Summary and Exam Tips

The Chapter in One Page

Growth is an irreversible permanent increase in the size of an organ, its parts, or even a single cell. It is generally accompanied by metabolic processes, both anabolic and catabolic, that occur at the expense of energy. That one sentence settles the chapter's opening question - the expansion of a leaf is growth; the swelling of a piece of wood placed in water is not, because it is reversible and has no metabolism behind it.

Plant growth is unique because plants retain the capacity for unlimited growth throughout their life, and they do so because of meristems at certain locations in the body, whose cells divide and self-perpetuate while the product soon loses the capacity to divide and makes up the plant body. That is the open form of growth, and it is why plant growth is generally indeterminate. The root apical and shoot apical meristems principally contribute to elongation along the axis; the lateral meristems, the vascular cambium and the cork cambium, appear later in life in dicotyledonous plants and gymnosperms and cause the increase in girth called secondary growth.

Growth is measurable, but not directly. At the cellular level growth is principally a consequence of an increase in the amount of protoplasm, and since protoplasm is difficult to measure directly, one measures a quantity more or less proportional to it - fresh weight, dry weight, length, area, volume or cell number. Which proxy you use depends on the system: a maize root apical meristem, giving more than 17,500 new cells per hour, is measured as cell number; watermelon cells, which may increase in size by up to 3,50,000 times, as cell size; a pollen tube as length; a dorsiventral leaf as surface area. Along a root the growth falls into three phases - meristematic, elongation and maturation: the constantly dividing cells at the apex, rich in protoplasm with large conspicuous nuclei and thin cellulosic primary walls with abundant plasmodesmatal connections; then the cells proximal to them, showing increased vacuolation, cell enlargement and new cell wall deposition; then, further away, the cells attaining their maximal size in wall thickening and protoplasmic modification.

Increased growth per unit time is the growth rate, and it may be arithmetic or geometric. In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures - a root elongating at a constant rate is the simplest example, and plotting length against time gives a linear curve. In geometric growth both progeny cells retain the ability to divide and continue to do so; the initial growth is slow, the lag phase, then increases rapidly at an exponential rate, the log phase, and then, with limited nutrient supply, slows to a stationary phase, giving a typical sigmoid or S-curve, characteristic of living organisms growing in a natural environment. Comparisons between systems are made in two ways: the measurement and comparison of total growth per unit time is the absolute growth rate, while the growth of a given system per unit time expressed on a common basis, for example per unit initial parameter, is the relative growth rate. Two leaves may add the same area in the same time and so have identical absolute growth rates, yet the smaller leaf has much the higher relative growth rate. Growth itself needs water, oxygen and nutrients: cells grow in size by cell enlargement, which requires water, turgidity helps in extension growth, water provides the medium for the enzymatic activities needed for growth, oxygen releases metabolic energy, and macro and micro essential elements are needed for the synthesis of protoplasm and as a source of energy. Every plant has an optimum temperature range, and light and gravity affect certain phases of growth.

Differentiation is the act leading to maturation of the cells derived from the apical meristems and the cambium, and during it cells undergo structural changes in both the cell wall and the protoplasm - to form a tracheary element the cells lose their protoplasm and develop a very strong, elastic, lignocellulosic secondary wall able to carry water to long distances even under extreme tension. Dedifferentiation is a living differentiated cell that had lost the power to divide regaining it - interfascicular cambium and cork cambium formed from fully differentiated parenchyma. Redifferentiation is the cells so produced losing that power once more and maturing to perform specific functions. Differentiation in plants is open, because cells arising out of the same meristem have different structures at maturity, and the final structure is also determined 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.

Development includes all the changes an organism goes through during its life cycle, from germination of the seed to senescence, and broadly, development is the sum of growth and differentiation. Plasticity is the ability of plants to follow different pathways in response to environment or phases of life, to form different kinds of structures - heterophylly in cotton, coriander and larkspur, where the juvenile leaves differ from the mature ones, and in buttercup, where leaves formed in air differ from those formed in water. Development is under the control of intrinsic and extrinsic factors - the intrinsic being intracellular genetic factors and the intercellular chemicals called plant growth regulators, and the extrinsic being light, temperature, water, oxygen and nutrition.

The plant growth regulators are small, simple molecules of diverse chemical composition, in five major groups. Auxins, gibberellins and cytokinins are the plant growth promoters, involved in cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. Abscisic acid belongs to the other group, acting in responses to wounds and stresses and in growth inhibiting activities such as dormancy and abscission. Ethylene could fit either group, but it is largely an inhibitor of growth activities. Every one of the five was found by accident, and each has many effects while each effect has many regulators - which is why this chapter is examined as a grid rather than as five separate lists.

Finally, light and temperature control the initiation of flowering. Photoperiodism is the response of plants to periods of day and night length, sorted by the critical day length, which is species-specific: short day plants flower when the day is shorter than that value, long day plants when it is longer, and day-neutral plants flower irrespective of day length. What actually decides the response is the length of the uninterrupted dark period, which is why a brief flash of light in the middle of the night stops a short day plant flowering and makes a long day plant flower. The leaves perceive the photoperiod, through the pigment phytochrome; flowering happens at the shoot apex; and a hormonal substance, florigen, migrates from leaf to apex - so a fully defoliated plant cannot respond to a photoperiodic cycle at all. Vernalisation is the promotion of flowering by a period of low temperature. It prevents precocious reproductive development late in the growing season and lets the plant reach maturity, and it explains the winter varieties of wheat, barley and rye, and the biennials sugarbeet, cabbages and carrots, in which a cold treatment stimulates a subsequent photoperiodic flowering response.

The Tables Worth Memorising

There are exactly two tables in this chapter, and between them they answer most of what gets asked. Learn them as tables, reading across a row, not as loose facts.

Table 1 - the five regulators: name, chemical nature, discovery, defining effect.

Table 2 - the promote-against-inhibit grid. This is the table the paper is built from. Read it down the left column - the examiner names the event and asks who does it.

Two lines of that grid answer more questions than any other lines in the chapter. Auxin causes apical dominance and cytokinin overcomes it, and abscisic acid is the antagonist of gibberellin in most situations. If time is short, learn those two first.

The Two Equations

The chapter contains exactly two equations, one for each kind of growth. Neither needs to be solved; both have to be read, and a written answer is expected to define every letter.

1. Arithmetic growth.

$$L_t = L_0 + rt$$

• $L_t$ - length at time $t$
• $L_0$ - length at time zero
• $r$ - growth rate, that is elongation per unit time
• $t$ - time

How to read it in one line: the organ gains the same amount of length in every equal interval of time, so length at any moment is the starting length plus a fixed rate multiplied by the time elapsed. Plot $L_t$ against $t$ and the curve is a straight line. The situation behind it is a root elongating at a constant rate, and its cell biology is that after mitosis only one daughter cell continues to divide while the other differentiates and matures.

2. Geometric, or exponential, growth.

$$W_1 = W_0 e^{rt}$$

• $W_1$ - final size - weight, height, number and so on
• $W_0$ - initial size at the beginning of the period
• $r$ - growth rate
• $t$ - time of growth
• $e$ - the base of natural logarithms

How to read it in one line: the bigger the system already is, the more it adds in the next interval, so the final size depends on the initial size - growth compounds instead of accumulating. The cell biology behind it is that both progeny cells retain the ability to divide and continue to do so, and the curve, once limited nutrient supply slows it, is the sigmoid or S-curve with its lag, log and stationary phases.

One more thing to say about $r$, because it is a marks-carrying sentence. In the exponential expression, $r$ is the relative growth rate, and is also the measure of the ability of the plant to produce new plant material, referred to as the efficiency index. Relative growth rate itself is defined as growth per unit time expressed on a common basis:

$$\text{RGR} = \frac{\text{growth per unit time}}{\text{initial parameter}}$$

And keep it apart from absolute growth rate, which is the measurement and comparison of total growth per unit time with no division by anything. Two leaves that both add 5 square centimetres in the same time have identical absolute growth rates, and the smaller one has by far the higher relative growth rate.

The Mistakes That Cost Marks in This Chapter

1. Counting swelling wood as growth. Growth is an irreversible permanent increase in size, generally accompanied by metabolic processes, both anabolic and catabolic, occurring at the expense of energy. A block of wood placed in water swells, but the change is reversible and no metabolism drives it, so it is not growth. Whenever a stem asks whether something is growth, test it against irreversible and metabolic in that order.
2. Confusing the three phases of a root with the three phases of a sigmoid curve. They are different kinds of thing. Meristematic, elongation and maturation are zones you can point to along the length of a growing root - the constantly dividing cells at the apex, then the cells proximal to them with increased vacuolation and cell enlargement, then the cells attaining maximal wall thickening. Lag, log and stationary are stretches of time on a growth curve, and they belong to geometric growth. A question that offers one ladder as the other is a straight trap, and the giveaway is whether the stem is talking about a place in the root or a period in time.
3. Thinking kinetin is natural. Kinetin does not occur naturally in plants. It was discovered as a modified form of adenine, a purine, from autoclaved herring sperm DNA, and it was the search for natural substances with cytokinin-like activity that led to the isolation of zeatin from corn-kernels and coconut milk. Zeatin is the natural one; kinetin is not.
4. Swapping cytokinin and auxin on apical dominance. Auxin causes apical dominance - in most higher plants the growing apical bud inhibits the growth of the lateral buds - and cytokinins help overcome apical dominance. Removal of shoot tips, that is decapitation, usually results in the growth of the lateral buds, which is why it is applied in tea plantations and hedge-making. Learn the pair as one sentence, in that order, and the reversal cannot happen.
5. Calling abscisic acid a synergist of gibberellin. In most situations ABA acts as an antagonist to gibberellins. The two pull against each other over and over - ABA inhibits seed germination and induces dormancy, gibberellin breaks it; ABA is a general growth inhibitor, gibberellin lengthens the axis. The word the examiner wants is antagonist, and the distractor is always synergist or complimentary.
6. Thinking the light period matters more than the dark period in photoperiodism. It does not. What decides the response is the length of the uninterrupted dark period. Interrupt the night of a short day plant with even a brief flash of light and it will not flower, while the same flash makes a long day plant flower. The names are the reason students get this wrong - a short day plant is really a long night plant. In a written answer, give the reason as the continuous dark period, never as the day length.
7. Giving secondary growth to a monocot. The lateral meristems - vascular cambium and cork cambium - appear later in life in dicotyledonous plants and gymnosperms, and it is these that cause the increase in girth. Monocots are not on that list. The same care applies to the herbicide fact that sits beside it: 2,4-D kills dicotyledonous weeds and does not affect mature monocotyledonous plants, which is why it makes a weed-free lawn of grass.

Writing the Chapter-End Exercises Well

Class 11 has no board paper, but the chapter-end exercises and your school tests are still written answers, marked by a person reading for particular words. Everything below is about putting those words on the page in the order a marker looks for them.

Name the regulator before you explain it. Every one of the six parts of the exercise that asks which regulator you would use for a given job is marked on the name first - auxin for rooting a twig, ethylene to quickly ripen a fruit, cytokinin to delay leaf senescence, cytokinin to induce growth in axillary buds, gibberellin to bolt a rosette plant, abscisic acid to induce immediate stomatal closure. Write the name on its own line, then one sentence of effect. A paragraph that describes the effect beautifully and never lands on the name scores nothing, and a bare name with one accurate sentence after it scores everything. The same order works for the longer four-part exercise on what would happen if - name the regulator or the process, then the outcome: $\mathrm{GA_3}$ applied to rice seedlings makes them elongate excessively, the bakanae or foolish seedling symptom; if dividing cells stopped differentiating, the plant would be a mass of undifferentiated dividing cells with no tissues, no organs and no function; a rotten fruit mixed with unripe ones ripens them, because it gives off large amounts of ethylene, and ethylene is a gas; and if you forget the cytokinin in a culture medium the callus will not form shoots, since the cytokinin-to-auxin ratio decides whether shoots or roots appear.

Give the organism and the person for a discovery question. The exercise that asks for a note on the discovery, physiological functions and applications of one regulator is marked in three parts, and the discovery part is the one students write vaguely. Anchor it with a person, a year where the chapter gives one, and an organism or a source material: the Darwins and the coleoptiles of canary grass, then F. W. Went and the tips of coleoptiles of oat seedlings; Gibberella fujikuroi and the bakanae disease of rice, then E. Kurosawa in 1926 with sterile filtrates of the fungus; kinetin from autoclaved herring sperm DNA, crystallised by Miller and co-workers in 1955, and zeatin from corn-kernels and coconut milk; H. H. Cousins in 1910, ripened oranges and stored unripened bananas; inhibitor-B, abscission II and dormin, three separate discoveries in the mid-1960s later proved chemically identical. Two names, one organism and a date carry the discovery marks.

For a definition question, quote the defining sentence rather than paraphrasing it. The first exercise asks for eight definitions in a row - growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate - and each is worth one mark, awarded for one sentence. Paraphrase loses those marks because it drops the qualifying word the marker is looking for. Write growth as an irreversible permanent increase in size of an organ, its parts or even a single cell; differentiation as the act leading to maturation of the cells derived from the apical meristems and the cambium; development as all the changes an organism goes through during its life cycle from germination of the seed to senescence, and broadly the sum of growth and differentiation; dedifferentiation as living differentiated cells that had lost the capacity to divide regaining it; redifferentiation as the cells so produced losing the capacity to divide again and maturing to perform specific functions; determinate growth as growth that stops after reaching a certain size, as in a leaf, a flower or a fruit; a meristem as a group of cells that have the capacity to divide and self-perpetuate; and growth rate as the increased growth per unit time. The words that carry the marks are irreversible, permanent, capacity to divide, self-perpetuate and per unit time - keep them in.

Three more habits that pay across the whole exercise set. For why no single parameter demonstrates growth throughout the life of a flowering plant, give the reason and then two examples - different organs grow in different ways, so the parameter must fit the organ, then cell number for a maize root apical meristem, cell size for a watermelon, length for a pollen tube, surface area for a dorsiventral leaf. For the pair of exercises on flowering, answer both from the dark period and from the site of perception - a short day plant and a long day plant can flower on the same day in the same place because critical day length is species-specific, and a defoliated plant will not respond to a photoperiodic cycle because the leaves are the site of perception and there is nothing left to perceive the photoperiod or to make the florigen. And for both growth and differentiation in higher plants are open, comment, give the two halves separately - growth is open because new cells are always being added by the meristem and differentiation is open because cells arising out of the same meristem have different structures at maturity, the final structure also being determined by the location of the cell within the organ.

The Night Before - What to Revise, in Order

Read in this order and stop when the list runs out. Nothing new goes in tonight.

1. Table 1, the five regulators, read across the rows. Twenty minutes, and the most valuable twenty in the chapter, because half of what gets asked is one of those four columns. Say each row aloud as one sentence - auxin, an indole compound, found by the Darwins on canary grass and isolated by Went from oat coleoptiles, causes apical dominance and roots a cutting.

2. Table 2, the promote-against-inhibit grid, read down the events. Ten minutes. Apical dominance, senescence, dormancy, abscission, stomatal closure, seed germination - for each one, name who does it and who undoes it, without looking. If only two lines will fit in your head, make them auxin causes apical dominance and cytokinin overcomes it, and abscisic acid is the antagonist of gibberellin in most situations.

3. The seven mistakes above. Ten minutes. These are the marks you are most likely to lose while knowing the material, which makes them the cheapest ones to save.

4. The two equations, letter by letter. Five minutes. Write $L_t = L_0 + rt$ and $W_1 = W_0 e^{rt}$ out from memory and label every letter, including $e$ as the base of natural logarithms and $r$ as the relative growth rate, the efficiency index.

5. The first half of the chapter, as six short answers. Fifteen minutes. Say aloud: what growth is and why swelling wood is not; why plant growth is indeterminate; the six parameters and the four systems they belong to; the three phases of a root against the three phases of a sigmoid curve; arithmetic against geometric, linear against sigmoid; absolute against relative growth rate, and which of two leaves has the higher relative rate.

6. Photoperiodism and vernalisation, as four one-word answers. Five minutes. Site of perception - the leaf. Site of flowering - the shoot apex. The transmissible flowering hormone - florigen. The photoperiodic pigment - phytochrome. Then the one sentence that governs the whole topic: it is the length of the uninterrupted dark period that decides the response, and vernalisation is the promotion of flowering by a period of low temperature.

If you have ten minutes and no more, read Table 2 twice. This chapter rewards the grid over everything else, and a student who can say who promotes and who inhibits each of the six events will out-score one who has read the whole chapter once more.