SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Plant Growth and Development

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Exercises 13.1–13.10

  1. Exercise 13.1

    Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.

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    Growth — an irreversible, permanent increase in the size of an organ, of its parts, or even of a single cell; it is generally accompanied by metabolic processes (both anabolic and catabolic) that occur at the expense of energy. Expansion of a leaf is growth.
    Differentiation — the act leading to maturation, by which cells derived from the root apical meristem, shoot apical meristem and cambium mature to perform specific functions. The cells undergo structural changes in their cell walls and protoplasm; e.g., to form a tracheary element a cell loses its protoplasm and develops a strong, elastic, lignocellulosic secondary wall.
    Development — all the changes an organism goes through during its life cycle, from germination of the seed to senescence. Broadly, development is the sum of growth and differentiation.
    Dedifferentiation — the regaining of the capacity to divide by living differentiated cells that had already lost it, e.g., formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
    Redifferentiation — the maturation of the cells produced by such dedifferentiated meristems; they once again lose the capacity to divide and mature to perform specific functions.
    Determinate growth — growth that ceases once a certain size or stage is reached, as in leaves, flowers and fruits, which have limited dimensions; it is the opposite of the indeterminate (open) growth of the plant axis.
    Meristem — a group of cells at certain locations in the plant body whose cells retain the capacity to divide and self-perpetuate; their activity keeps adding new cells to the plant body (root and shoot apical meristems, vascular cambium, cork cambium).
    Growth rate — the increased growth per unit time. It may be arithmetic, \(\displaystyle L_{t} = L_{0} + rt\), or geometric, \(\displaystyle W_{1} = W_{0}e^{rt}\).

  2. Exercise 13.2

    Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?

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    Because different organs, and the same plant at different stages, grow in different ways, and each way is best measured by a different quantity.
    Growth at the cellular level is really an increase in the amount of protoplasm, but protoplasm cannot be measured directly; one has to measure some quantity roughly proportional to it — fresh weight, dry weight, length, area, volume or cell number.
    In a maize root apical meristem, growth shows up as an increase in cell number (a single apical meristem can give rise to more than $\displaystyle 17,500$ new cells per hour).
    In a watermelon, growth shows up as an increase in cell size (cells may increase in size up to $\displaystyle 3,50,000$ times). One parameter clearly cannot describe both.
    Growth of a pollen tube is measured by its length, whereas growth of a dorsiventral leaf is measured by its increase in surface area.
    The plant also grows differently at different times — elongation along the axis by the apical meristems, increase in girth by the lateral meristems — so a set of parameters, not any one of them, is needed to follow growth through the whole life of a flowering plant.

  3. Exercise 13.3

    Describe briefly:
    (a)
    Arithmetic growth
    (b)
    Geometric growth
    (c)
    Sigmoid growth curve
    (d)
    Absolute and relative growth rates

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    (a) Arithmetic growth
    Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures.
    The simplest example is a root elongating at a constant rate.
    On plotting the length of the organ against time, a linear curve is obtained.
    \(\displaystyle L_{t} = L_{0} + rt\), where \(\displaystyle L_{t}\) = length at time 't', \(\displaystyle L_{0}\) = length at time zero, and r = growth rate or elongation per unit time.
    (b) Geometric growth
    Both progeny cells following mitotic division retain the ability to divide and continue to do so.
    The initial growth is slow (lag phase), then increases rapidly at an exponential rate (log or exponential phase), and finally, with limited nutrient supply, slows down to a stationary phase.
    \(\displaystyle W_{1} = W_{0}e^{rt}\), where \(\displaystyle W_{1}\) = final size, \(\displaystyle W_{0}\) = initial size at the beginning of the period, r = growth rate, t = time of growth, e = base of natural logarithms.
    Here r is the relative growth rate and is also a measure of the ability of the plant to produce new plant material, the efficiency index; the final size \(\displaystyle W_{1}\) therefore depends on the initial size \(\displaystyle W_{0}\).
    (c) Sigmoid growth curve
    The S-shaped curve obtained when the parameter of growth is plotted against time in geometric growth.
    It has three parts: a flat lag phase, a steeply rising exponential (log) phase, and a flattening stationary phase.
    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 (Figure $\displaystyle 13.6$).
    (d) Absolute and relative growth rates
    Absolute growth rate — the measurement and comparison of total growth per unit time.
    Relative growth rate — the growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter.
    In the book's example (Figure $\displaystyle 13.7$), two leaves A and B of different sizes both increase their area by $\displaystyle 5$ \(\displaystyle cm^{2}\) in the same time to give \(\displaystyle A^{1}\) and \(\displaystyle B^{1}\). Their absolute growth rates are the same, but the smaller leaf has the much higher relative growth rate, because the same increase is a far larger fraction of its initial area.
    NCERT_Solution_Class11_Biology_Ch13_Q13-3

  4. Exercise 13.4

    List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.

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    The five main groups of natural plant growth regulators
    Auxins — indole compounds, e.g., indole-$\displaystyle 3$-acetic acid (IAA).
    Gibberellins — terpenes, e.g., gibberellic acid (\(\displaystyle GA_{3}\)).
    Cytokinins — adenine derivatives, e.g., \(\displaystyle N^{6}\)-furfurylamino purine (kinetin).
    Abscisic acid (ABA) — a derivative of carotenoids.
    Ethylene — a gas, \(\displaystyle C_{2}\)\(\displaystyle H_{4}\).
    The first three are growth promoters; ABA is a growth inhibitor; ethylene could fit either group but is largely an inhibitor of growth activities.
    A note on auxinsDiscovery
    The name is from the Greek 'auxein', to grow; auxin was first isolated from human urine.
    Charles Darwin and his son Francis Darwin observed that coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism).
    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 (Figure $\displaystyle 13.10$).
    Auxin was finally isolated by F.W. Went from the tips of coleoptiles of oat seedlings.
    IAA and indole butyric acid (IBA) are natural auxins isolated from plants; NAA (naphthalene acetic acid) and $\displaystyle 2,4$-D ($\displaystyle 2,4$-dichlorophenoxyacetic acid) are synthetic auxins.
    Physiological functions
    Produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.
    Cause apical dominance — the growing apical bud inhibits the growth of the lateral (axillary) buds; removal of the shoot tip (decapitation) releases the lateral buds into growth.
    Prevent fruit and leaf drop at early stages, but promote the abscission of older, mature leaves and fruits.
    Control xylem differentiation and help in cell division.
    Induce parthenocarpy, e.g., in tomatoes.
    Agricultural and horticultural applications
    Initiate rooting in stem cuttings — the basis of plant propagation.
    Promote flowering, e.g., in pineapples.
    Produce seedless (parthenocarpic) tomatoes.
    Used widely as herbicides: $\displaystyle 2,4$-D kills dicotyledonous weeds but does not affect mature monocotyledonous plants, so gardeners use it to prepare weed-free lawns.
    Decapitation, which works by removing the auxin source, is applied in tea plantations and in hedge-making to get bushy growth.
    NCERT_Solution_Class11_Biology_Ch13_Q13-4

  5. Exercise 13.5

    Why is abscisic acid also known as stress hormone?

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    Because ABA is the regulator that carries a plant through unfavourable conditions — it increases the tolerance of plants to various kinds of stresses.
    It stimulates the closure of stomata, which checks transpirational water loss when water is short.
    It acts as a general plant growth inhibitor and an inhibitor of plant metabolism, so growth is shut down rather than continued under adverse conditions.
    It inhibits seed germination and induces dormancy, which helps seeds withstand desiccation and other factors unfavourable for growth; it also plays an important role in seed development and maturation.
    In most situations ABA acts as an antagonist to the gibberellins.

  6. Exercise 13.6

    ‘Both growth and differentiation in higher plants are open’. Comment.

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    Both are open because neither the number of cells a plant will make, nor the fate of any one of them, is fixed in advance.
    Growth is open
    Plants retain the capacity for unlimited growth throughout their life because of meristems at certain locations in the body, whose cells can divide and self-perpetuate.
    New cells are always being added to the plant body by the activity of the meristem — this is called the open form of growth.
    Root and shoot apical meristems add length; the lateral meristems (vascular cambium and cork cambium) add girth, so growth continues in more than one direction and does not stop at a fixed size.
    Differentiation is open
    Cells and tissues arising out of the same meristem have different structures at maturity.
    The final structure at maturity is decided by the location of the cell within the organ: cells positioned away from the root apical meristem differentiate as root-cap cells, while those pushed to the periphery mature as epidermis.
    The fate of a cell is not even permanent — a differentiated cell may dedifferentiate (interfascicular cambium, cork cambium from parenchyma) and then redifferentiate.
    Consequence
    Since differentiation is open, development is also flexible; development is the sum of growth and differentiation.
    This flexibility shows up as plasticity, e.g., heterophylly in cotton, coriander and larkspur, where juvenile leaves differ in shape from mature ones, and in buttercup, where leaves formed in air differ from those formed in water.

  7. Exercise 13.7

    ‘Both a short day plant and a long day plant can produce can flower simultaneously in a given place’. Explain.

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    Yes — a short day plant and a long day plant growing in the same place can be in flower on the same day.
    All this chapter supplies is the general control: flowering is one of the developmental events governed by extrinsic factors, chiefly light and temperature, and the chapter states that these extrinsic factors act on growth and development via plant growth regulators.
    One place on one date offers a single day length, but two species need not respond to that one day length in the same way — what each plant does with the day depends on the plant, not on the day.
    Not from this chapter: the usual explanation is that 'short day' and 'long day' name each species' own light-duration requirement (the critical duration, in editions that still carry the Photoperiodism section) rather than any fixed number of hours, so a single photoperiod can fall below one plant's requirement and above the other's at the same time. Treat that as background; do not quote it, or any hour-figures, as this chapter's.
    Note on the source: the Reprint $\displaystyle 2026$-$\displaystyle 27$ chapter runs only from $\displaystyle 13.1$ to $\displaystyle 13.4$ and stops at the plant growth regulators. It promises a discussion of light and temperature on the initiation of flowering and never delivers one, so no critical day length, no definition of a short day or long day plant, and no site of photoperiodic perception exists anywhere in the text this exercise sits at the end of.

  8. Exercise 13.8

    Which one of the plant growth regulators would you use if you are asked to:
    (a)
    induce rooting in a twig
    (b)
    quickly ripen a fruit
    (c)
    delay leaf senescence
    (d)
    induce growth in axillary buds
    (e)
    ‘bolt’ a rosette plant
    (f)
    induce immediate stomatal closure in leaves.

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    (a) Induce rooting in a twig — Auxins (IBA or NAA). Auxins initiate rooting in stem cuttings, an application widely used for plant propagation.
    (b) Quickly ripen a fruit — Ethylene (in practice ethephon, which is readily absorbed, transported within the plant and releases ethylene slowly). Ethylene is highly effective in fruit ripening and enhances the respiration rate during ripening (respiratory climactic).
    (c) Delay leaf senescence — Cytokinins. They promote nutrient mobilisation, which helps delay leaf senescence.
    (d) Induce growth in axillary buds — Cytokinins. They help overcome apical dominance and promote lateral shoot growth.
    (e) 'Bolt' a rosette plant — Gibberellins. They promote bolting, i.e., internode elongation just prior to flowering, in beet, cabbages and many plants with the rosette habit.
    (f) Induce immediate stomatal closure in leaves — Abscisic acid (ABA). It stimulates the closure of stomata and increases tolerance to stress.

  9. Exercise 13.9

    Would a defoliated plant respond to photoperiodic cycle? Why?

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    No — a plant stripped of all its leaves would not respond to the photoperiodic cycle.
    This chapter cannot supply the reason. It names light as an extrinsic factor that controls flowering and other developmental events through plant growth regulators, announces a discussion of light and temperature on the initiation of flowering, and then ends — the Photoperiodism section is absent from the Reprint $\displaystyle 2026$-$\displaystyle 27$ text.
    Not from this chapter: the standard reason is that the leaf is the organ that perceives the photoperiod, so a plant carrying no leaves has no organ with which to register the light and dark periods. Editions that still carry Photoperiodism say this; this one does not, so do not cite it as this chapter's.
    Two things often written into this answer have no support here at all, and should not be offered as book fact: that a hormonal floral stimulus is made in the leaf and migrates to the shoot apices, and that a single leaf left on the plant is enough for the response. Neither statement appears anywhere in this chapter.

  10. Exercise 13.10

    What would be expected to happen if:
    (a)
    GA3\displaystyle GA_{3} is applied to rice seedlings
    (b)
    dividing cells stop differentiating
    (c)
    a rotten fruit gets mixed with unripe fruits
    (d)
    you forget to add cytokinin to the culture medium.

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    (a) \(\displaystyle GA_{3}\) is applied to rice seedlings
    The seedlings grow abnormally elongated — the symptoms of the 'bakanae' or foolish seedling disease of rice.
    This is the same effect produced naturally by the fungal pathogen Gibberella fujikuroi; E. Kurosawa ($\displaystyle 1926$) reproduced the symptoms by treating rice seedlings with sterile filtrates of the fungus, and the active substances were later identified as gibberellic acid.
    Elongation is what gibberellins do generally — they cause an increase in the length of the axis, as when they lengthen grape stalks, increase the stem length of sugarcane, and drive the bolting internodes of rosette plants.
    (b) Dividing cells stop differentiating
    No mature tissues or organs would form; the plant would remain an undifferentiated, continuously dividing mass of cells — a callus or tumour-like growth.
    Differentiation is what gives a cell the structure that matches its function: without it there would be no tracheary elements to carry water under tension, no epidermis, no root cap.
    Growth (increase in cell number) would go on, but development would stop, since development is the sum of growth and differentiation.
    (c) A rotten fruit gets mixed with unripe fruits
    The unripe fruits would ripen much faster.
    A ripening or senescing fruit synthesises large amounts of ethylene, and ethylene hastens the ripening of fruits stored with it — exactly what H.H. Cousins ($\displaystyle 1910$) confirmed, when a volatile substance released from ripened oranges hastened the ripening of stored unripened bananas.
    Ethylene also enhances the respiration rate during ripening (respiratory climactic) and promotes senescence and abscission, so the ripening quickly runs on to over-ripening and spoilage of the whole lot.
    (d) You forget to add cytokinin to the culture medium
    The tissue would not proliferate: cytokinesis stops, so the callus does not grow and no shoots are formed.
    F. Skoog and his co-workers showed that callus from internodal segments of tobacco stems proliferated only if, in addition to auxins, the nutrient medium was supplemented with a cytokinin source — extracts of vascular tissues, yeast extract, coconut milk or DNA; Miller et al. ($\displaystyle 1955$) later crystallised the active substance, kinetin.
    Without cytokinin the cells may enlarge but they will not divide, and the effects cytokinin promotes — new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation — will not appear.