Plant Growth and Development Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1Give the eight definitions in NCERT's order, keeping each to one or two lines, since the marks here are for precision of language and not for length. Growth, differentiation, development and meristem are the four big ones and the other four are their corollaries.
2. 2Growth is the irreversible permanent increase in the size of a plant organ, and it may be measured as an increase in length, in area, in volume, in mass or in the number of the units, that is the leaves, the flowers and the branches. The essential properties to state are that it is by an irreversible and permanent increase, that it is measured as a quantitative increase in some parameter, and that it is the result of cell division in the meristem followed by the cell enlargement. And two qualifications to note, because they are examinable: growth is a quantitative phenomenon, so an increase in size, whereas differentiation is qualitative, and only the meristematic cells divide, so that the growth of a plant is localised at the meristems.
3. 3Differentiation is the maturation of the cells of a tissue to perform a specific function, and the NCERT wording is that the cells mature to perform specific functions, so that the meristematic cells, which are the undifferentiated and actively dividing cells, come to give rise to the specialised and permanent tissues, and the change of the meristematic cell into a permanent tissue cell is the differentiation. The point to emphasise is that differentiation is a qualitative process, a change in the nature of the cell and not merely in its size, so the contrast with growth is exact, growth being quantitative and differentiation qualitative. The examples to quote are the meristematic cell becoming a permanent tissue cell, the guard cell of the epidermis differentiated from the epidermal cell, the protoxylem being a differentiation of the xylem, and the cambium forming the secondary xylem and the secondary phloem, and a fourth that NCERT gives for this chapter is the tracheary element, in which the cell loses its protoplasm and develops a very strong, elastic, lignocellulosic secondary wall.
4. 4Development is the sum of all the changes that a plant undergoes during its whole life from the germinating seed to the senescent plant, and the NCERT definition is that it is the sum of all the changes that an organism undergoes, so it is a wider word than either growth or differentiation and it includes both of them as the two components of the change. The clearest statement is that development is growth plus differentiation plus the interaction of the two, and that a cell that grows, differentiates and then redifferentiates in response to a signal has undergone a development. And it is worth adding that the whole of the cellular and molecular biology of the plant, the signalling, the gene expression and the hormone action, is best understood as a study of the development.
5. 5Dedifferentiation is the reacquisition by a mature, permanent and fully differentiated tissue cell of the capacity to divide again, so that the cell reverts to a less specialised and more meristematic-like state, and the NCERT wording is that the cells of the permanent tissue regain the capacity to divide. Redifferentiation is the reverse, that is the conversion of the dedifferentiated cell once more into a specialised tissue cell, that is the restoration of the differentiated state. And the definition of determinate growth is that the growth in which the extent of growth is limited to a fixed or a predetermined limit, and it is determinate in the sense that the organ stops growing once the limit is reached and then undergoes senescence, so the whole of the growth of the plant is determinate in the sense that it has a limit, though some organs such as the root and the shoot apex may show indeterminate growth. And the definition of a meristem is that it is a tissue of the actively dividing cells which produces the new cells for the growth and the replacement of the lost or the worn-out tissues, and the two kinds are the apical meristem, at the tips of the stem and the root, and the lateral meristem, the vascular cambium and the cork-cambium, in the girth.
6. 6Finally the growth rate, and this one is a definition that carries a formula with it, so give it precisely. The increased growth per unit time is termed as the growth rate, and NCERT expresses that rate in two ways. The absolute growth rate is the measurement and the comparison of the total growth per unit time, so it is simply the actual increase in the parameter in a given time, that is the increase in the length or the mass, so that a plant gaining 5 cm in a week has an absolute growth rate of 5 cm a week. The relative growth rate is the growth of the given system per unit time expressed on a common basis, for example per unit initial parameter, so that two plants of very different size can be compared, and the same quantity can then be compared between a large plant and a small one. And the comparison of the two is the closing point: because the relative rate is expressed on a common basis, it is the only one of the two that allows a fair comparison between a large plant and a small one, which is exactly the point that Q2 and Q3 turn on.

Step-by-step solution

1. 1The answer is that the rate of growth is not constant and the parameter chosen goes up and down, so no single parameter can show growth through the whole life. Give the parameters and then the graph, and then the working example, which is the best part of the answer.
2. 2The parameters that could be used, and the point about each, are as follows. A length can be measured, the height of the plant or the length of one organ, and it is convenient but it does not show the total growth, since a plant may be getting taller without gaining any mass. The area of the leaf can be measured, the diameter of the stem, the fresh mass or the dry mass, the number of the leaves, the flowers and the branches, and the volume. The problem is not that these are difficult to measure but that none of them shows growth continuously, and the reason is that the rate of growth of a plant is not constant through its life.
3. 3The demonstration, and this is what the examiner wants. If a plant is grown from a seed and its height is measured at intervals and the values are plotted against time, the curve obtained is not a straight line. In the early period, the seedling grows only very slowly, because the seed is small and the plant has few leaves and so little photosynthetic surface, and the plant is in fact living on the reserve of the seed. The plant then enters a phase of very rapid growth, a young plant growing exponentially, and this is the logarithmic or the exponential curve, so that the growth is fastest in this phase. Then the growth slows down and the plant enters a phase of declining growth, in which the rate falls progressively, and finally the growth ceases altogether, the plant reaches maturity, the leaves begin to fall, the flowers open and the plant enters the phase of senescence and dies. So the plant passes through an initial slow phase, an phase of most vigorous growth, a phase of declining growth, a period of maturity and a period of senescence, and the graph of any single parameter against time has the same shape, a curve that rises slowly, rises fastest, rises slowly again and finally falls.
4. 4The conclusion, and it should be stated in one sentence. No single parameter is adequate because the rate of growth of a plant is not constant through its life, since the plant passes through these successive phases, and any parameter that measures the increase will show first a slow increase, then a rapid one, then a slowing one, then no increase, and finally a decrease as the plant ages. So the demonstration of growth throughout the life requires the measurement of the plant at successive intervals of time, and the plotting of the parameter against time, and the interpretation of the phases of the curve, and not a single measurement. And it is worth adding that this is the reason the growth curve of a plant is studied as a curve at all, since the curve is the only honest picture of what the plant is doing, whereas a single figure would be true only of one moment.

Step-by-step solution

1. 1Take the four in the order given. The first two are the two maths, the third is the natural curve, and the fourth is the two measures, so the answer should show that the third is derived from the first two and the fourth is how the rate is expressed in each.
2. 2(a) Arithmetic growth. In arithmetic growth the increase in the measured parameter is constant in unit time, that is the rate of growth is constant, so that the total growth is linear with time. NCERT expresses it as Lt equals L0 plus rt, where L0 is the length at time zero, Lt is the length at time t and r is the growth rate, that is the elongation per unit time, and the graph of the length against time is a straight line. And the biological situation in which this occurs, and the one NCERT gives, is a root elongating at a constant rate, and the point to make is that in a plant this condition is found only over a limited range and generally does not hold for the whole life.
3. 3(b) Geometric growth. In geometric growth the increase in the parameter is itself proportional to the value already reached, that is the rate of growth is not constant but itself keeps increasing, so the growth is exponential. NCERT writes it as W equals W0 times e raised to the power rt, where W0 is the initial size, the weight, the height or the number, W is the final size, r is the growth rate, which here is the relative growth rate, and e is the base of the natural logarithms, and the graph is a curve of the exponential or the logarithmic type that rises ever more steeply. And the biological situation in which this occurs is the early phase of the growth of a cell, tissue or organ, in which the daughter cells following each mitotic division retain the ability to divide and go on dividing, and the point to make is that a constant relative rate of increase can continue only for a limited time, since the plant cannot grow indefinitely at an increasing rate and the tissue would become impossibly large, so the geometric growth is the growth of the young phase only.
4. 4(c) The sigmoid growth curve. The sigmoid, that is the S-shaped, growth curve is the sum of the two above, that is the arithmetic growth phase followed by the geometric growth phase followed by the arithmetic growth phase again, and the whole of it is the growth curve of an organ or of a whole plant through its life. Its three phases, which should be named, are: first the initial or the lag or the slow phase, the arithmetic phase, in which the growth is slow and the plant is living on the reserve of the seed and has little photosynthetic surface; second the the log or the exponential or the acceleration phase, the geometric phase, in which the growth is at its maximum and is proportional to the existing size, and the rate of growth is increasing and the curve is rising steeply; and third the deceleration or the declining or the stationary phase, the arithmetic phase again, in which the rate falls progressively, the growth slows, the curve flattens, and the organ reaches full size. And the sigmoid is so called because of its S shape, the first inflection at the transition from the slow to the rapid phase and the second at the transition from the rapid to the slow phase, and it should be noted that the growth of a plant organ is a good example, that the entire plant passes through such a curve, and that the practical importance is that the final size of an organ, which is the point at which the curve flattens, is determined by the balance of the factors that increase the growth and of those that limit it.
5. 5(d) The absolute and the relative growth rates. The absolute growth rate is the measurement and the comparison of the total growth per unit time, and it is obtained simply by subtraction, the change in the value of the parameter with time being the difference between the two measurements, so a plant gaining 5 cm in a week has an absolute growth rate of 5 cm a week. The relative growth rate is the growth of the given system per unit time expressed on a common basis, for example per unit initial parameter, and its advantage is precisely that common basis, since it allows two systems of very different size to be compared. And the comparison to close with is the one NCERT illustrates in its figure, in which two leaves, A and B, of different sizes are drawn and both show the same absolute increase in area, five square centimetres, in the same time to give the leaves A1 and B1, so the two plants have the same absolute growth rate; but only one of them shows a much higher relative growth rate, and that is the smaller leaf, the leaf B, because the same increase of five square centimetres is a much larger fraction of its smaller initial area than it is of the large initial area of the leaf A. So the leaf with the smaller initial size has the higher relative growth rate, and the relative rate is therefore the measure that allows a fair comparison between a large plant and a small one, whereas the absolute rate on its own would make them look alike.

Step-by-step solution

1. 1Give the five groups first with their common names, since that list alone earns marks, and then take the auxin for the detailed note, because it is the one with the clearest discovery story, the Darwin phototropism experiment.
2. 2The five groups, with the common name and the principal natural representative of each, are as follows. The auxins, from the Greek auxein meaning to grow, of which the commonest natural auxin is the indole 3-acetic acid, the IAA, the term auxin being applied to the IAA and to the other natural and the synthetic compounds having growth-regulating properties, and the IAA and the indole butyric acid, the IBA, having been isolated from plants, while the naphthaleneacetic acid, the NAA, and the 2,4-D are synthetic. The gibberellins, of which there are more than a hundred, denoted GA1, GA2, GA3 and so on, the gibberellic acid, GA3, being the one first discovered and the most intensively studied, and all of them being acidic. The cytokinins, which were discovered as the kinetin, a modified form of the adenine, a purine, and which do not occur naturally in plants, the natural substance with cytokinin-like activity being the zeatin, isolated from the corn-kernels and the coconut milk, and they are synthesised in the regions of the rapid cell division, the root apices, the developing shoot buds and the young fruits. The abscisic acid, the ABA, a derivative of the carotenoids, which acts as a general plant growth inhibitor and an inhibitor of the plant metabolism. And the ethylene, a gas, C2H4, which could fit either of the two groups but is largely an inhibitor of the growth activities. So the five are the auxins, the gibberellins, the cytokinins, the abscisic acid and the ethylene, and the grouping to remember is that the auxins, the gibberellins and the cytokinins are the growth promoters, while the ABA and the ethylene belong to the inhibiting side.
3. 3Then the discovery of the auxin, and here the NCERT account is the one to give. It started with the observation of Charles Darwin and his son Francis Darwin, that the coleoptiles of the canary grass responded to unilateral illumination by growing towards the light source, the phototropism, and after a series of experiments it was concluded that the tip of the coleoptile was the site of a transmittable influence that caused the bending of the entire coleoptile, so that the tip was the source of the auxin. And the auxin itself was then isolated by F. W. Went from the tips of the coleoptiles of the oat seedlings, which is the experiment NCERT figures, and this is the isolation of the auxin proper. The chemical note, which is examinable, is that the auxin, from the Greek auxein meaning to grow, was first isolated from the human urine, and that the term auxin is applied to the indole-3-acetic acid, the IAA, and to the other natural and synthetic compounds having certain growth-regulating properties, that the auxins are generally produced by the growing apices of the stems and the roots, from where they migrate to the regions of their action, and that the auxins like the IAA and the indole butyric acid, the IBA, have been isolated from the plants, while the NAA and the 2,4-D are synthetic. And the point to close the discovery on is that the discovery of each of the five major groups of the PGRs has been accidental, so the auxin was no exception, and it was the tip of the coleoptile and not the whole coleoptile that proved to be the source.
4. 4Then the physiological functions of the auxin, and these should be listed since the question asks for them. The auxin initiates the rooting in the stem cuttings, which is the basis of the propagation and is exploited in the layering and in the grafting. The auxin controls the xylem differentiation and helps in the cell division. The auxin helps to prevent the fruit and the leaf drop at the early stages, but it promotes the abscission of the older mature leaves and fruits, so this is a two-sided effect and the balance between the two is what matters. The auxin promotes the flowering, for example in the pineapples, and it also induces the parthenocarpy, that is the development of the fruit in the absence of pollination, as in the tomatoes, and this is what gives the seedless fruit. And the auxin is widely used as the herbicide, that is the weedicide, the 2,4-D being used to kill the dicotyledonous weeds, which it does affect, without affecting the mature monocotyledonous plants, so the gardeners use the 2,4-D to prepare the weed-free lawns. And the auxin also induces the parthenocarpy in the tomato.
5. 5The agricultural and horticultural applications, which is the third head asked for and which should be kept separate from the functions, since the question names them separately. The auxin promotes the rooting of the stem cuttings, so it is used in the propagation of the plants, in the layering and in the grafting. It is used to promote the flowering, for example in the pineapples, and to produce the parthenocarpic and so the seedless fruit in the tomatoes. It helps to prevent the fruit and the leaf drop at the early stages. And as the 2,4-D it is used as the weedicide to kill the dicotyledonous weeds without affecting the mature monocotyledonous plants, so it is used to prepare the weed-free lawns. And the two synthetic auxins to name are the naphthaleneacetic acid, the NAA, and the 2,4-dichlorophenoxyacetic acid, the 2,4-D, and the NCERT point is that all of these auxins have been used extensively in the agricultural and the horticultural practices. And the closing point to make is that the auxin is the growth regulator that is most widely used in the agriculture and in the horticulture, and the reason is that it is the one with the widest range of effects, from the rooting to the flowering to the fruit set to the weed control.

Step-by-step solution

1. 1Give the reason in the first line, that the ABA accumulates in the plant under any stress and so the name follows, and then give the two stresses in NCERT, the water stress and the salinity, with the mechanism, since the mechanism is the mark.
2. 2The reason is that the abscisic acid accumulates in the plant whenever the plant is subjected to any kind of stress, and it is a general response to the stress rather than being specific to any one, so it came to be called the stress hormone. The history that NCERT gives is that the abscisic acid was discovered for its role in regulating the abscission and the dormancy, and that during the mid-1960s three independent researches reported the purification and the chemical characterisation of three different kinds of inhibitors, inhibitor-B, abscission II and dormin, which were later all proved to be chemically identical, and it was then named abscisic acid, the ABA. And the point of the second name is that, like the other PGRs, the ABA has a wide range of effects on the plant growth and the development, so that it acts as a general plant growth inhibitor and an inhibitor of the plant metabolism, it inhibits the seed germination, it stimulates the closure of the stomata, and it increases the tolerance of the plants to the various kinds of stresses, and it is for that last effect that it is also called the stress hormone. It also plays an important role in the seed development, the maturation and the dormancy, and by inducing the dormancy the ABA helps the seeds to withstand the desiccation and the other factors unfavourable for the growth.
3. 3Then the two stresses of NCERT, and the mechanism is what earns the marks. The first is the water stress, and the drought, and the sequence is as follows. When the plant is short of water, there is a fall in the water content of the leaf, so that the leaf cells lose turgidity, and this loss of turgidity is sensed, and it triggers a rise in the concentration of the ABA, which is made principally in the mature leaves and in the roots. The ABA then brings about the rapid and immediate closure of the stomata, within minutes, by causing the guard cells to lose their turgor, so that the loss of water by transpiration is checked and the water content of the leaf is restored. And the closure of the stomata also checks the entry of the carbon dioxide, so the photosynthesis is reduced, but this is a secondary sacrifice made in order to save the plant from desiccation, and that is the point, that the plant sacrifices its carbon fixation to save its water. And the ABA also induces the seeds to synthesise the proteins of the germination inhibitors in the maturing seed, so that the seed is held in dormancy and does not germinate until the water is available, which is a second water-stress function.
4. 4The second is the high salinity, and the sequence is the same in essence but with a different initial trigger. When the plant is subjected to a high salt concentration, there is a decrease in the osmotic potential of the soil, so the movement of the water from the soil into the root is much reduced, or the water may even move out of the root and into the soil, so the root cells lose turgidity and a similar sequence of events follows. The ABA is synthesised and the concentration rises, the stomata close, and the transpiration is checked, and in the tolerant species the ABA also induces the synthesis of the proteins of the salt tolerance, so that the cell accumulates the solutes and the osmotic potential is brought down again and the turgidity restored. So the ABA is the hormone of the stress response in general, of the water deficit and of the salinity, and the two are the examples that the question expects.

Step-by-step solution

1. 1The words to explain are open, meaning that both are not fixed and can be modified, so the answer should define the two and then give the evidence for each being open, and the strongest evidence is the tissue culture. But the NCERT reason must come first, and it is that even differentiation in the plant is open because the cells and the tissues arising out of the same meristem have different structures at maturity, and because the final structure at maturity of a cell or a tissue is also determined by the location of the cell within the organ.
2. 2What open means. To say that the growth and the differentiation in the higher plants are open is to say that neither is final, neither is irrevocably fixed at the moment it is produced, and that a cell which has already grown and differentiated can be induced to change its fate, to divide again and to form a different tissue, and this is unlike the animal, in which the fate of the cell is largely fixed and irreversibly determined. So the word open is used in the sense of being modifiable and reversible, and both the growth and the differentiation in the plant are open in this sense.
3. 3The evidence that the growth is open. The meristem is the obvious case, since a meristematic cell retains the capacity to divide for the whole life of the plant and to produce new cells, whereas in an animal most of the cells lose the power of division after differentiation, and this is retained in the meristem. And the NCERT illustration of the open differentiation is the position of the cell, since the cells positioned away from the root apical meristems differentiate as the root-cap cells, while those pushed to the periphery mature as the epidermis, so the same meristem gives the root-cap on one side and the epidermis on the other, which is only possible if the differentiation is not irrevocably fixed. But the more striking evidence is that the differentiated cells can revert. The tracheary element, that is the xylem vessel, is a dead cell, the mature vessel having lost its nucleus and its protoplasm, so that it is the most differentiated and the least living tissue in the plant, and yet it can be induced to revert to a living, dividing, meristematic-like cell and to produce a whole plant, and this is the experimental proof that the differentiation is open and is not final, and it is done in the tissue culture by culturing a small piece of the tissue on a suitable medium. Similarly the mature sieve tube element, which has lost its nucleus, has been shown to be induced to divide, and the leaf cells and the cells of the cortex and of the pith can be induced to divide and to form a callus.
4. 4Then the evidence that the differentiation is open, which is the same evidence read the other way, and the technique to describe, because the tissue culture is the standard example. In the tissue culture, a small piece of the plant tissue, the explant, is taken and cultured on a sterile nutrient medium containing the sugars, the mineral salts and the growth regulators. The cells of the explant, which were fully differentiated, divide to form an undifferentiated mass of cells called the callus, and this is the dedifferentiation, so the differentiation has been shown to be open. And the dedifferentiation in the plant body itself is not a laboratory event, since the living differentiated cells that have lost the capacity to divide can regain it, the examples NCERT gives being the formation of the interfascicular cambium and the cork cambium from the fully differentiated parenchyma cells. The callus is then transferred to a fresh medium containing the cytokinins and the auxins in the proportions that are appropriate, and the ratio of auxin to cytokinin determines what the callus gives rise to: a high cytokinin to auxin ratio promotes the shoot formation and the shoot buds are induced, a high auxin to cytokinin ratio promotes the root formation, and an intermediate balanced ratio promotes the callus itself. So the same undifferentiated mass of cells can be made to form roots or shoots simply by changing the proportion of the two hormones, and this is the strongest possible demonstration that the differentiation in the higher plants is open, since the same cells can be directed to become several quite different tissues. And the last step of the argument is that a whole plant can be regenerated from this tissue, so that a single explant of a single leaf can be made to give a complete new plant, and a large number of genetically identical plants can be produced from a single explant, which is the clonal propagation of the horticulture. And the qualification that closes the answer is that it is not unlimited: the genome remains the same and the cell cannot be made to form a tissue of an entirely different type, so it is open within the limits set by the genetic programme of the plant.

Step-by-step solution

1. 1This is a question about the critical photoperiod, and the answer is that a plant flowers when the day is longer or the shorter than a critical value, so the same day can be a short day for one species and a long day for another, and a single day can therefore induce the flowering of both. Write the definition of the two types first, then the critical photoperiod, then the mechanism, then the example.
2. 2The definitions. A short day plant is one in which the flowering is induced by a day shorter than a critical duration, that is the photoperiod must be shorter than the critical photoperiod, and a long day plant is one in which the flowering is induced by a day longer than the critical duration. And a neutral or day-neutral plant is one in which the photoperiod has no influence at all, so the two categories are contrasted with the third rather than being exhaustive. The short day plants are the ones that flower when the day is short, and this is not paradoxically a response to the day being favourable but to the day being shorter than the critical value, so that a long day is a long night in effect, since the two are complements of one another.
3. 3Then the critical photoperiod, which is the key to the question. Every photoperiodic plant has its own critical photoperiod, that is its own critical duration of the light, and this is a characteristic of the species and not of the individual, and it is fixed and genetically determined, and a plant is induced to flower only if the day it receives is longer than its critical photoperiod, in the case of a long day plant, or shorter than it, in the case of a short day plant. So the critical photoperiod is the threshold, and the flowering is a comparison of the received day with the threshold, and nothing about the day is short or long in itself, it is short or long only in relation to the critical value of the particular plant. So the two terms do not describe the day but describe the plant's relation to the day, and this is the whole answer to the question.
4. 4The consequence and the example. Since short and long are defined only relative to the critical photoperiod, and since the critical photoperiod differs from species to species, the same day of the same length can be longer than the critical photoperiod of one species and shorter than the critical photoperiod of another, so that the same day is a long day for the second and a short day for the first. And so a short day plant and a long day plant growing in the same place on the same day receive the same photoperiod and yet one of them experiences it as a short day and the other as a long day, and both are induced to flower, and they flower simultaneously. The classic illustration is that a short day plant with a critical photoperiod of, say, ten hours will flower in a day of nine hours, while a long day plant with a critical photoperiod of, say, eight hours will flower in that same day of nine hours, since for the first the day is shorter than its critical value and for the second it is longer than its critical value. And the reason for the comparison is agronomic, since the sowing time of a crop is decided by the critical photoperiod of that crop, and so the knowledge of the critical photoperiod is what lets a farmer choose the date of sowing so that the crop flowers at the right time, and the phytochrome and the florigen are the same in both, since the photoperiod is perceived by the leaves and the florigen is produced in the leaves and transported to the shoot apex.

Step-by-step solution

1. 1This is the single regulator to each treatment, so give one hormone for each of the six with a one-line reason, and add the specific example in each case since the example is what secures the mark.
2. 2(a) Rooting in a twig: the auxin. The auxin at the appropriate concentration initiates the rooting in the stem cuttings, and the low concentration of the synthetic auxin, the NAA, is applied to the basal end of the twig, and this is the standard practice in the propagation of the plants by the cuttings, in the layering and in the grafting, and the rooting is from the adventitious and the lateral roots formed at the cut surface.
3. 3(b) Quickly ripening a fruit: the ethylene. The ethylene is the gaseous hormone, and it is the fruit ripener, being used commercially to ripen the fruits that are picked unripe, as in the banana and the mango and in the citrus and in the tomato, and it is also used in the canned and in the stored fruit, and the mechanism is that the ethylene initiates the changes of the wall, of the colour and of the texture that together constitute the ripening, so a single dose of the gas will ripen a whole consignment at once. And the action of the auxin in the same circumstance is the opposite, since the auxin prevents the ripening and the senescence of the fruit and is used to hold the fruit on the tree, so it is a useful contrast to note here.
4. 4(c) Delaying the senescence of the leaf: the cytokinin. The cytokinin is the anti-senescence hormone, and it markedly delays the ageing of the leaves, so that a leaf that would normally fall at a given time is held on the plant, and the mechanism is the mobilisation of the nutrients, the amino acids and the mineral solutes, out of the ageing leaf and into the sink, that is the young growing parts, which is the delayed senescence, so the cytokinin delays the senescence of the leaves and even the mobilisation of the nutrients out of them. The rich sources of the cytokinins, which are the regions of rapid cell division and the the fruits and the seeds, are consistent with this, for they are the parts of the plant that are drawing on the nutrients of the old leaves.
5. 5(d) Inducing the growth of the axillary buds: the cytokinin. The cytokinins promote the growth of the lateral, that is the axillary, buds, and this is what releases the apical dominance, the auxin of the apical bud having suppressed the lateral buds, so the two are in antagonism and the cytokinin overcomes the auxin. The application is the basis of the technique of the tissue culture, in which a large cytokinin to auxin ratio in the medium promotes the shoot formation, so a single explant yields a large number of shoots, and the practical horticultural consequence is that a plant from which the apical bud has been removed becomes bushy, since the cytokinin supplied to the axillary buds releases them.
6. 6(e) Bolting a rosette plant: the gibberellin. Bolting is the rapid internodal elongation that just prior to flowering, so that a rosette of leaves at the ground level is carried up on a stem and the flowers are lifted into the light and into the air for the pollination, and it is induced by the gibberellin, the GA3, which causes this rapid elongation of the internodes. So the answer is the gibberellic acid, and the species that is quoted is the rosette of the cabbage, the Beta vulgaris of the beet and the Hyoscyamus, and the commercial use that follows is the bolting of the onion and of the seed production in the rosette plants, for the flowering is a precondition of the seed formation.
7. 7(f) Inducing the immediate closure of the stomata in the leaves: the abscisic acid. The ABA stimulates the closure of the stomata, and it does so rapidly and immediately, within minutes of the onset of the water stress, by causing the guard cells to lose their turgor, and this is the mechanism that checks the transpiration and so saves the water. And the point to add is the contrast with the ethylene, which is the gaseous PGR that is largely an inhibitor of the growth activities and which is used in the horticulture to break the seed and the bud dormancy and to initiate the germination, but it is the ABA and not the ethylene that closes the stomata.

Step-by-step solution

1. 1The answer is that it cannot, and the reason is the site of the perception, which is the leaf. So identify the leaf, the phytochrome and the florigen, and the consequence of defoliation, and the qualification that the meristem and the leaves already formed are still present.
2. 2The answer is that a defoliated plant cannot respond to the photoperiodic cycle, and the reason is that the site of the perception of the photoperiod is the leaf, so the defoliation removes the very organ that is required. The mechanism of the photoperiodic response is as follows. The light is perceived by a photoreceptor in the leaf, and this photoreceptor is the phytochrome, which exists in the two interconvertible forms, the Pr that absorbs the red light of about 660 nanometres and is converted to the Pfr, and the Pfr that absorbs the far-red light of about 730 nanometres and is converted back to the Pr, and the Pfr is the physiologically active form that initiates the photoperiodic response. The activated phytochrome then acts on the leaf to induce the production of a floral stimulus, the florigen, in the leaf, and this is the hormone-like molecule that is transported from the leaf through the phloem to the shoot apical meristem, and there it brings about the flowering when the day is of the required length.
3. 3Then the consequence of the defoliation. Since the phytochrome and therefore the perception of the photoperiod is in the leaf, and since the florigen is also produced in the leaf and there is nowhere else for it to be made, a plant that has had all its leaves removed has neither the receptor nor the source of the florigen, so it cannot perceive the length of the day and cannot form the florigen, and so it will not flower in response to the photoperiod. The two steps, the perception and the production of the florigen, are both lost with the leaves, so the whole photoperiodic response is lost. And the experiment to quote in support is the one in which a single leaf of a short day plant is kept on a plant that has otherwise been defoliated, and the plant then does respond, and it is this that demonstrates that the leaf is the site of the perception and that the response of the shoot apex is mediated by a translocated stimulus from the leaf.
4. 4The qualification, which is what makes the answer complete without weakening it. A defoliated plant is not in every respect inert. The shoot apical meristem is still present, and it is the organ that will eventually carry the flowers, so the plant is not physically incapable of flowering. And a plant that had already been induced to flower before it was defoliated will go on to develop and open its flowers and set its fruit, because the induction has already been completed and the leaves are no longer needed for that step; the leafless plant may even continue to respire and to draw on the reserves of its stem for a while. And a partly defoliated plant may still respond, since one retained, photosynthetically active leaf is enough to perceive the photoperiod and to produce the florigen. But none of this changes the answer to the question as it is asked: the induction of the flowering, the photoperiodic response itself, cannot occur in a fully defoliated plant, because the two steps that the response consists of, the perception of the day length and the production of the florigen, are both carried out in the leaf, and the defoliation removes both of them.

Step-by-step solution

1. 1Four short predictions, one hormone or one principle each, and each should be stated with the mechanism in one or two lines. The four are: elongation without flowering in the rice, dedifferentiation and a callus in the second, ripening of all of the fruit in the third, and the failure of the shoot formation with only the roots in the fourth.
2. 2(a) GA3 applied to the rice seedlings. The rice is a monocot and the internodes of the rice plant are short, so the plant is in a rosette-like condition, and the gibberellin, the GA3, causes the rapid elongation of the internodes, so the rice seedlings would show a dramatic increase in the length of the internodes and would grow tall and slender, that is they would show the bolting effect. But there is a more specific and more examinable consequence, since the GA3 would cause the internodes to elongate without the plant flowering, for the elongation is promoted independently of the flowering, and so the GA3 applied to the rice seedlings would cause an elongation of the internodes in the absence of any flowering. The practical significance to add is that this is a hazard in the rice, since the lodging, the falling over, of the crop is caused by the elongation of the internodes, so that if the GA3 is applied at the wrong time to the rice, the plant elongates, does not flower properly and falls over, and the crop is lost, and this is the reason the GA3 must be used with care in the rice.
3. 3(b) The dividing cells stop differentiating. If the cells that are dividing cease to differentiate, then the plant is deprived of the differentiated cells that it needs, since the growth of the plant is normally a balance between the division and the differentiation, and so the meristematic cells would continue to divide and accumulate, and the result would be a mass of undifferentiated cells, a callus, with no organs and no normal structure. The plant would therefore be unable to form its tissues and its organs, since the meristematic cells would persist as such and the process of the maturation to the specific functions, which is the differentiation, would be abolished, and so the plant would show an abnormal growth, a proliferative mass of cells, and it might be described as a tumour-like growth or a callus formation, since the whole of the tissue culture technique depends on the maintenance of this undifferentiated state. So the answer is that the plant would produce a mass of undifferentiated cells, a callus, instead of the normal differentiated tissues and organs.
4. 4(c) A rotten fruit is mixed with the unripe fruits. The rotten fruit gives off the ethylene, which is the gaseous hormone and the ripening hormone, and the ethylene diffuses through the atmosphere of the container and reaches the unripe fruits, and the unripe fruits are induced to ripen, so the whole lot ripens together and quickly. And the point to add is that this is exactly what happens in practice, for the fruits that are to be ripened are kept with a ripe or a rotten fruit in a closed box, or an ethylene gas is injected into the store, and the reason is that the ethylene is a gas and so it spreads through the packed fruit and reaches every one of them, and a rotten fruit produces it in quantity. So the practical advice that follows is that a single rotten apple spoils the whole lot, and the rotten fruit should be removed from the store immediately, which is the reason the fruits are graded and the damaged ones are culled.
5. 5(d) The cytokinin is omitted from the culture medium. The ratio of the auxin to the cytokinin in the tissue-culture medium determines what the callus gives rise to, and it is the cytokinin that promotes the formation of the shoot buds. So if the cytokinin is omitted, the medium will have a high auxin to cytokinin ratio, and this promotes the formation of the roots and not the shoots, so the callus would give rise to roots only, or at best a poor root formation with no shoot buds at all, and the plantlets would fail to develop and there would be no complete new plant. The consequence in practice is that the culture would produce a mass of root-like tissue and no aerial shoot, so the propagation fails and the experiment is wasted, and the remedy is to add the cytokinin, for the shoots require it and the roots do not. And it is worth adding that a completely cytokinin-free medium with only the auxin gives the root, so the cytokinin is the specific requirement of the shoot, which is exactly what the ratio experiment of Skoog and Miller demonstrated.