NCERT Solutions for Class 9th Science Chapter 11 End-of-chapter questions — Revise, Reflect, Refine

Book page 225 – 226 Updated on2026-09-08

Q1.

A flower’s anthers are removed before it matures. Later, pollen from another plant of the same species is dusted onto its stigma and seeds are produced. Which process has been ensured here? (i) Self-pollination (ii) Cross-pollination (iii) Fertilisation (iv) Tissue culture

Answer

(ii) Cross-pollination.

Why: the two steps of the treatment do exactly the two things needed to guarantee a cross.

Removing the anthers before the flower matures takes away the flower's own pollen before it can be shed — so self-pollination is now impossible.
Dusting pollen from another plant of the same species onto the stigma is, word for word, the chapter's definition of cross-pollination: pollen transferred from the anther of a flower of one plant to the stigma of a flower of another plant of the same type.

Why (iii) is the tempting wrong answer: fertilisation certainly did happen — seeds were produced, and seeds cannot form without it. But fertilisation is not something the experimenter ensured; it followed on its own once the pollen was there. What the treatment ensured, by design, is that the pollen came from a different plant. (i) is ruled out because the anthers were removed, and (iv) is unrelated — tissue culture is a method of asexual propagation and involves no pollen at all.

Tip: this procedure has a name — artificial hybridisation. The chapter describes it in the Bridging Science and Society box on page 217: remove the stamens, bag the flower to keep out stray pollen, then hand-transfer pollen carrying the desired characters. It is how plant breeders create new high-yielding and disease-resistant varieties.

Q2.

Arrange the following stages of sexual reproduction in plants in the correct order: (i) Pollen germination on stigma (ii) Fertilisation (iii) Pollination (iv) Formation of zygote

Answer

(iii) Pollination → (i) Pollen germination on stigma → (ii) Fertilisation → (iv) Formation of zygote.

The order is fixed by physical necessity — each stage supplies the condition the next one needs.

Why no other order is possible:

The pollen must first be on the stigma before it can germinate there — so (iii) precedes (i).
The male gamete cannot reach the ovule until the tube has grown down the style — so (i) precedes (ii).
The zygote is the product of that fusion. It cannot exist before the fusion — so (ii) precedes (iv).

Tip: (ii) and (iv) are almost the same instant, which is what makes this pair the trap. Fertilisation is the event — two gametes fusing. The zygote is the result of that event. Cause must be written before effect.

Q3.

Assertion (A): The zygote formed after fertilisation immediately attaches to the uterus wall. Reason (R): The uterus wall is always prepared to receive the zygote. (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

Answer

(iv) A is false, but R is true.

Why A is false. The zygote does not attach immediately. Fertilisation happens in the oviduct, not in the uterus. The chapter is explicit: the zygote undergoes a series of mitotic divisions while travelling to the uterus, and only then implants into the inner lining. Several days pass between fertilisation and implantation, and the structure that implants is no longer a single-celled zygote but a ball of cells.

fertilisation in the oviduct → repeated mitotic divisions during the journey
→ the dividing mass reaches the uterus
→ implantation in the thickened inner lining = beginning of pregnancy

Why R is taken as true. The uterus does not wait for a zygote to arrive before getting ready — it prepares in advance, in every single cycle. As the chapter says, before ovulation the uterus starts to prepare itself and the inner lining becomes thick, and after ovulation it becomes thicker still and richer in blood vessels. So a lining is made ready each month whether or not a zygote is ever formed.

Why R still does not explain A: even a fully prepared lining cannot make the attachment happen sooner, because the zygote is not in the uterus yet. The delay is set by the journey down the oviduct, not by the readiness of the lining. So the reason, while true in itself, is not the reason for the assertion — and in this case the assertion is false anyway.

A note on the wording: the word "always" in R deserves care. The lining is prepared in every cycle, which is the sense in which R is true. It is not literally present at all times — during menstruation (days 1 – 5) that very lining is being shed, and it is rebuilt over days 6 – 14. Read R as "in every cycle the wall is prepared in advance", and option (iv) is the correct choice.

Q4.

Why does asexual reproduction produce offsprings that are genetically identical to the parent?

Answer

Because there is only one parent and only one kind of cell division involved — mitosis, which copies the chromosome set exactly instead of reshuffling or halving it.

parent cell (chromosome number 2n)
↓ DNA replicated: every chromosome copied once
↓ mitosis: the two copies of each chromosome separated into different daughter cells
two daughter cells, each with 2n chromosomes carrying the same information

Three things that would create differences are all absent:

No meiosis, so chromosome pairs never separate independently and gametes are never formed — the 2ⁿ reshuffling of Activity 11.4 simply does not occur.
No second parent, so no chromosome from anywhere else is ever brought in.
No fertilisation, so no two different sets are ever combined.

The result is a group of individuals with identical genetic information — what the chapter calls clones. It applies equally to a budding yeast cell, a hydra bud, a spore from Rhizopus, a potato tuber and a grafted rose.

Why "identical" is not quite absolute: DNA copying is extremely accurate but not perfect. A rare copying error — a mutation — can slip in and be passed to that daughter cell and all of its descendants. This is the only source of variation available to a purely asexual line, and it is far too slow and rare to compare with the reshuffling of sexual reproduction. That difference is exactly what makes clonal crops vulnerable, as Question 7 explores.

Q5.

Explain why the menstrual cycle stops during pregnancy.

Answer

Because menstruation is the shedding of a uterine lining that is no longer needed — and during pregnancy that lining is very much needed, so it is kept instead of being shed.

Compare the two situations side by side:

Stage	No fertilisation	Pregnancy
The egg	Stays viable about a day, then degenerates	Fused with a sperm — a zygote forms
The uterine lining	Thick and rich in blood vessels, but with nothing to nourish	Thick and rich in blood vessels, and the embryo is implanted in it
What happens next	The lining sheds with some blood — menstruation	The lining is maintained and thickened further to nourish the embryo
Ovulation	Occurs again in the next cycle	Stops for the whole pregnancy — no new egg is released

Why the body knows the difference: once the embryo implants, it and the tissues that grow around it release hormones into the mother's blood. Hormones, as the chapter defines them, are chemicals that regulate different functions elsewhere in the body. These particular hormones do two things at once — they keep the uterine lining intact instead of letting it break down, and they stop the ovary from releasing another egg. So both halves of the cycle, ovulation and shedding, are switched off together and stay off for the whole nine months.

Tip: this is why a missed period is usually the first sign of pregnancy. It is not a coincidence — it is the direct consequence of the lining being retained. The cycle resumes some time after childbirth, often later in mothers who are breastfeeding.

Q6.

Why are flowers that bloom at night white or light in colour as compared to flowers that bloom during the day?

Answer

Because their pollinators fly at night, and in the dark a white surface is the only one that can still be seen. A brightly coloured petal would be invisible.

Why white works and colour does not, in the dark: a coloured petal looks coloured because its pigment absorbs most wavelengths and reflects only a narrow band. In daylight that is fine — the reflected band is still bright. At night there is very little light to start with, so after most of it has been absorbed almost nothing comes back to the moth's eye. A white petal has no such pigment and reflects nearly all of the little light there is, so it appears as a pale patch against dark leaves. In moonlight a white flower is often the brightest object in the plant.

Why the eye cannot help either: in dim light both our eyes and an insect's respond mainly to brightness rather than to colour. So even a moth that can distinguish colours by day is judging contrast, not hue, at night. Brightness is exactly what a white petal maximises.

What night-bloomers use instead of colour. They spend on scent. Raat ki rani, harsingar (parijat), jasmine, moonflower and the night-blooming cactus all release strong fragrance after sunset. Scent molecules travel on air and can be followed in complete darkness, so smell replaces sight as the long-range signal, while the pale petal serves for the final approach. Many such flowers also produce plenty of nectar, since night-flying moths and bats are large and need a good reward.

Did you know? The signal is timed as well as coloured. Raat ki rani releases its fragrance only after dark and stops by morning — there is no point advertising to visitors that are not flying.

Q7.

Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?

Answer

Because they are clones. Every plant in the field carries the same genetic information, so a pathogen that can defeat one plant can defeat all of them.

Two separate reasons, and both matter:

No genetic variation to fall back on. Vegetative propagation uses mitosis from a single parent, so there is no reshuffling and no second parent. If the parent lacks a gene for resistance to a particular fungus, so does every one of its clones. A disease that gets into the field spreads through it without meeting a single plant that can stop it.
The disease travels inside the planting material itself. A cutting, tuber, sucker or graft is a piece of living stem, and it carries whatever viruses, bacteria and fungi were inside the parent straight into the new plant. A seed usually does not — the embryo forms afresh inside the ovule and is comparatively clean. So clonal propagation multiplies not only the plant but also its infections.

Why sexual reproduction protects a population: the offspring of a sexually reproduced crop differ from one another, because meiosis and fertilisation give every seed a new combination of characters. When a disease arrives, some plants happen to carry a combination that resists it. Those survive and set seed, so the crop is damaged but not destroyed, and the resistant character becomes commoner in the next generation. In a clonal field that mechanism has nothing to work with.

Did you know? This is not a theoretical worry. The Gros Michel banana, once the world's main commercial variety, was propagated clonally and was wiped out by Panama disease. Its replacement, the Cavendish, is also a clone and is now under attack from a new strain of the same fungus. It is exactly to break this cycle that the chapter's example of virus-free tissue-culture banana plantlets raised from the shoot tip matters — the apical meristem is usually free of virus, so the clones at least start clean.

Q8.

If all flowers in a type of plant were only capable of self-pollination, how would it affect the genetic diversity over several generations? Explain.

Answer

Genetic diversity would fall sharply, and after a few generations the population would be almost uniform.

Why: in self-pollination both gametes come from the same plant, so a gene pair can only be matched with copies of itself. Any pair that was mixed (one form from each parent) has a one-in-two chance of staying mixed in the next generation, and a one-in-two chance of becoming a matched pair — and once matched, it can never become mixed again by selfing.

fraction of mixed gene pairs left after n generations of selfing = (½)ⁿ
after 1 generation = 50%
after 3 generations = (½)³ = 12.5%
after 5 generations = (½)⁵ = 1/32 = 3.1%
after 10 generations = (½)¹⁰ ≈ 0.1%

So within about five to ten generations the plants become almost entirely uniform — what breeders call a pure line.

What follows from that loss:

Hidden harmful characters get exposed. A harmful form of a gene that was masked while the pair was mixed shows up once the pair is matched. Plants become weaker, smaller and less fertile — this is inbreeding depression.
No new combinations. The reshuffling of Activity 11.4 still happens inside the plant, but since both gametes come from the same plant there is no new material to shuffle with. The population stops generating novelty.
No ability to adapt. When a new pest, a new disease or a change in rainfall arrives, there is no variant plant that happens to cope. A single epidemic can take the whole population, exactly as in the clonal crops of Question 7.

Tip: uniformity is not always a disadvantage, which is why some crops do it deliberately. Wheat, rice and pea are largely self-pollinated: every plant in the field ripens together and yields alike, and no pollinator is needed at all, so seed set is assured. Breeders even use forced selfing on purpose — first to fix a pure line, and then to cross two such lines to produce a vigorous hybrid.

Q9.

A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.

Answer

He should use vegetative propagation — above all tissue culture from the shoot tip, and, depending on the crop, cutting, grafting or layering. All of these are asexual, so all of them give clones.

Method	Best for	Why it suits this farmer
Tissue culture from the shoot tip (apical meristem)	Banana, sugarcane, orchids, potato	Thousands of identical plantlets from one small piece of tissue, in a sterile nutrient medium, in any season and in a small space; the apical meristem is usually free of virus, so the plantlets are healthy
Cutting	Sugarcane, money plant, rose, hibiscus	Simplest and cheapest; a cutting with 3 – 5 nodes roots in two to three weeks
Grafting	Mango, citrus, rose, apple	Combines a chosen variety's shoot with a hardy, disease-resistant root system
Layering	Lemon, guava, jasmine, pomegranate	Very reliable, because the twig stays attached to the parent and cannot wilt while rooting

Why these methods are effective for what he wants:

Genetically identical — only mitosis is involved, from a single parent, so every plant carries exactly the parent's characters: the same yield, the same fruit size, the same sweetness, the same ripening date. A whole orchard behaves as one plant, which makes spraying, harvesting and marketing far simpler.
Quick — the plant skips flowering, pollination, seed set, seed dormancy and germination. A cutting already has nodes and stored food and starts growing at once, so the crop is ready months or years sooner than one raised from seed.
Large numbers — tissue culture in particular multiplies one shoot tip into thousands of plantlets. The chapter's own example is banana farming, where mass-produced healthy plantlets have revolutionised the practice and ensured high yields.
The only option for some crops — banana, seedless grape and sugarcane produce no usable seed, so they cannot be raised any other way.

Tip to give the farmer: uniformity is also a risk. A field of clones has no plant that can resist a new disease. Advise him to plant more than one variety, to buy certified virus-free planting material, and to rotate his crop.

Q10.

Suresh prepares slides with pollen grains in different sugar concentrations (0%, 2.5%, 5%, 7.5%, 10%) to study the germination of pollen. (i) What are the different hypotheses which can be tested using this set-up? (ii) What parameters should be kept the same in this set-up?

Answer

(i) Hypotheses that this set-up can test. Each must be a statement that the experiment could prove wrong.

Pollen grains need an external sugar supply to germinate. Prediction: little or no germination at 0% (plain water), germination at the higher concentrations.
The percentage of grains that germinate depends on the sugar concentration. Prediction: the count of germinated grains changes from slide to slide.
The length of the pollen tube depends on the sugar concentration. Prediction: tubes are measurably longer on some slides than others.
There is an optimum concentration, with germination falling on both sides of it. Prediction: the graph of germination against concentration rises, peaks and then falls.

Why an optimum is expected rather than "more sugar, more growth": sugar plays two roles at once. It is the respiratory substrate that supplies energy for building the pollen tube, so too little of it starves the grain. It is also an osmotic agent. In plain water the solution outside is more dilute than the contents of the grain, so water rushes in and the grain may burst. In a very concentrated solution the reverse happens — water is drawn out of the grain, it shrinks, and germination stops. Somewhere between the two lies the concentration that both feeds the grain and balances its water, and that is where germination peaks.

(ii) Parameters that must be kept the same — everything except the one factor being tested.

Type of variable	What it is here
Independent variable (the only thing changed)	Sugar concentration: 0%, 2.5%, 5%, 7.5%, 10%
Dependent variables (what is measured)	Percentage of grains germinated; length of the pollen tube
Controlled variables (kept identical)	Same plant species and preferably the same flower; pollen of the same age and freshness; roughly the same number of grains per slide; same volume and size of drop; same slide and coverslip; same temperature; same humidity; same light; same waiting time before counting; same microscope magnification; same definition of "germinated" (say, a tube at least as long as the grain is wide); same person counting

Tip: the 0% slide is not a wasted slide — it is the control. Without it Suresh could not claim that sugar is what caused germination, because he would have nothing to compare the other four against. Also, he should count several fields of view on each slide and take the average; a single field can mislead.

Q11.

Look at the picture given below and think in line with the given prompts and find out which type(s) of pollination might have been followed in these flowers — Tomato: Stamens cover the stigma. Wheat: Flowers open after pollination. Papaya: Male and female flowers are often borne on different papaya trees.

Answer

Tomato and wheat are self-pollinated; papaya must be cross-pollinated. Each prompt is a structural clue that settles the question by itself.

Flower	Clue given	Type of pollination	Why the clue decides it
Tomato	Stamens cover the stigma	Self-pollination	The anthers form a close cone around the style, so when they split, the pollen falls straight down onto the flower's own stigma. Pollen from outside cannot easily get past the cone
Wheat	Flowers open after pollination	Self-pollination (with a little wind cross-pollination possible)	If pollination is already over by the time the flower opens, it must have happened inside the closed flower — and the only pollen present in there is its own
Papaya	Male and female flowers are often on different trees	Cross-pollination	A female flower has no stamens and so has no pollen of its own. The pollen must be carried from a male tree to a female tree, by insects (chiefly moths) and by wind

Why the same logic reappears in the pea of Activity 11.6: the pea also pollinates itself inside the closed bud, which is why removing the stamens from an already-open pea flower still gave a pod. Wheat behaves the same way. Whenever a book says a flower "opens after pollination", read it as a guarantee of self-pollination.

Tip: once wheat's florets do open, some pollen escapes into the air and can land on a neighbouring plant, so a small percentage of cross-pollination occurs — wheat is a grass, and the chapter lists grasses among the wind-pollinated plants. And a tomato grower knows that a bumblebee vibrating the flower shakes far more pollen out of the anther cone, which is why greenhouses keep bees even for a self-pollinating crop.

Q12.

In the lower Himalayan region of northern India, apples are an important cash crop that contribute significantly to farmer’s livelihoods. The fruit yield in apple cultivation is declining continuously, associated with climate change and a significant decline in the population of natural pollinators. A researcher-farmer group set up two experimental apple orchards at two distinct locations: Places A and B. In apple orchards at Place A, they allowed natural pollinators to pollinate the flowers of the apple. In apple orchards at Place B, they applied mixed farming techniques of beekeeping. Along with honey, the farmer yielded apples. The yield of apples is depicted in Fig. 11.24, in terms of fruit setting (number of fruits/the total number of corresponding fruit-bearing branches) and fruit drop (premature falling of developing fruits) in the two types of experimental places of apple orchards. (i) What are the hypotheses the researcher-farmers group has thought of for this investigation? (ii) What are the different parameters in the experiment? (iii) Compare and analyse the data of two experimental orchards Places A and B, in terms of high yields of apple fruits. (iv) Based on your analysis, what do you infer from the data?

Answer

(i) The hypotheses. Each is a testable statement about cause and effect.

The fall in apple yield is caused mainly by a shortage of pollinators, not only by climate change.
Adding a managed honeybee colony to an orchard will raise fruit setting, because more flowers get pollinated.
Better-pollinated flowers give better-fertilised, well-seeded fruits, which the tree holds on to — so fruit drop will fall.
Mixed farming with beekeeping raises the total income, since the farmer harvests honey as well as a larger apple crop.

(ii) The parameters.

Type	In this experiment
Independent variable	The pollination treatment — natural pollinators only (Place A) vs a bee colony added (Place B)
Dependent variables	Per cent fruit setting, and per cent fruit drop
Controlled variables (must be kept the same)	Apple variety; age, size and number of trees; the number of fruit-bearing branches counted; soil and altitude; irrigation, manure and pruning; pesticide use and its timing; the season; the date and method of counting

(iii) Comparing the data. Reading Fig. 11.24 at the printed page:

Fig. 11.24 redrawn with its axis labels. Note: the English edition prints this graph without the two x-axis category labels and without the 0 at the origin; the Hindi edition prints them as “फल लगना” (fruit setting) and “फल गिरना” (fruit drop). The bar values are identical in both editions.

Measurement	Place A — natural pollination	Place B — with bee colony	Change
Fruit setting	26%	40%	+14 percentage points, a 54% relative rise
Fruit drop	35%	8%	−27 percentage points, a 77% relative fall
Fruit actually retained	16.9%	36.8%	about 2.2 times more

rise in fruit setting = 40% − 26% = 14 percentage points
relative rise = (14 ÷ 26) × 100 = 53.8% ≈ 54%

fall in fruit drop = 35% − 8% = 27 percentage points
relative fall = (27 ÷ 35) × 100 = 77.1% ≈ 77%

fruit retained = fruit setting × (100 − fruit drop) ÷ 100
Place A = 26 × (100 − 35) ÷ 100 = 26 × 0.65 = 16.9%
Place B = 40 × (100 − 8) ÷ 100 = 40 × 0.92 = 36.8%
ratio = 36.8 ÷ 16.9 = 2.2 times

(iv) Inference. Introducing a honeybee colony improves the crop at both stages of the process — more flowers become fruits, and far more of those fruits are held to maturity — so the final harvest at Place B is a little over twice that at Place A. This supports hypotheses 1 to 3: the decline in apple yield is largely a pollination-limitation problem, and it can be fixed. Mixed farming with beekeeping is therefore a practical, low-cost remedy for the Himalayan apple grower, and it pays twice over because honey is harvested as well.

Why more pollination reduces fruit drop and not just fruit setting: an apple fruit develops normally only if enough of its ovules have been fertilised. A flower that received only a few pollen grains sets a fruit with few seeds, and the tree tends to shed such poorly-seeded fruits while they are still small. Thorough pollination by bees fertilises more ovules per flower, so the fruits that form are well-seeded and are retained. The two bars in Fig. 11.24 are therefore two views of the same cause.

Tip — a weakness in the design worth naming: the two orchards are at two different places, so soil, altitude, weather and the surrounding vegetation are not truly controlled. Some of the difference could come from the sites rather than from the bees. A stronger design would repeat the comparison at several paired locations, or place bee colonies in half of one orchard and leave the other half untreated.

Q13.

A student claims, “In humans, ovulation always happens on day 14 of the menstrual cycle”. Critically examine this claim and state whether the claim is correct or not. Give at least two reasons for your answer.

Answer

The claim is not correct. Day 14 is a fair average for a 28-day cycle, but the word "always" makes the statement false.

Reason 1 — cycle length itself varies from person to person. The chapter states that the cycle repeats typically every 21 – 35 days. Day 14 is the midpoint only of a 28-day cycle. What is much steadier is the gap between ovulation and the next period, which is about 14 days. Counting backwards from that:

day of ovulation ≈ cycle length − 14 days
21-day cycle: 21 − 14 = day 7
28-day cycle: 28 − 14 = day 14
35-day cycle: 35 − 14 = day 21

Ovulation is fixed relative to the next period, not to the last one — so the day it falls on shifts with the length of the cycle.

Reason 2 — even in the same person the cycle is not fixed. Its length changes from month to month with illness, stress, travel, a change in weight, heavy physical training, thyroid problems, PCOS and breastfeeding. The cycle is typically irregular in the first year or two after it begins (the chapter puts that at ages 10 – 14) and again in the years before menopause around 50. Some cycles pass without any egg being released at all.

How to state the claim correctly: "In a typical 28-day cycle, ovulation occurs at about day 14." The chapter's own Fig. 11.21 is careful to label itself as the key stages across a typical 28-day period — a representative case, not a rule.

Tip: this is not a quibble about wording. Calculating a "safe period" from the calendar assumes ovulation is on a fixed day, and it is precisely because that assumption is wrong that such calculation is an unreliable method of avoiding pregnancy.

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