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Chapter 11 Reproduction: How Life Continues class 9th (Exploration) ncert solution

Class 9 · Science (Exploration) · Chapter 11: Reproduction: How Life Continues · All Board · ENGLISH · 14 views

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Grade 9 · Science · Curiosity

Reproduction: How Life Continues

Chapter 11 — full, step-by-step solutions for every In-text prompt (Think It Over, Activities, Pause & Ponder, Threads of Curiosity) and the end-of-chapter “Revise, Reflect, Refine” exercise, with the chapter’s own diagrams.

Asexual & Sexual Vegetative Propagation Pollination & Fertilisation Human Reproduction
1

In-Text — Think It Over & Activities

The reflective “Think It Over” prompts and the questions raised inside the chapter’s activities.

Think It Over
TIO 1
When does a farmer prefer asexual or sexual methods of reproduction for crop production?
Solution

Asexual (vegetative) methods are preferred when the farmer wants to:

  • Keep a prized variety exactly — clones are genetically identical, so fruit quality, taste and colour stay the same.
  • Get faster results — cuttings/grafts flower and fruit earlier than seed-grown plants.
  • Multiply plants with few or no viable seeds — e.g. banana, sugarcane, grapes, rose, potato.

Sexual methods (seeds) are preferred when the farmer wants:

  • Variation to develop new/improved varieties (selective breeding, hybridisation).
  • Disease resistance and adaptability across generations.
  • Easy-to-store, easy-to-transport seeds for large-scale crops like wheat, rice, maize.
AnswerAsexual → fast multiplication of one prized, identical variety; sexual → variation and new, hardier varieties.
TIO 2
Why do most complex animals and flowering plants use sexual reproduction, while simple organisms like yeast and hydra mainly reproduce asexually?
Solution

Complex organisms → sexual reproduction. It mixes genetic material from two parents (meiosis + fertilisation), creating variation. Variation lets a population adapt to changing environments, resist diseases and evolve — a big long-term advantage for long-lived organisms.

Simple organisms → asexual reproduction. Yeast and hydra are short-lived and often live in stable conditions where multiplying fast matters more than variation. Asexual reproduction is quick, needs only one parent, no mate and no gametes, so numbers can explode when conditions are good.

AnswerSexual = variation & adaptability (worth the effort for complex life); asexual = speed & certainty (ideal for simple, fast-breeding life).
Activity Questions
Act 11.2
Do you observe small, round outgrowths (buds) on the parent yeast cells? Do they indicate the yeast is duplicating? How do these observations help you understand reproduction in yeast?
Yeast cells with bud outgrowths under microscope
Fig. 11.6 — Yeast cells with outgrowths (buds)
Solution

Yes — rounded outgrowths (buds) appear on the parent cells, and they do show the yeast is reproducing. In this process, called budding, a bud grows out from the parent, enlarges, and finally separates as a new, independent yeast cell.

AnswerYeast reproduces asexually by budding; via mitosis the new cells are identical to the parent, explaining the rapid multiplication seen.
Act 11.4
Using three pairs of contrasting characters (beads), how many combinations are possible? And how many with 23 pairs of chromosomes?
Beads showing segregation of characters during gamete formation
Fig. 11.9 — Segregation of characters during gamete formation
Solution

For each pair you pick one of two beads, independently — so the total doubles with every pair.

$$\underbrace{2}_{\text{pair 1}}\times\underbrace{2}_{\text{pair 2}}\times\underbrace{2}_{\text{pair 3}} = 2^{3} = 8 \text{ combinations}$$
$$\text{With 23 pairs:}\quad 2^{23} = 8{,}388{,}608 \ \text{(over 8 million)}$$
Answer3 pairs → 8 combinations; 23 pairs → 2²³ ≈ 8.4 million — and crossing over adds even more, so every child is genetically unique.
Act 11.6
In which treatment(s) of the pea-flower experiment are flowers replaced by fruits? What can we infer?
Experimental set-up for pollination using muslin bags on pea flowers
Fig. 11.12 — Experimental set-up for pollination in pea
Solution

Fruits form in every treatment where pollen could still reach the stigma (flowers/buds that kept their stamens, and the open uncovered flower). No fruit forms in the treatment where the stamens were removed and the flower was bagged — there was no pollen and the bag blocked any from outside.

AnswerInference: the transfer of pollen from anther to stigma — pollination — is necessary for fruit (and seed) formation.
Act 11.7
Compare wind- and insect-pollination using the data. (1) Pollen-to-seed ratio & efficiency. (2) Why is producing a very large number of pollen grains still an effective strategy?
Solution
StrategyPollen / flowerSeeds formed
Wind (maize, wheat)5,00,000 – 10,00,00050 – 200
Insect (sunflower)20,000 – 40,000800 – 1,000

(1) Pollen-to-seed ratio (rough mid-values):

$$\text{Wind:}\ \frac{10{,}00{,}000}{100}\approx 10{,}000:1 \qquad \text{Insect:}\ \frac{30{,}000}{900}\approx 33:1$$

Insect pollination is far more efficient per grain; wind pollination wastes most of its pollen.

(2) Why it still works: wind pollination relies on chance — most pollen never lands on a compatible stigma. By releasing millions of light grains, the plant raises the odds that enough reach a stigma, without spending on nectar, scent or bright petals.

AnswerLow efficiency per grain is offset by sheer numbers, so wind pollination remains a successful strategy.
2

In-Text — Pause & Ponder + Threads of Curiosity

Every “Pause and Ponder” question, plus the “Threads of Curiosity” prompt on sex determination.

Pause & Ponder
1
In a china-rose (gudhal) plant, a pollen tube grows through the style after pollen lands on the stigma. Which process is about to happen next?
Pollen germinating on the stigma, pollen tube reaching the egg cell
Fig. 11.14 — Pollen tube growing down to the egg cell
Solution

The pollen tube carries the male gamete down through the style to the ovule. The next step is the male gamete fusing with the egg cell.

AnswerNext process → Fertilisation (fusion of male gamete with egg, forming a zygote).
2
Look at the seeds of calotropis (madar) and dandelion. What kind of seed dispersal are these adapted for?
Madar seeds and dandelion seeds with hairy tufts
Fig. 11.16 — (a) Madar and (b) Dandelion seeds
Solution

Both seeds are very light and carry fine, silky/feathery hairs (a tuft or “parachute”) that catch moving air, letting them float far from the parent plant.

AnswerAdapted for dispersal by wind — the hairy tufts increase surface area so the wind carries them over long distances.
3
A farmer plants two maize varieties side by side, but seeds form only when pollen from one variety reaches the stigma of the other. What type of pollination is this?
Solution

Pollen travels from a flower on one plant to the stigma of a flower on another plant of the same kind.

AnswerCross-pollination.
4
Why do animals with external fertilisation generally produce more eggs than animals with internal fertilisation?
Life cycle of a butterfly: egg, larva, pupa, adult
Fig. 11.17 — Life cycle of a butterfly
Solution

In external fertilisation eggs and sperm are released into water, where fertilisation is left to chance. Many gametes never meet, and exposed eggs/young are swept away or eaten, so survival is very low. Producing thousands of eggs raises the chance some survive. In internal fertilisation, fertilisation is more certain and the embryo is protected inside the body, so few eggs suffice.

AnswerLow survival in water is balanced by producing very large numbers of eggs.
5
In animals, in which fertilisation method are the gametes more protected?
Solution
AnswerInternal fertilisation — gametes meet, and the zygote/embryo develops, sheltered inside the female’s body.
6
Ravi notices he is growing taller rapidly, his shoulders are broadening, and his voice cracks. What stage of life is he entering?
Solution

A growth spurt, broadening shoulders and a deepening (cracking) voice are secondary sexual characteristics in boys, caused by hormones as the reproductive organs mature.

AnswerHe is entering puberty / adolescence (sexual maturation).
7
Rina’s period occurs every 28 days. Her last period was on the 5th of March. On which day is she most likely to get her next period?
Solution
  1. Next period = 28 days after the last onestart from 5 March.
  2. Days left in March after the 5th$31 - 5 = 26$ days.
  3. Remaining days fall into April$28 - 26 = 2$ → 2 April.
$$5\ \text{March} + 28\ \text{days} = 2\ \text{April}$$
AnswerMost likely on 2 April.
8
A human zygote has just formed. How many chromosomes does it have?
Solution
  1. Each gamete is haploidegg = 23, sperm = 23.
  2. Fertilisation adds them$23 + 23 = 46$.
Answer46 chromosomes (23 pairs) — the normal diploid number for humans.
9
What protective devices can be used during sexual activity to reduce the spread of STIs?
Solution
AnswerCondoms (a barrier method) — they reduce STI transmission and also help prevent pregnancy.
10
If a couple uses oral contraceptive pills but not condoms, which risks remain and why?
Solution

Oral pills change hormones to stop ovulation, so they help prevent pregnancy — but they do nothing to block the contact and fluid exchange that spread infections.

AnswerThe risk of Sexually Transmitted Infections (STIs) — gonorrhoea, syphilis, herpes, genital warts, HIV — remains. Only a barrier method (condom) reduces STI transmission.
11
Many animals’ young can walk or find food soon after birth, but human babies are dependent for a long time. What are some advantages and disadvantages of this for humans?
Solution

Advantages:

  • The long, protected childhood gives the large human brain time to develop.
  • Lots of learning — language, social skills, problem-solving, tool use, culture passed from elders.
  • Strong family and social bonds, improving survival and the success of the species.

Disadvantages:

  • Heavy investment of parental time, energy and resources.
  • Parents can usually raise fewer offspring at a time.
  • The helpless infant is vulnerable and needs constant protection.
Threads of Curiosity
ToC
Females have XX and males have XY chromosomes. Who determines the biological sex of a baby?
Solution

The mother always gives an X; the father gives either an X or a Y:

Egg (mother)Sperm (father)Baby
XXXX → Girl
XYXY → Boy
AnswerThe father determines the baby’s biological sex, because the X-or-Y always comes from the sperm.
3

Exercise — Revise, Reflect, Refine

Full solutions to all 13 end-of-chapter questions.

1
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?
  • Self-pollination
  • Cross-pollination
  • Fertilisation
  • Tissue culture
Solution

Removing the anthers (emasculation) prevents the flower’s own pollen being used, and the pollen comes from another plant of the same species.

AnswerCorrect option: (ii) Cross-pollination. (Fertilisation follows later, but the step deliberately ensured is cross-pollination.)
2
Arrange these stages of sexual reproduction in plants in the correct order:
  • Pollen germination on stigma
  • Fertilisation
  • Pollination
  • Formation of zygote
Solution
  1. (iii) Pollinationpollen reaches the stigma.
  2. (i) Pollen germinationpollen tube grows down the style.
  3. (ii) Fertilisationmale gamete fuses with the egg.
  4. (iv) Zygote formationfertilised egg becomes the zygote.
AnswerCorrect order: (iii) → (i) → (ii) → (iv).
3
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.
  • Both A and R true, R explains A.
  • Both true, R does not explain A.
  • A true, R false.
  • A false, R true.
Solution

A is false: the zygote does not attach immediately — it undergoes several mitotic divisions while travelling down the oviduct over days, and only then implants.

R is false: the uterus lining is not always prepared — it thickens only after ovulation and is shed during menstruation if no fertilisation occurs.

AnswerBoth A and R are false. (None of the four printed options matches “both false”; the scientifically correct verdict is that A is false and R is false.)
4
Why does asexual reproduction produce offspring that are genetically identical to the parent?
Solution

Asexual reproduction involves a single parent and uses mitosis, which produces daughter cells with the same number and identical copies of chromosomes as the parent. With no fusion of gametes and no mixing of genetic material, offspring receive an exact copy of the parent’s genes.

AnswerSingle parent + mitosis + no genetic mixing → offspring are identical clones.
5
Explain why the menstrual cycle stops during pregnancy.
Solution

The cycle prepares the uterus for pregnancy and sheds the lining if none occurs. When an embryo implants, the thick, blood-rich lining is needed to nourish it, so it is maintained — not shed. Pregnancy hormones also stop further ovulation and prevent the lining from breaking down.

AnswerThe uterine lining is kept to nourish the embryo and ovulation stops, so there is no menstruation until the pregnancy ends.
6
Why are flowers that bloom at night white or light-coloured compared to flowers that bloom during the day?
Solution

Night flowers are pollinated by night-active pollinators (moths, bats). In the dark, bright colours are invisible, but white/pale flowers reflect faint moonlight and stand out, and they also rely on strong fragrance. Day flowers can use bright colours because bees, butterflies and birds see colours well in sunlight.

AnswerLight colour + scent make night flowers easy for nocturnal pollinators to find.
7
Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?
Solution

They are clones — genetically identical with no variation. If a disease can infect one plant, it can infect all of them the same way, so a single outbreak can destroy the whole crop. Sexually reproduced plants have variation, so some individuals may carry resistance and survive.

AnswerLack of genetic variation makes identical clones far more vulnerable to disease.
8
If all flowers in a type of plant were only capable of self-pollination, how would it affect genetic diversity over several generations?
Solution

Self-pollination uses pollen and eggs from the same plant, so little new genetic material is mixed in. Over generations, offspring become more and more genetically similar and genetic diversity steadily falls.

AnswerDiversity decreases; the plants become poorly adaptable to changing conditions, new diseases and pests, weakening long-term survival.
9
A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.
Plant tissue culture producing many identical plantlets
Fig. 11.5 — Tissue culture mass-produces identical, disease-free plantlets
Solution

Methods (asexual / vegetative): tissue culture (micropropagation), cuttings, grafting and layering.

  • They use a single parent and mitosis, so each plant is an identical clone with the same desirable traits.
  • They are faster than growing from seed.
  • Tissue culture can produce thousands of healthy, disease-free plantlets quickly from a tiny piece of tissue (e.g. banana).
AnswerVegetative propagation — especially tissue culture — gives uniform, true-to-type plants in large numbers, rapidly.
10
Suresh prepares slides with pollen grains in different sugar concentrations (0%, 2.5%, 5%, 7.5%, 10%) to study germination. (i) What hypotheses can be tested? (ii) What parameters should be kept the same?
Solution

(i) Hypotheses:

  • Sugar concentration affects the percentage/rate of pollen germination.
  • There is an optimum sugar concentration where germination is maximum — too low or too high reduces it.
  • In 0% (plain water) pollen germinates poorly or bursts.

(ii) Kept the same (controlled variables): same species and freshness of pollen, same number of pollen grains, same temperature, time, volume and pH of solution, same light/humidity, and the same microscope magnification — only the sugar % is varied.

AnswerVary only sugar concentration; control everything else; test whether (and at what optimum) sugar level affects germination.
11
Which type(s) of pollination are followed by: Tomato (stamens cover the stigma), Wheat (flowers open after pollination), Papaya (male & female flowers often on different trees)?
Tomato, Wheat and Papaya flowers
Tomato, Wheat and Papaya flowers (from the question)
Solution
  • Tomato → Self-pollination. Stamens surround the stigma, so its own pollen falls on it.
  • Wheat → Self-pollination. Pollination happens inside the flower before it opens.
  • Papaya → Cross-pollination. Male and female flowers are on different trees, so pollen must travel between trees (by wind/insects).
AnswerTomato & Wheat → self-pollination; Papaya → cross-pollination.
12
Apple orchards: Place A used natural pollinators; Place B added bee colonies. The graph shows fruit set and fruit drop. (i) Hypotheses? (ii) Parameters? (iii) Compare the data. (iv) What do you infer?
Bar graph: fruit set and fruit drop for natural pollination vs with bee colony
Fig. 11.24 — Fruit set & fruit drop: Natural vs With bee colony
Solution
PlaceFruit set (%)Fruit drop (%)
A — Natural pollination≈ 26%≈ 40%
B — With bee colony≈ 35%≈ 8%

(i) Hypotheses: adding bee colonies increases pollination, raising fruit set and lowering fruit drop → higher yield (and the decline of natural pollinators lowers yield).

(ii) Parameters: independent = pollination method (natural vs bee colony); dependent = fruit set % and fruit drop %; controlled = apple variety, climate/season, orchard care, number of fruit-bearing branches.

(iii) Comparison: with bees, fruit set is higher (~35% vs ~26%) and fruit drop is much lower (~8% vs ~40%) — Place B gives a clearly higher effective yield.

Answer(iv) Adding bee colonies greatly improves pollination → more fruits set, fewer drop → higher yield (plus honey). Where natural pollinators have declined, beekeeping effectively boosts apple production.
13
A student claims, “In humans, ovulation always happens on day 14 of the menstrual cycle.” Critically examine this claim and give at least two reasons.
Solution

The claim is not (always) correct. “Day 14” is only an average for an idealised 28-day cycle.

  • Cycle length varies (commonly 21–35 days) and even month to month. Ovulation occurs about 14 days before the next period, not always 14 days after the last — so in a 30- or 35-day cycle it is later.
  • Many factors — stress, illness, hormones, age, diet, irregular cycles around puberty — shift ovulation timing.
  • So “always day 14” is an over-generalisation that fits only a perfect 28-day cycle.
AnswerThe claim is incorrect as a general rule — day-14 ovulation is just a typical average, not a fixed fact.

How life continues…

From a single budding yeast cell to a developing human embryo — reproduction is nature’s way of passing life, and variation, to the next generation.

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