Class 9 Science · Chapter 11

11. Reproduction: How Life Continues

Complete solutions to Revise, Reflect, Refine. CBSE Class 9 Science (NCERT Exploration).

Revise, Reflect, Refine

1A 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?

Answer: (ii) Cross-pollination

Removing the anthers stops self-pollination. Pollen from another plant is then placed on the stigma. This ensures cross-pollination. Fertilisation follows, so seeds are formed.

2Arrange 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

(iii) Pollination — pollen reaches the stigma.

(i) Pollen germination on stigma — a pollen tube grows to the ovule.

(ii) Fertilisation — male and female gametes fuse.

(iv) Formation of zygote

Correct sequence: (iii), (i), (ii), (iv)

3Assertion (A): The zygote formed after fertilisation immediately attaches to the uterus wall. Reason (R): The uterus wall is always prepared to receive the zygote.

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

The zygote does not attach immediately. It first divides many times while travelling to the uterus, and then implants. The uterus prepares itself every month by making its inner lining thick.

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

Asexual reproduction involves only one parent. There is no fusion of gametes. New individuals are formed by mitosis, which makes exact copies of the parent’s DNA. So the offspring are genetically identical to the parent.

5Explain why the menstrual cycle stops during pregnancy.

During pregnancy, the embryo is implanted in the thick inner lining of the uterus. This lining nourishes the growing baby. So the lining must not break down. Menstruation, which is the shedding of this lining, therefore stops. Ovulation also stops during pregnancy.

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

At night there is very little light. White or light-coloured flowers can be seen easily in dim light. This attracts night pollinators like moths and bats. Such flowers usually also have a strong smell. Day flowers can be brightly coloured because there is enough light to see them.

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

Vegetatively propagated plants are genetically identical. They have no variation. If one plant cannot resist a disease, none of them can. So a single disease can destroy the whole crop. Sexually reproduced plants show variation, so some of them may resist the disease.

8If all flowers in a type of plant were only capable of self-pollination, how would it affect the genetic diversity over several generations? Explain.

Genetic diversity would decrease. In self-pollination, gametes come from the same plant. No new genetic material enters from other plants. Over many generations, the plants would become very similar. They would be less able to adapt to new diseases or changes in climate.

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

Tissue culture: thousands of plants can be grown from a small piece of tissue in a short time.
Cutting, layering and grafting: new plants grow from parts of the parent plant.

These are methods of vegetative propagation. They are effective because they do not need seeds. They are faster, and all new plants are identical to the parent. So the good qualities of the parent are kept.

10Suresh 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?

Sugar is needed for pollen grains to germinate.
The rate of pollen germination changes with sugar concentration.
There is a best (optimum) sugar concentration for pollen germination.
Very high sugar concentration reduces germination (as water leaves the pollen by osmosis).

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

Type and age of pollen, amount of pollen and solution on each slide, temperature, light, time of observation and the method of counting germinated pollen.

11Find out which type(s) of pollination might have been followed in tomato (stamens cover the stigma), wheat (flowers open after pollination) and papaya (male and female flowers are often borne on different trees).

Tomato: Self-pollination — the stamens cover the stigma, so pollen falls on the same flower.

Wheat: Self-pollination — pollination happens before the flower opens.

Papaya: Cross-pollination — male and female flowers are on different trees, so pollen must come from another plant.

12Apple orchards at Place A used natural pollinators, while orchards at Place B used beekeeping. The yield is shown in Fig. 11.24 in terms of fruit set and fruit drop.

From the graph: fruit set — natural pollination 26%, bee colony 40%. Fruit drop — natural pollination 35%, bee colony 8%.

(i) What are the hypotheses the researcher-farmers group has thought of?

Keeping bee colonies in orchards increases pollination of apple flowers.
Better pollination increases fruit set and reduces fruit drop, giving a higher yield.

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

Changed parameter: type of pollination (natural pollinators vs bee colonies).

Measured parameters: fruit set (%) and fruit drop (%).

Parameters to be kept same: apple variety, age of trees, soil, water, fertiliser, weather and care of the orchards.

(iii) Compare and analyse the data of the two orchards in terms of high yields.

Place B (bee colony) had a higher fruit set (40% vs 26%). It also had a much lower fruit drop (8% vs 35%). So Place B gave a much higher yield of apples than Place A.

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

Beekeeping improves pollination and increases apple yield. It also gives honey as extra income. So mixed farming with beekeeping can help farmers where natural pollinators are declining.

13A student claims, “In humans, ovulation always happens on day 14 of the menstrual cycle”. Critically examine this claim and state whether it is correct or not. Give at least two reasons.

The claim is not correct. Day 14 is only an average for a 28-day cycle.

The length of the menstrual cycle differs from person to person. It may be shorter or longer than 28 days.
The cycle of the same person can change from month to month.
Stress, illness, diet and age can change the time of ovulation.

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