Exploration - Science solution
Class 9 - Chapter 11: Reproduction: How Life Continues
Exercise Solution
Question 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?
Correct Answer: (ii) Cross-pollination
Explanation:
Removing the anthers before they mature prevents self-pollination. Dusting pollen from another plant of the same species onto the stigma ensures that pollen comes from a different plant. This process is called cross-pollination. Cross-pollination increases genetic variation in the offspring. :contentReference[oaicite:0]{index=0}
| Option | Correct / Incorrect | Reason |
|---|---|---|
| (i) Self-pollination | Incorrect | Anthers were removed, preventing self-pollination. |
| (ii) Cross-pollination | Correct | Pollen from another plant of the same species reaches the stigma. |
| (iii) Fertilisation | Incorrect | Fertilisation occurs after pollination. |
| (iv) Tissue culture | Incorrect | Tissue culture is an artificial method of vegetative propagation. |
Question 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
Answer:
Correct Order:
(iii) Pollination → (i) Pollen germination on stigma → (ii) Fertilisation → (iv) Formation of zygote
| Stage | What Happens? |
|---|---|
| 1. Pollination | Pollen grains are transferred from the anther to the stigma. |
| 2. Pollen germination | Pollen grain germinates and forms a pollen tube. |
| 3. Fertilisation | Male gamete fuses with the egg cell. |
| 4. Formation of zygote | The fertilised egg develops into a zygote. |
The zygote later develops into an embryo. :contentReference[oaicite:1]{index=1}
Question 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.
Correct Answer: (iii) A is true, but R is false.
Explanation:
The zygote undergoes several mitotic divisions while travelling through the oviduct before it implants in the uterus. Therefore, the word "immediately" in the assertion is not scientifically correct. Also, the uterus is not always prepared to receive the zygote. Its lining thickens only during a particular phase of the menstrual cycle before implantation. :contentReference[oaicite:2]{index=2} :contentReference[oaicite:3]{index=3}
| Statement | Correctness | Reason |
|---|---|---|
| Assertion (A) | False | Implantation occurs after several cell divisions, not immediately after fertilisation. |
| Reason (R) | False | The uterus prepares itself only during specific stages of the menstrual cycle. |
Question 4
Why does asexual reproduction produce offspring that are genetically identical to the parent?
Answer:
Asexual reproduction involves only one parent and occurs through mitosis. During mitosis, the genetic material is copied accurately without mixing genes from another parent. Therefore, the offspring receive the same genetic information as the parent and are called clones. :contentReference[oaicite:4]{index=4}
| Reason | Explanation |
|---|---|
| One parent involved | No fusion of gametes occurs. |
| Mitosis | Produces daughter cells with identical chromosomes. |
| No genetic recombination | Genes are copied without variation. |
| Result | Offspring are genetically identical (clones). |
Question 5
Explain why the menstrual cycle stops during pregnancy.
Answer:
During pregnancy, the fertilised egg implants in the uterus and develops into an embryo. The uterine lining is needed to nourish and protect the developing embryo, so it is not shed. Therefore, menstruation stops throughout pregnancy. :contentReference[oaicite:5]{index=5} :contentReference[oaicite:6]{index=6}
| During Normal Cycle | During Pregnancy |
|---|---|
| If fertilisation does not occur, the uterine lining breaks down. | The uterine lining is maintained for embryo development. |
| Menstruation occurs. | Menstruation stops. |
Question 6
Why are flowers that bloom at night white or light in colour as compared to flowers that bloom during the day?
Answer:
Night-blooming flowers are usually white or light coloured because they are more easily visible in dim light. They often produce a strong fragrance to attract nocturnal pollinators such as moths and certain insects. In contrast, many day-blooming flowers are brightly coloured to attract bees and butterflies. The chapter explains that flowers possess special adaptations to attract different pollinators. :contentReference[oaicite:7]{index=7}
| Night-Blooming Flowers | Day-Blooming Flowers |
|---|---|
| Usually white or light coloured. | Usually brightly coloured. |
| Often strongly fragrant. | May produce nectar and bright colours. |
| Attract night pollinators. | Attract bees, butterflies and birds. |
Question 7
Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?
Answer:
Vegetative propagation produces genetically identical plants (clones). Since all plants have nearly the same genetic makeup, a disease affecting one plant can easily affect all others. Sexual reproduction creates genetic variation, increasing the chances that some plants possess natural resistance to diseases. :contentReference[oaicite:8]{index=8} :contentReference[oaicite:9]{index=9}
| Vegetative Propagation | Sexual Reproduction |
|---|---|
| Produces clones. | Produces genetically varied offspring. |
| Low genetic diversity. | High genetic diversity. |
| Higher disease susceptibility. | Some individuals may resist diseases. |
Question 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.
Answer:
If only self-pollination occurred, the exchange of genes between different plants would be greatly reduced. As a result, genetic variation would decrease over generations. Lower genetic diversity makes the population less adaptable to changing environmental conditions and diseases. :contentReference[oaicite:10]{index=10} :contentReference[oaicite:11]{index=11}
| Effect | Result |
|---|---|
| Only self-pollination | Less genetic mixing. |
| Reduced variation | Offspring become more genetically similar. |
| Lower adaptability | Greater risk from diseases and environmental changes. |
Question 9
A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.
Answer:
The farmer should use vegetative propagation. Suitable methods include cutting, grafting, layering and tissue culture. These methods produce genetically identical plants rapidly and help maintain desirable characteristics such as high yield, disease resistance and fruit quality. Tissue culture is especially useful for producing large numbers of healthy plants in a short time. :contentReference[oaicite:12]{index=12} :contentReference[oaicite:13]{index=13}
| Method | Why It Is Effective |
|---|---|
| Cutting | Produces new plants quickly from stems or shoots. |
| Grafting | Combines desirable qualities of two plants. |
| Layering | Produces healthy plants while attached to the parent plant. |
| Tissue Culture | Produces a large number of disease-free, genetically identical plants rapidly. |
Question 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?
Answer:
The experiment is designed to study the effect of different sugar concentrations on pollen germination. The following hypotheses can be tested:
| Hypothesis | Explanation |
|---|---|
| 1. Sugar concentration affects pollen germination. | Pollen grains may germinate differently in different sugar concentrations. |
| 2. There is an optimum sugar concentration for pollen germination. | Maximum pollen germination may occur at one particular sugar concentration. |
| 3. Very low sugar concentration reduces pollen germination. | Insufficient sugar may not provide favourable conditions for pollen tube growth. |
| 4. Very high sugar concentration also reduces pollen germination. | Excess sugar may inhibit water absorption and pollen tube development. |
| 5. Pollen germination percentage varies with sugar concentration. | Different concentrations may produce different numbers of germinated pollen grains. |
(ii) What parameters should be kept the same in this set-up?
Answer:
To obtain reliable results, only the sugar concentration should be changed. All other conditions should remain constant.
| Parameter | Reason |
|---|---|
| Type of pollen grains | Ensures a fair comparison. |
| Number of pollen grains | Maintains equal sample size. |
| Volume of sugar solution | Provides identical experimental conditions. |
| Temperature | Temperature influences pollen germination. |
| Duration of observation | Each slide should be observed after the same time interval. |
| Light conditions | All slides should receive similar light exposure. |
| Glass slides and cover slips | Identical apparatus ensures accuracy. |
Independent Variable: Sugar concentration
Dependent Variable: Percentage of pollen germination (or pollen tube growth)
Controlled Variables: Type of pollen, temperature, observation time, volume of solution, number of pollen grains and light conditions.
Question 11
Look at the picture given below and identify which type(s) of pollination might have occurred in these flowers.
| Plant | Observation Given | Type of Pollination | Reason |
|---|---|---|---|
| Tomato | Stamens cover the stigma. | Self-pollination | The close position of stamens and stigma makes transfer of pollen within the same flower easy. |
| Wheat | Flowers open after pollination. | Self-pollination (Cleistogamy) | Pollination occurs before the flowers open, preventing pollen from other plants from reaching the stigma. |
| Papaya | Male and female flowers are borne on different papaya trees. | Cross-pollination | Pollen must be transferred from a male plant to a female plant. |
Summary
| Plant | Pollination Type |
|---|---|
| Tomato | Self-pollination |
| Wheat | Self-pollination |
| Papaya | Cross-pollination |
Conclusion:
Tomato and wheat possess floral adaptations that favour self-pollination, whereas papaya bears male and female flowers on separate plants, making cross-pollination essential for seed formation.
Question 12
In the lower Himalayan region of northern India, apples are an important cash crop. Two experimental apple orchards were established to study the effect of pollination on fruit yield. Based on the given graph and information, answer the following questions.
(i) What are the hypotheses the researcher-farmers group has thought of for this investigation?
Answer:
The investigation compares natural pollination with managed bee colonies to determine their effect on apple production.
The following hypotheses can be tested:
| Hypothesis | Explanation |
|---|---|
| 1. Managed bee colonies increase fruit set. | Bee pollination may improve the number of fruits formed. |
| 2. Bee pollination reduces fruit drop. | Better pollination may help developing fruits remain attached. |
| 3. Decline in natural pollinators reduces apple yield. | Fewer natural pollinators may result in poor pollination and lower production. |
| 4. Scientific beekeeping can improve apple production. | Introducing bee colonies may increase the overall fruit yield. |
(ii) What are the different parameters in the experiment?
Answer:
| Parameter | Description |
|---|---|
| Independent Variable | Method of pollination (Natural pollination or Bee colony pollination). |
| Dependent Variable | Fruit set (%) and Fruit drop (%). |
| Controlled Variables | Apple variety, age of trees, irrigation, soil conditions, fertilizers, climate, observation period and orchard management practices. |
(iii) Compare and analyse the data of two experimental orchards (Places A and B) in terms of high yields of apple fruits.
Answer:
The graph clearly shows that orchards managed with bee colonies performed much better than orchards depending only on natural pollination.
| Observation | Natural Pollination | With Bee Colony |
|---|---|---|
| Fruit Set (%) | About 26% | About 40% |
| Fruit Drop (%) | About 35% | About 8% |
Analysis:
- Fruit set is much higher in orchards provided with bee colonies.
- Fruit drop is greatly reduced when managed bee pollination is used.
- Higher fruit set combined with lower fruit drop results in a much greater apple yield.
- Bee colonies improve pollination efficiency and increase successful fertilisation.
(iv) Based on your analysis, what do you infer from the data?
Answer:
The data indicate that bee-mediated pollination significantly improves apple production. Introducing managed bee colonies increases the percentage of fruit set and reduces premature fruit drop, leading to higher yields.
The study also highlights the ecological importance of pollinators. A decline in natural pollinator populations due to climate change can reduce fruit production, whereas scientific beekeeping can help overcome this problem and improve farmers' income.
| Inference | Evidence from the Data |
|---|---|
| Bee pollination improves fruit production. | Fruit set increases from about 26% to about 40%. |
| Bee pollination reduces fruit loss. | Fruit drop decreases from about 35% to about 8%. |
| Managed pollination increases yield. | Higher fruit set and lower fruit drop together produce more apples. |
| Pollinator conservation is important. | Healthy pollinator populations are essential for sustainable agriculture. |
Question 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.
Answer:
The claim is not completely correct. Although ovulation is often described as occurring around the 14th day of a 28-day menstrual cycle, it does not always occur exactly on day 14 in every woman. The timing of ovulation can vary depending on the length of the menstrual cycle and individual differences. :contentReference[oaicite:0]{index=0}
Reasons
| Reason | Explanation |
|---|---|
| Variation in menstrual cycle length | Not all women have a 28-day menstrual cycle. Some have shorter or longer cycles, so ovulation may occur earlier or later than the 14th day. |
| Ovulation is approximate | In a normal 28-day cycle, ovulation usually occurs around the middle of the cycle, not necessarily exactly on day 14. |
| Individual differences | Factors such as age, health, stress and hormonal changes can influence the timing of ovulation. |
| Hormonal regulation | Ovulation depends on the coordinated action of reproductive hormones, so its timing may vary from one cycle to another. |
Conclusion
The statement "ovulation always happens on day 14" is incorrect. Ovulation generally occurs around the middle of the menstrual cycle, but the exact day varies among individuals and even between different menstrual cycles of the same individual. Therefore, the timing of ovulation should be considered approximate rather than fixed. :contentReference[oaicite:1]{index=1}