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NCERT Class 9 Science Reproduction: How Life Continues: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 9 Science book, chapter “Reproduction: How Life Continues”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   A flower's anthers are removed before they mature; later pollen from another plant of the same species is dusted on its stigma and seeds form. Which process has been ensured? (i) self-pollination (ii) cross-pollination (iii) fertilisation (iv) tissue culture

   1. Removing the anthers before they mature stops the flower's own pollen from reaching its stigma, so self-pollination cannot happen.
   2. Dusting pollen from another plant of the same species on the stigma is cross-pollination done by hand.
   3. Seeds formed afterwards, which shows that the male and female gametes fused: fertilisation took place.
   4. The book's answer key marks fertilisation as the process ensured by this set-up (the procedure itself is a hand cross-pollination).

   Answer: (iii) fertilisation (seeds formed); the set-up itself is a hand cross-pollination

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2. Exercises Q2

   Put these stages of sexual reproduction in plants in order: (i) pollen germinates on the stigma (ii) fertilisation (iii) pollination (iv) the zygote forms.

   1. Pollination is the first step where pollen grains are transferred to the stigma.
   2. Once on the stigma, the pollen grain germinates and grows a pollen tube.
   3. The pollen tube carries the male gamete to the ovule for fertilisation.
   4. Fertilisation is the fusion of the male and female gametes.
   5. This fusion results in the formation of a zygote.

   Answer: (iii), (i), (ii), (iv)

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3. Exercises Q3

   Assertion: the zygote attaches to the uterus wall straight after fertilisation. Reason: the uterus wall is always ready to receive it. Options: (i) both true, R explains A (ii) both true, R does not explain A (iii) A true, R false (iv) A false, R true

   1. The assertion is false: after fertilisation the zygote divides several times while it moves down to the uterus, and only then does the embryo attach to the uterus wall.
   2. The reason is taken as true: each month the uterus wall gets ready to receive a fertilised egg, becoming thick and rich in blood vessels.
   3. So the assertion is false while the reason is true.

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

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4. Exercises Q4

   Why does asexual reproduction give offspring genetically identical to the parent?

   1. Asexual reproduction involves only one parent, so there is no mixing of genetic material from two individuals.
   2. The central process behind this method is mitosis, a type of cell division.
   3. Mitosis produces two daughter cells that have the same number of chromosomes as the parent cell.
   4. Because the chromosomes are identical to those of the parent, the resulting offspring are genetically identical clones.

   Answer: Asexual reproduction produces genetically identical offspring because it relies on mitosis, which creates daughter cells with the same chromosome number and genetic makeup as the single parent.

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5. Exercises Q5

   Why does the menstrual cycle stop during pregnancy?

   1. The menstrual cycle prepares the body for a new pregnancy by releasing an egg and changing the lining of the uterus.
   2. When a sperm meets the egg, the body starts supporting the growing baby instead of preparing for another cycle.
   3. Since the body is already nurturing the developing embryo, it stops the regular monthly changes.
   4. This pause ensures the mother's energy is used for the baby's growth rather than shedding the uterine lining.

   Answer: The menstrual cycle stops during pregnancy because the body is already supporting a fertilised egg and no longer needs to prepare for a new one.

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6. Exercises Q6

   Why are flowers that bloom at night white or pale compared with day-blooming flowers?

   1. Day-blooming flowers are often brightly coloured to attract pollinators like insects and birds during daylight hours.
   2. Night-blooming flowers rely on pollinators that are active in the dark, such as moths.
   3. White or pale colours are more visible in low light conditions, helping these nocturnal pollinators locate the flowers.

   Answer: Night-blooming flowers are white or pale to be visible to nocturnal pollinators in low light, unlike day-blooming flowers which use bright colours to attract daytime pollinators.

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7. Exercises Q7

   Why are vegetatively propagated plants more open to disease than plants from sexual reproduction?

   1. Vegetative propagation is a form of asexual reproduction where offspring are exact copies of the parent plant.
   2. Because these plants lack genetic variation, they do not have the mixed characteristics that help in adapting to changing environments.
   3. Sexual reproduction involves the mixing of characteristics from two parents, which can lead to the development of disease-resistant varieties.
   4. Without this genetic diversity, vegetatively propagated plants are more susceptible to diseases compared to those produced sexually.

   Answer: Vegetatively propagated plants are more open to disease because they are genetically identical to the parent and lack the variation that sexual reproduction provides for adaptation and resistance.

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8. Exercises Q8

   If a plant's flowers could only self-pollinate, how would its genetic diversity change over generations? Explain.

   1. Self-pollination involves the transfer of pollen from the anther to the stigma of the same flower or another flower on the same plant.
   2. This process results in offspring that are genetically very similar to the parent plant because no new genetic material is introduced.
   3. In contrast, cross-pollination involves the transfer of pollen between different plants, which mixes genetic traits and increases variation.
   4. If a plant species relies solely on self-pollination, the genetic diversity within its population will decrease over several generations.
   5. The lack of genetic variation makes the population more vulnerable to diseases and environmental changes, as seen in vegetatively propagated plants.

   Answer: Genetic diversity would decrease over generations because self-pollination produces offspring with identical genetic makeup to the parent, unlike cross-pollination which introduces variation.

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9. Exercises Q9

   A farmer wants many genetically identical plants quickly. Suggest methods and why they work.

   1. Vegetative propagation methods like cutting, grafting, layering, and tissue culture are suitable for this purpose.
   2. These methods are forms of asexual reproduction that involve only one parent plant.
   3. Because they do not involve the fusion of gametes, the new plants are genetically identical to the parent.
   4. Tissue culture is particularly effective for quickly producing a large number of healthy plantlets from shoot tips.
   5. These techniques allow farmers to cultivate desirable crops on a large scale efficiently.

   Answer: Use vegetative propagation methods such as cutting, grafting, layering, or tissue culture, as they are asexual processes that produce genetically identical offspring rapidly.

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10. Exercises Q10

    Suresh studies pollen germination on slides with 0%, 2.5%, 5%, 7.5% and 10% sugar. (i) What hypotheses can this test? (ii) What should be kept the same?

    1. (i) The setup can test if pollen germination requires a specific sugar concentration to occur.
    2. (i) It can also test whether higher sugar concentrations lead to better or faster germination rates.
    3. (ii) The type of pollen grains used must be kept the same across all slides.
    4. (ii) Environmental conditions like temperature and humidity should remain constant for all trials.

    Answer: (i) Hypotheses include the necessity of sugar for germination and the effect of concentration levels on success; (ii) Pollen type and environmental conditions must be kept constant.

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11. Exercises Q11

    Which type of pollination is likely: tomato (stamens cover the stigma), wheat (flowers open after pollination), papaya (male and female flowers often on different trees)?

    Uses a figure printed in the NCERT book — open the chapter above to see it.

    1. (i) In tomato, the stamens cover the stigma, which allows pollen to transfer directly to the stigma of the same flower, making self-pollination likely.
    2. (ii) In wheat, the flowers open only after pollination has occurred, indicating that self-pollination takes place before the flower is exposed to external agents.
    3. (iii) In papaya, male and female flowers are often on different trees, so pollen must move from one plant to another, making cross-pollination likely.

    Answer: (i) Self-pollination; (ii) Self-pollination; (iii) Cross-pollination

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12. Exercises Q12

    Apple orchards in the lower Himalayas: Place A relied on natural pollinators; Place B kept bee colonies. From Fig. 11.24, fruit set was about 26% at A and 40% at B; fruit drop about 35% at A and 8% at B. (i) What hypotheses were tested? (ii) What are the parameters? (iii) Compare A and B for yield. (iv) What do you infer?

    Uses a figure printed in the NCERT book — open the chapter above to see it.

    1. (i) The group tested if beekeeping improves apple yield compared to relying solely on natural pollinators.
    2. (ii) The parameters measured were fruit setting percentage and fruit drop percentage.
    3. (iii) Place B showed higher fruit setting (40%) and lower fruit drop (8%) than Place A (26% and 35%).
    4. (iv) We infer that beekeeping enhances pollination efficiency, leading to better fruit retention and higher yields.

    Answer: (i) Beekeeping improves yield over natural pollination; (ii) Fruit setting and fruit drop; (iii) B has higher setting and lower drop than A; (iv) Beekeeping boosts pollination and yield.

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13. Exercises Q13

    A student says ovulation always happens on day 14 of the menstrual cycle. Is this right? Give at least two reasons.

    1. The claim is incorrect because the menstrual cycle length varies between individuals, typically ranging from 21 to 35 days.
    2. Ovulation occurs around the 14th day only in a typical 28-day cycle, not as a fixed date for everyone.
    3. Since the cycle duration differs, the specific day of ovulation will also vary accordingly for different women.

    Answer: The claim is not correct; ovulation timing depends on the individual's cycle length, which varies between 21 and 35 days.

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