NCERT Class 9 Science Chapter 11 Reproduction: How Life Continues

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NCERT Class 9 Science Chapter 11 Reproduction: How Life Continues

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Reproduction: How Life Continues

Chapter – 11

Pause And Ponder

1. In a china-rose (hibiscus or gudhal) plant, a pollen tube grows and continues through the style after pollen lands on the stigma. Which process is about to happen next?

Ans: The next process is fertilisation. The male gamete will fuse with the female gamete inside the ovule to form a zygote.

2. Look at the pictures (Fig. 11.16) of calotropis (madar) seeds and dandelion seeds given below. Can you guess what kind of seed dispersal these seeds are adapted for?

(a) Madar Seeds.

(b) Dandelion Seeds.

Ans: These seeds are adapted for dispersal by wind because they have hairy or feathery structures that help them float in the air.

3. A farmer plants two varieties of maize side by side, but notices that seeds from one variety reach the stigma of the other. What type of pollination is this?

Ans: This is cross-pollination, because the pollen from one plant reaches the stigma of a flower on another plant.

Pause And Ponder

1. Why do animals with external fertilisation generally produce more eggs than animals with internal fertilisation?

Ans: Animals with external fertilisation produce more eggs because many eggs and sperms are lost or destroyed in the external environment. Producing a large number of eggs increases the chances of successful fertilisation and survival of the offspring.

2. In animals, which fertilisation method are the gametes more protected?

Ans: In internal fertilisation, the gametes are more protected because fertilisation takes place inside the body of the female, where the gametes are safe from environmental hazards.

Pause And Ponder

1. Ravi suddenly notices that he is growing taller rapidly, his shoulders are broadening, and his voice cracks. What stage of life is he entering?

Ans: Ravi is entering adolescence (puberty).

2. 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?

Ans: Her next period is most likely to occur on 2nd April.

3. A human zygote has just formed. How many chromosomes does it have?

Ans: A human zygote has 46 chromosomes.

Pause And Ponder

1. What protective devices can be used during sexual activity to reduce the spread of STIs?

Ans: Condoms (male or female condoms) can be used during sexual activity to reduce the spread of sexually transmitted infections (STIs).

2. If a couple uses oral contraceptive pills but not condoms, which risks remain and why?

Ans: The risk of sexually transmitted infections (STIs) remains because oral contraceptive pills prevent pregnancy but do not protect against the spread of STIs.

Pause And Ponder

1. In many animals, the young ones can walk or find food soon after birth but human babies are completely dependent on adults for a long time. What might be some advantages and disadvantages of this for humans as a species?

Ans: Human babies remain dependent on adults for a long time, which allows them to receive proper care, protection, and education. This helps in the development of intelligence, language, and social skills. However, it also means that parents must spend a long time caring for their children, making them dependent on adults for many years.

Pause And Ponder

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?

(i) Self-pollination.

(ii) Cross-pollination. 

(iii) Fertilisation. 

(iv) Tissue culture.

Ans: (ii) Cross-pollination.

The transfer of pollen from the anther of a flower of one plant to the stigma of a flower of another plant of the same type is known as cross-pollination.

2. 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.

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

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.

(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.

Ans: Assertion (A) is false and Reason (R) is false.

The zygote does not attach immediately; it undergoes a series of mitotic divisions while travelling to the uterus before it implants into the inner lining. Furthermore, the uterus wall is not always prepared; the inner lining thickens in a cycle and sheds during menstruation if fertilisation does not occur.

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

Ans: Asexual reproduction relies on mitosis, a type of cell division that produces two daughter cells having the exact same number of chromosomes identical to the parent cell. Because the genetic material is exactly duplicated without the mixing of traits from two parents, the resulting offspring are genetically identical clones.

5. Explain why the menstrual cycle stops during pregnancy.

Ans: During the menstrual cycle, the inner lining of the uterus becomes thick and rich with blood vessels to prepare for a potential embryo. If an egg is not fertilised, this lining sheds, causing menstruation. However, if fertilisation occurs, the zygote implants into this thickened uterine lining to receive vital nourishment for development. Shedding the lining would terminate the pregnancy, therefore menstruation stops to protect and support the growing embryo.

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

Ans: Flowers that bloom during the day are often brightly coloured to attract day-active insect pollinators like bees and butterflies. In contrast, night-blooming flowers are typically white or light-coloured so that they are more visible in the dark (under moonlight). To attract nocturnal pollinators, they likely rely heavily on producing strong fragrances rather than bright visual colours.

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

Ans: Vegetative propagation is a form of asexual reproduction, meaning it produces plants that are genetically identical (clones) to the parent plant. Unlike sexually reproduced plants, which benefit from genetic variations created by the random mixing of chromosomes from two parents, cloned plants lack genetic diversity. Consequently, if a disease targets a specific genetic weakness, the entire population of identical plants is highly vulnerable and could be wiped out.

8. 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.

Ans: Self-pollination involves the transfer of pollen within the same flower or to another flower on the same plant. Relying solely on self-pollination restricts the mixing of characteristics from two different parent individuals. Over several generations, this lack of genetic mixing would severely reduce genetic diversity and variation, making it much harder for the plant species to adapt to changing environments and survive.

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

Ans: The farmer should use vegetative propagation methods such as cutting, grafting, layering, or tissue culture. These methods utilize asexual reproduction, which is very fast and allows for the rapid mass production of plants that are genetically identical to the parent. Tissue culture, for example, is highly effective for mass-producing healthy plantlets from shoot tips, ensuring high yields and disease-free crops.

10. 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?

Ans: The following hypotheses can be tested using this set-up:

(a) Different sugar concentrations affect the rate of pollen germination.

(b) There is an optimum sugar concentration at which maximum pollen germination and pollen tube growth occur.

(c) Very low or very high sugar concentrations reduce pollen germination and pollen tube growth.

(ii) What parameters should be kept the same in this set-up?

Ans: The parameters that must remain constant are the type/species of pollen grains used, the temperature of the environment, the total time allowed for germination, and the quantity of pollen placed on each slide.

11. 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	Wheat	Papaya
Stamens cover the stigma	Flowers open after pollination	Male and female flowers are often borne on different papaya trees

Ans: 

Plant	Type of Pollination	Reason
Tomato	Self-pollination	Stamens surround the stigma and pollination occurs before the flower opens.
Wheat	Self-pollination	Pollination usually occurs before the flowers open.
Papaya	Cross-pollination	Male and female flowers are usually on different plants.

12. 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?

Ans: The hypothesis is that introducing artificial bee colonies (mixed farming) will result in more efficient pollination, thereby increasing the fruit setting percentage and decreasing the premature fruit drop percentage compared to relying only on natural pollinators. 

(ii) What are the different parameters in the experiment?

Ans: The independent variable is the type of pollination strategy used (natural pollination in Place A vs. beekeeping/mixed farming in Place B). The dependent variables are the percentage of fruit set and the percentage of fruit drop. 

(iii) Compare and analyse the data of two experimental orchards Places A and B, in terms of high yields of apple fruits.

Ans: According to the graph data, Place A (natural pollination) has a relatively low fruit set (around 26%) and a high fruit drop rate (around 35%). Conversely, Place B (with bee colonies) shows a significantly higher fruit set (around 40%) and a drastically lower fruit drop rate (around 8%).

(iv) Based on your analysis, what do you infer from the data?

Ans: We can infer that applying mixed farming techniques with beekeeping dramatically improves pollination efficiency, which directly leads to higher overall apple yields by maximising fruit setting and minimising the loss of developing fruits.

13. 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.

Ans: The claim is incorrect.

(i) The length of the menstrual cycle varies significantly from person to person, typically ranging anywhere between 21 to 35 days. Because of this variation, ovulation does not strictly occur on day 14 for everyone.

(ii) Day 14 is only cited as an average or expected ovulation day specifically for a “typical 28-day period”. For individuals with shorter or longer cycles, the day of ovulation naturally shifts accordingly.