces xylem towards the inside and phloem towards the outside, and a shoot meristem can produce leaves, branches or flowers depending on position and signals.

3. Flexibility that makes it open. Differentiated cells can dedifferentiate (parenchyma forming interfascicular cambium and cork cambium) and then redifferentiate; plants can follow different developmental pathways in response to environment or phase of life — plasticity, as in the heterophylly of cotton, coriander, larkspur and buttercup. Development is therefore flexible: it is the sum of growth and differentiation, both being open.

Extra depth. This openness is why a plant can regenerate after pruning, form new roots on a cutting, thicken year after year, and be cultured from a single cell. It is also why the same genetic plant can look different in air and water, as in buttercup.

Conclusion: Growth in higher plants is open because meristems keep adding new cells throughout life, and differentiation is open because cells from the same meristem mature into different structures according to their position, so plant development remains flexible and plastic.

Concept. The response of plants to the relative lengths of day and night is called photoperiodism. NCERT notes that light and temperature are extrinsic factors that influence the initiation of flowering through PGRs (the dedicated photoperiodism section is not printed in this edition, so the explanation below uses standard plant physiology). Plants are classified by their flowering response: short day plants (SDP) flower when the day length is shorter than a critical period, long day plants (LDP) flower when it is longer than a critical period, and day-neutral plants (DNP) flower irrespective of day length.

The key idea: the critical day length is different for each plant. A short day plant does not need “short days” in an absolute sense but days shorter than its own critical day length; a long day plant needs days longer than its own critical day length. These critical values are not the same for all species. If the critical day length of the SDP is longer than that of the LDP, then there is a band of day lengths that is shorter than the SDP’s critical value and longer than the LDP’s critical value, and in that band both plants receive their flowering stimulus.

1. Short day plant (SDP): flowers when the photoperiod is less than its critical day length (more precisely, when the uninterrupted dark period exceeds a critical length). Example: Xanthium (cocklebur), whose critical day length is about 15.5 hours, so it flowers on any day shorter than this.
2. Long day plant (LDP): flowers when the photoperiod is more than its critical day length. Example: Hyoscyamus niger (henbane), with a critical day length of roughly 11 hours, so it flowers on any day longer than this (approximate textbook values).
3. Overlap: on a day of, say, 12 to 14 hours, the day is longer than the LDP’s 11 hours (so the LDP flowers) and shorter than the SDP’s 15.5 hours (so the SDP flowers). Both therefore flower at the same place and the same time.
4. Illustration with simple numbers: let the critical day length of an SDP be 14 hours and of an LDP be 10 hours. A 12-hour day satisfies both: it is shorter than 14 hours (SDP) and longer than 10 hours (LDP).

Extra depth. Flowering is controlled by the length of the uninterrupted dark period, perceived by the leaf pigment phytochrome; the “short” and “long” labels are therefore relative to each species’ critical value and not to a universal number of hours. This is also why the same SDP and LDP flower in different seasons at different latitudes, but may share a flowering window in the transition seasons (spring and autumn) when day length lies between their critical values.

Conclusion: A short day plant flowers when the day is shorter than its critical day length and a long day plant when the day is longer than its own critical value; because these critical values differ, at an intermediate day length both conditions are satisfied and both plants flower simultaneously in the same place.

Concept. Each PGR has a characteristic set of physiological effects, and these effects are exploited in agriculture and horticulture. To choose the right regulator for a job, match the required effect with the PGR whose NCERT-listed effect it is. The six parts of this question cover all five groups: auxin, ethylene, cytokinin (and GA), gibberellin and abscisic acid.

1. (a) Induce rooting in a twig — AUXIN (IBA or NAA). Auxins initiate rooting in stem cuttings and are widely used for plant propagation; the cut end of the twig is dipped in an auxin solution or powder, and adventitious roots arise from it.
2. (b) Quickly ripen a fruit — ETHYLENE (usually applied as ethephon). Ethylene is highly effective in fruit ripening and enhances the respiration rate during ripening (respiratory climacteric). Ethephon in aqueous solution is absorbed, transported in the plant and releases ethylene slowly; it hastens ripening in tomatoes and apples.
3. (c) Delay leaf senescence — CYTOKININ. Cytokinins promote nutrient mobilisation which helps in delaying leaf senescence, and they stimulate chloroplast formation in leaves. (NCERT also notes that gibberellins delay senescence of fruits, but for leaves cytokinin is the answer.)
4. (d) Induce growth in axillary buds — CYTOKININ (or decapitation). Cytokinins help overcome apical dominance and promote lateral shoot growth, whereas auxin from the apical bud inhibits the lateral buds; removal of the shoot tip has the same effect.
5. (e) ‘Bolt’ a rosette plant — GIBBERELLIN (GA₃). Gibberellins promote bolting, the internode elongation just prior to flowering in beet, cabbages and many plants with a rosette habit.
6. (f) Induce immediate stomatal closure in leaves — ABSCISIC ACID (ABA). ABA stimulates the closure of stomata and increases tolerance to stresses such as water deficit.

Extra depth. The two obvious pairs of antagonism are auxin versus cytokinin on apical dominance (auxin maintains it, cytokinin overcomes it) and GA versus ABA on dormancy and growth. Knowing such pairs makes it easy to predict which regulator achieves a given effect.

Conclusion: Auxin is used for rooting, ethylene for quick ripening, cytokinin for delaying leaf senescence and inducing axillary buds, gibberellin for bolting of rosette plants and abscisic acid for immediate stomatal closure.

Concept. Defoliated means stripped of its leaves. In photoperiodism the plant’s flowering depends on the length of the day or, more exactly, the length of the uninterrupted dark period. The next question is: which part of the plant actually senses this? (As in Q7, the photoperiodism section is not in the printed chapter, so the explanation uses standard plant physiology.)

Answer: No, a defoliated plant would not respond to the photoperiodic cycle. The reason is that the leaves are the sites of perception of the photoperiodic stimulus.

1. Leaves perceive the signal. The photoreceptor pigment phytochrome is present in the leaves and detects the light/dark cycle. Without leaves there is no organ to perceive the length of day and night.
2. Leaves produce the flowering stimulus. When the correct photoperiod is perceived, the leaves synthesise a flowering stimulus (the hypothetical hormone florigen, proposed by Chailakhyan in 1936) which is transported to the shoot apex, where the vegetative meristem is converted into a floral meristem.
3. Evidence. Experiments on Xanthium show that exposing even a single leaf to the inductive photoperiod makes the whole plant flower, whereas the plant exposed to the same cycle after removal of all its leaves does not flower; the apex (shoot tip) itself, when treated alone, does not perceive photoperiod.
4. Therefore the leaf is the “receptor and generator”, and the shoot apex is only the “responder”. Remove the leaves and the chain breaks at its very first link.

Extra depth. If even one leaf is retained, the plant can still respond, showing that a small area of leaf is sufficient. This explains why in commercial flower production (e.g. chrysanthemum) the leaves are exposed to controlled day lengths to time flowering. NCERT itself notes that extrinsic factors such as light and temperature control flowering via PGRs, and that vernalisation, flowering, dormancy and seed germination are such events.

Conclusion: A defoliated plant would not respond to the photoperiodic cycle because leaves are the sites that perceive the day/night length through phytochrome and produce the flowering stimulus; without leaves there is neither perception nor stimulus, so the shoot apex receives no signal to flower.

Concept. This question tests whether the physiological effects of PGRs and the processes of growth and differentiation are understood well enough to predict an outcome. For each situation identify (i) the process or hormone involved, (ii) its normal action and (iii) the consequence of its presence or absence.

(a) GA₃ applied to rice seedlings. Gibberellins increase the length of the axis. Rice seedlings treated with GA₃ would show excessive elongation — tall, thin, weak and pale (yellowish) seedlings. This is exactly what happens in the bakanae (“foolish seedling”) disease caused by the fungus Gibberella fujikuroi: E. Kurosawa (1926) reproduced the symptoms by treating healthy rice seedlings with the sterile filtrate of the fungus, and the active substance was later identified as gibberellic acid. Such seedlings are weak and tend to fall over (lodge), so grain yield falls.

(b) Dividing cells stop differentiating. Differentiation turns dividing meristematic cells into mature cells with specific structure and function. If dividing cells never differentiated, they would continue to divide and form an undifferentiated mass of cells, like a callus or a tumour; no tracheary elements, sieve tubes, epidermis or other specialised tissues would form, so the plant could not build a functional body (no organs, no conduction, no protection). Conversely, a plant whose growth is open needs the balance between division and differentiation. Development, the sum of growth and differentiation, would be incomplete.

(c) A rotten fruit gets mixed with unripe fruits. Ripening and senescing tissues synthesise ethylene in large amounts. Ethylene is a gas and diffuses to neighbouring fruits, where it hastens ripening and raises their respiration (respiratory climacteric). Thus the unripe fruits would ripen quickly and then themselves rot, hence the saying that one rotten fruit spoils the whole basket. The historical basis is Cousins’ 1910 observation that a volatile substance from ripe oranges hastened ripening of stored unripe bananas.

(d) Cytokinin is forgotten in the culture medium. Cytokinins have specific effects on cytokinesis. In Skoog’s experiments the tobacco stem callus proliferated only when, in addition to auxin, the medium was supplemented with a cytokinin source (vascular-tissue extract, yeast extract, coconut milk or DNA). Without cytokinin in the culture medium the explant would fail to divide and proliferate properly: cell division (cytokinesis) is inadequate, little or no callus mass forms, and there is no regeneration of new shoots or leaves; the tissue may only enlarge or die.

Extra depth. Notice how each prediction links back to a discovery story in the chapter — Kurosawa (GA), Cousins (ethylene), Skoog and Miller (cytokinin) — and to the idea that development depends on a correct balance of hormones and of division versus differentiation.

Conclusion: GA₃ would make rice seedlings overgrow and become weak (bakanae), cells that never differentiate would form an unspecialised mass without functional tissues, a rotten fruit would release ethylene and hasten ripening of the unripe fruits, and omission of cytokinin would stop cytokinesis so that the cultured tissue would not proliferate.