1. Growth — an irreversible permanent increase in size of an organ or its parts, or even of an individual cell. It is generally accompanied by metabolic processes, both anabolic and catabolic, that occur at the expense of energy. (Note the two tests: irreversible, and metabolic. The swelling of a wooden block in water fails both.)

2. Differentiation — the act leading to maturation of the cells derived from root apical and shoot apical meristems and cambium, so that they perform specific functions. During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm. (Example: to form a tracheary element the cell loses its protoplasm and develops a strong, elastic, lignocellulosic secondary wall.)

3. Development — a term that includes all changes that an organism goes through during its life cycle, from germination of the seed to senescence. Broadly, development is the sum of growth and differentiation.

4. Dedifferentiation — the phenomenon in which living differentiated cells, that by now have lost the capacity to divide, can regain the capacity of division under certain conditions. (Example: formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.)

5. Redifferentiation — the process by which such dedifferentiated meristems or tissues divide and produce cells that once again lose the capacity to divide but mature to perform specific functions. (Example: secondary xylem, secondary phloem and cork.)

6. Determinate growth — growth that is limited, stopping after a certain size or stage is reached, because the meristematic activity ceases. It is the opposite of the indeterminate growth of plant axes. (Examples: leaves, flowers and fruits, which attain a fixed size and then stop.)

7. Meristem — a tissue whose cells have the capacity to divide and self-perpetuate, present at certain locations in the plant body, and the site of growth in plants. Apical meristems (root and shoot, sometimes with intercalary meristem) contribute to elongation growth; lateral meristems (vascular cambium, cork cambium) cause increase in girth. Their products soon lose the capacity to divide and make up the plant body.

8. Growth rate — the increased growth per unit time. It can be expressed mathematically, and may be arithmetic (L_t = L₀ + rt) or geometrical (W₁ = W₀e^rt).

Because growth at the cellular level is principally a consequence of increase in the amount of protoplasm, and increase in protoplasm is difficult to measure directly. One therefore measures some quantity which is more or less proportional to it — but which quantity is proportional changes with the organ, the tissue and the stage of life. No single parameter remains appropriate throughout.

The chapter's own illustrations make the point:

• One single maize root apical meristem can give rise to more than 17,500 new cells per hour. Here growth is best expressed as an increase in cell number — the cells are not getting bigger.
• Cells in a watermelon may increase in size by up to 3,50,000 times. Here growth must be expressed as an increase in size of the cell — counting cells would show almost no change.
• The growth of a pollen tube is measured in terms of its length, since it extends as a narrow tube in one direction.
• An increase in surface area denotes the growth in a dorsiventral leaf, since a flat organ expands in two dimensions and length alone would miss most of it.

Further reasons a single parameter fails:

• Fresh weight can change without growth at all. A wilted plant watered overnight gains weight by imbibition and turgidity, not by growth, and a plant in water stress loses weight without shrinking permanently.
• Dry weight can fall during genuine growth. A germinating seed grows vigorously while its total dry weight decreases, because stored food is being respired away before the seedling becomes photosynthetic.
• Different organs grow in different directions. A root and stem elongate, a leaf spreads, a fruit swells in volume, a cambium adds girth. One measurement cannot capture all four.
• Volume or height is useless for a rosette plant until it bolts, even though it is growing all along.

Conclusion. Since a flowering plant passes through germination, vegetative elongation, leaf expansion, secondary thickening, flowering, fruiting and senescence — each with a different dominant kind of change — growth must be measured by whichever parameter is proportional to protoplasmic increase at that stage. This is exactly why NCERT lists fresh weight, dry weight, length, area, volume and cell number together rather than naming one.

(a) Arithmetic growth. In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The simplest expression is a root elongating at a constant rate. On plotting the length of the organ against time, a linear curve is obtained.

(b) Geometric growth. Here both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. In most systems, the initial growth is slow (lag phase), and it increases rapidly thereafter at an exponential rate (log or exponential phase). However, with limited nutrient supply, the growth slows down, leading to a stationary phase.

Here r is the relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index. Hence the final size of W₁ depends on the initial size W₀.

(c) Sigmoid growth curve. If the parameter of growth is plotted against time in geometric growth, we get a typical sigmoid or S-curve, with the three phases in sequence:

• Lag phase — initial growth is slow;
• Log or exponential phase — growth increases rapidly, at an exponential rate;
• Stationary phase — with limited nutrient supply the growth slows down and levels off.

A sigmoid curve is a characteristic of living organisms growing in a natural environment. It is typical for all cells, tissues and organs of a plant. (The chapter summary refers to the three principal phases as the lag, the log and the senescent phase.)

(d) Absolute and relative growth rates. Quantitative comparisons between the growth of living systems can be made in two ways:

• 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.

The chapter's illustration: two leaves A and B of different sizes both show the same absolute increase in area, 5 cm², in the same time to give A₁ and B₁. Yet one shows a much higher relative growth rate — the smaller leaf A. If A began at 5 cm² and B at 50 cm², then A's relative growth is 5/5 = 100% while B's is 5/50 = 10%. The absolute rates are identical; the relative rates differ tenfold.

The five main groups of natural plant growth regulators:

1. Auxins — indole compounds, e.g. indole-3-acetic acid (IAA)
2. Gibberellins — terpenes, e.g. gibberellic acid (GA₃)
3. Cytokinins — adenine derivatives, e.g. kinetin (N⁶-furfurylamino purine), zeatin
4. Abscisic acid (ABA) — a derivative of carotenoids
5. Ethylene — a gas, C₂H₄

Of these, auxins, gibberellins and cytokinins are the plant growth promoters; ABA belongs to the group involved in stress responses and growth inhibition; ethylene could fit either group but is largely an inhibitor of growth activities.

A note on gibberellins

Discovery. Like all five groups, the discovery was accidental. The ‘bakanae’ or foolish seedling disease of rice seedlings was caused by a fungal pathogen, Gibberella fujikuroi. Infected seedlings grew abnormally tall and weak. E. Kurosawa (1926) reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus — the crucial step, since a sterile filtrate contains no fungus, proving that a chemical and not the organism produced the effect. The active substances were later identified as gibberellic acid.

Nature and nomenclature. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants, denoted GA₁, GA₂, GA₃ and so on. Gibberellic acid (GA₃) was one of the first to be discovered and remains the most intensively studied form. All GAs are acidic.

Physiological functions.

• Cause an increase in length of the axis;
• cause fruits to elongate and improve their shape;
• delay senescence;
• promote bolting — internode elongation just prior to flowering — in rosette plants;
• hasten the maturity period of juvenile plants;
• in most situations they are antagonised by ABA, notably over seed germination and dormancy.

Agricultural and horticultural applications.

• Increasing the length of grape stalks, using their ability to elongate the axis;
• making apples elongate and improving their shape;
• delaying senescence so that fruits can be left on the tree longer, extending the market period;
• GA₃ is used to speed up the malting process in the brewing industry;
• spraying sugarcane with gibberellins increases the length of the stem, raising the yield by as much as 20 tonnes per acre — since sugarcane stores its carbohydrate as sugar in the stem;
• spraying juvenile conifers with GAs hastens the maturity period, leading to early seed production;
• promoting bolting in beet, cabbages and many plants with a rosette habit.

Because ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. That is the chapter's direct reason, and it should open the answer.

How the two parts of that sentence work:

• Stomatal closure. Most of a plant's water is lost through the stomata. When water is short, ABA accumulates and closes them, cutting transpiration immediately. It is the fastest defence a plant has against drought, and it is a defence no other PGR provides.
• General stress tolerance. ABA raises tolerance to a range of stresses, not to drought alone — which is why the name is stress hormone rather than drought hormone.

Its other stress-related roles support the name:

• It acts as a general plant growth inhibitor and an inhibitor of plant metabolism — shutting growth down is the correct response when conditions are hostile, since growth is expensive and irreversible.
• It inhibits seed germination, so a seed does not germinate into unfavourable conditions.
• It plays an important role in seed development, maturation and dormancy, and by inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.

One further point worth adding. In most situations ABA acts as an antagonist to GAs. The gibberellins push for germination and elongation; ABA holds them back. The plant's actual behaviour is decided by the balance between the two, and under stress the balance tips towards ABA.

The statement is correct, and ‘open’ means something slightly different in each half.

Growth is open

Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This is due to the presence of meristems at certain locations in their body, whose cells have the capacity to divide and self-perpetuate. This form of growth, wherein new cells are always being added to the plant body by the activity of the meristem, is called the open form of growth. Its products soon lose the capacity to divide and make up the plant body, while the meristem itself continues.

So growth is open in the sense that it is never finished: there is no fixed final body, and new organs can be added indefinitely. Note the chapter's own qualification, though — growth in plants is open, i.e. it can be indeterminate or determinate. Axes grow indeterminately; leaves, flowers and fruits are determinate.

Differentiation is open

Even differentiation in plants is open, because cells or tissues arising out of the same meristem have different structures at maturity. The fate of a cell is not settled in advance by its ancestry. Instead, the final structure at maturity of a cell or tissue is also determined by the location of the cell within.

The chapter's example: cells positioned away from the root apical meristem differentiate as root-cap cells, while those pushed to the periphery mature as epidermis — identical starting cells, different destinies decided purely by position.

Further examples of the same principle: from the vascular cambium, the same meristem produces secondary xylem on its inner face and secondary phloem on its outer face; in a leaf, mesophyll cells near the upper surface become palisade and those below become spongy; in the root, the same apical meristem yields epidermis, cortex, endodermis, pericycle and vascular tissue according to radial position alone.

And openness goes one step further

Differentiation in plants is not merely flexible at the outset but reversible afterwards. Living differentiated cells that have lost the capacity to divide can regain it under certain conditions — dedifferentiation, as in the interfascicular cambium and cork cambium formed from fully differentiated parenchyma — and their products then redifferentiate. Whole plants can be regenerated from a callus for the same reason.

The consequence

Since differentiation in plants is open, the development could also be flexible — i.e., development is the sum of growth and differentiation. Plants exhibit plasticity in development. Heterophylly in cotton, coriander, larkspur and buttercup is the visible proof: the same genotype produces different leaf forms according to the phase of life or the environment. A plant's final form is therefore negotiated with its surroundings rather than fixed in advance — the deepest single contrast between plant and animal development.

Yes, this is entirely possible, and the reason is that the terms “short day” and “long day” do not name two different day lengths — they name two different relationships to a critical duration, and that critical duration is different for every species.

The definitions, stated carefully

• A long day plant (LDP) flowers when the light period is longer than its own critical duration.
• A short day plant (SDP) flowers when the light period is shorter than its own critical duration.

Neither definition fixes an actual number of hours. So a day length can be simultaneously above one species' critical duration and below another's.

A worked illustration

Suppose at a given place on a given date the photoperiod is 13 hours of light. Suppose a long day plant X has a critical duration of 12 hours, and a short day plant Y has a critical duration of 14 hours. Then:

• For X (LDP): 13 > 12, so the light period exceeds its critical duration → X flowers.
• For Y (SDP): 13 < 14, so the light period is shorter than its critical duration → Y flowers.

Both flower on the same day, in the same field, under exactly the same photoperiod. There is no contradiction at all — the same 13 hours is a “long day” to X and a “short day” to Y.

Points to add for full marks

• It is the length of the uninterrupted dark period that is actually critical, not the light period. Some authors therefore prefer long night and short night plants. A short day plant given a brief flash of light in the middle of the night will not flower, even though the total light hours barely changed.
• Day-neutral plants (tomato, cucumber, sunflower) flower irrespective of photoperiod, and so can of course flower alongside both.
• Other factors also decide the actual date: vernalisation requirement, age of the plant, temperature and nutrition. Two species reaching the flowering stage at the same time need only both satisfy their own photoperiodic condition.

The one-sentence answer: because the critical duration is species-specific, a single photoperiod at a given place can be above the critical duration of a long day plant and below that of a short day plant at the same time, so both are induced to flower together.

A pattern worth noticing across the six answers. Parts (c) and (d) both take cytokinins, and parts (a) and (e) take different hormones for what looks like the same thing (making a plant grow longer or bushier). This is precisely the chapter's point that any PGR has diverse physiological effects, and diverse PGRs also manifest similar effects. Answer each part from the specific effect named in the text rather than from a general idea of “the growth hormone”.

No. A completely defoliated plant would not respond to the photoperiodic cycle.

Why not

Because the leaf is the site at which the photoperiod is perceived. The sequence has two separate locations:

• The leaves perceive the relative lengths of day and night and, in response, produce a hormonal substance responsible for floral induction.
• That substance then migrates from the leaves to the shoot apices, where it transforms the vegetative apex into a flowering apex.

Remove all the leaves and you remove the organ of perception and the source of the floral signal. The shoot apex, which is where flowering actually happens, cannot detect the photoperiod itself — so there is nothing to trigger the change, however favourable the day length may be. The plant may stay vegetative indefinitely.

The supporting evidence

This is not an assumption but a well-established experimental result. A plant kept under an unfavourable photoperiod will flower if even a single leaf is given the correct photoperiod — showing that one leaf is enough to supply the signal, and that the signal travels. Conversely, if the shoot is given the correct photoperiod while all leaves are kept under the wrong one, or are removed, flowering does not occur.

A necessary qualification

The answer depends on complete defoliation. If even one mature leaf remains and receives the inductive photoperiod, the plant can still flower, because that single leaf will produce and export the floral hormone. Very young, immature leaves are much less effective. So the accurate statement is: a fully defoliated plant cannot respond, because it has lost the organ that perceives the photoperiod and produces the transmissible floral signal.

A wider point. This is a clear instance of the chapter's general principle that many of the extrinsic factors such as temperature and light control plant growth and development via PGRs — light does not act on the apex directly; it acts through a chemical messenger made in the leaf.