Reproduction: How Life ContinuesClass 9 Science NCERT Solutions
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Q1Revise, Reflect, Refine
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
Solution
Answer: (ii) Cross-pollination
Explanation:
Cross-pollination is the transfer of pollen from the anther of a flower on one plant to the stigma of a flower on another plant of the same species. In this case, the anthers of the first flower were removed to prevent self-pollination. Then, pollen from a different plant was manually transferred to its stigma. This procedure ensures that cross-pollination occurs.
Q2Revise, Reflect, Refine
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
Solution
The correct order of the stages of sexual reproduction in plants is:
iii → i → ii → iv
- (iii) Pollination: The process begins with the transfer of pollen grains from the anther to the stigma of a flower.
- (i) Pollen germination on stigma: Once on a compatible stigma, the pollen grain germinates and grows a pollen tube down through the style.
- (ii) Fertilisation: The male gamete travels through the pollen tube to the ovule and fuses with the female gamete (egg cell).
- (iv) Formation of zygote: The fusion of the male and female gametes results in the formation of a fertilised egg, called a zygote.
Q3Revise, Reflect, Refine
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.
Solution
Answer: The question is flawed as both Assertion (A) and Reason (R) are false based on the provided text. However, if forced to choose the best option, the most accurate part is that Assertion (A) is definitively false.
Let's analyze both statements:
- Assertion (A) is false: Fertilisation occurs in the oviduct (fallopian tube). The zygote then travels for several days towards the uterus, undergoing multiple cell divisions (mitosis) to form an embryo before it implants in the uterine wall. The attachment is not immediate.
- Reason (R) is false: The uterus wall is not 'always' prepared. It prepares cyclically. The inner lining (endometrium) thickens to receive an embryo after ovulation. If fertilisation does not occur, this lining is shed during menstruation. Therefore, the uterus is only prepared for a specific period in each menstrual cycle.
Since no option for 'Both A and R are false' is available, the question is problematic. But the most significant error in the sequence of events is in the Assertion.
Q4Revise, Reflect, Refine
Why does asexual reproduction produce offsprings that are genetically identical to the parent?
Solution
Asexual reproduction produces offspring that are genetically identical to the parent for two main reasons:
- Single Parent: It involves only one parent. Therefore, the offspring inherit the genetic material from that single parent only, with no mixing of genes from another individual.
- Mitotic Cell Division: The underlying process of cell division in asexual reproduction is mitosis. Mitosis is a type of cell division that produces two daughter cells that are genetically identical to the parent cell, having the same number and type of chromosomes.
Because there is no fusion of gametes or mixing of genetic information from two different individuals, the resulting offspring are clones of the parent.
Q5Revise, Reflect, Refine
Explain why the menstrual cycle stops during pregnancy.
Solution
The menstrual cycle stops during pregnancy because the thickened inner lining of the uterus, which is rich in blood vessels, is essential for the development of the embryo. Here is a step-by-step explanation:
- Implantation: After fertilisation, the resulting embryo travels to the uterus and implants itself into the uterine wall.
- Nourishment: This uterine lining provides nourishment and support to the growing embryo and foetus throughout the pregnancy.
- Hormonal Control: The body produces specific hormones (like progesterone) after implantation. These hormones maintain the uterine lining and prevent it from breaking down and being shed.
Since menstruation is the shedding of this uterine lining when fertilisation does not occur, its maintenance during pregnancy causes the menstrual cycle to pause until after childbirth.
Q6Revise, Reflect, Refine
Why are flowers that bloom at night white or light in colour as compared to flowers that bloom during the day?
Solution
Flowers that bloom at night are often white or light in colour to attract nocturnal pollinators, such as moths. The reasons are:
- Visibility: In low-light conditions or moonlight, white or pale colours are much more visible than bright colours like red or purple. This helps pollinators locate the flowers in the dark.
- Fragrance: In addition to their light colour, night-blooming flowers are often highly fragrant. The strong scent can travel long distances and helps pollinators find the flowers using their sense of smell.
In contrast, flowers that bloom during the day use bright colours (like red, yellow, blue) to stand out against the green foliage in bright sunlight and attract daytime pollinators like bees and butterflies, which have good colour vision.
Q7Revise, Reflect, Refine
Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?
Solution
Vegetatively propagated plants are more vulnerable to diseases because they lack genetic diversity. Here's why:
- Genetically Identical (Clones): Vegetative propagation is a form of asexual reproduction that produces offspring that are genetically identical to the single parent plant. These offspring are essentially clones.
- Uniform Susceptibility: Because all the plants in a population are genetically the same, they share the same weaknesses. If a particular disease or pest can infect one plant, it can likely infect and devastate the entire population, as no individuals possess genetic resistance.
In contrast, sexually reproduced plants inherit a mix of genes from two different parents. This creates genetic variation within the population. Some individuals may happen to have genes that make them resistant to a specific disease, allowing them to survive and reproduce, ensuring the survival of the species.
Q8Revise, Reflect, Refine
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.
Solution
If a plant species were only capable of self-pollination, its genetic diversity would significantly decrease over several generations.
Explanation:
Self-pollination involves the fusion of gametes from the same parent plant. This means there is no introduction of new genetic material from other individuals. While new traits can arise very slowly through random mutations, the primary effect is the reshuffling of the same set of genes.
Over several generations, this process leads to a highly uniform population where individuals are genetically very similar to each other and to their ancestors. This lack of genetic diversity is risky for the species' long-term survival. If environmental conditions change or a new disease emerges, the entire population could be wiped out because no individuals would possess the different genetic traits that might have allowed them to adapt and survive.
Q9Revise, Reflect, Refine
A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.
Solution
To produce a large number of genetically identical plants quickly, a farmer should use methods of vegetative propagation. Suitable methods mentioned in the chapter include:
- Cutting: A piece of the stem or branch of a parent plant is cut and planted in the soil. It develops roots and grows into a new, identical plant. This is common for plants like sugarcane and rose.
- Layering: A flexible branch of a parent plant is bent and a portion is buried in the soil while still attached to the parent. Once this portion develops roots, it is cut from the parent to grow as an independent, identical plant. This is used for plants like lemon and jasmine.
- Tissue Culture: This is a modern laboratory technique where a small piece of plant tissue is grown on an artificial nutrient medium under sterile conditions. This can produce thousands of genetically identical plantlets in a short time.
Why these methods are effective:
These methods are all forms of asexual reproduction. They are effective because they involve only one parent and produce offspring that are genetically identical (clones). This ensures that desirable traits of the parent plant, such as high yield, good fruit quality, or disease resistance, are preserved in all the new plants. These methods are also generally much faster than growing plants from seeds.
Q10Revise, Reflect, Refine
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?
(ii)
What parameters should be kept the same in this set-up?
Solution
(i)
Hypotheses that can be tested:
- Hypothesis 1: Pollen grains require a sugar solution to germinate.
- Hypothesis 2: The rate or percentage of pollen germination is dependent on the concentration of the sugar solution.
- Hypothesis 3: There is an optimal sugar concentration (e.g., 5% or 7.5%) that results in the maximum germination of pollen grains.
- Hypothesis 4: Very high sugar concentrations (e.g., 10%) inhibit or reduce pollen germination.
(ii)
Parameters to be kept the same (controlled variables):
To ensure that only the sugar concentration is affecting the results (a fair test), the following parameters should be kept constant for all slides:
- Source of Pollen: All pollen grains should be from the same type of flower and preferably from the same plant, collected at the same time.
- Temperature: The slides should be kept at a constant temperature, as temperature affects biological processes.
- Amount of Solution: The volume of the sugar solution added to each slide should be the same.
- Time: The duration for which the pollen is allowed to germinate before observation should be identical for all samples.
- Light Conditions: All slides should be kept under the same light conditions (e.g., in a dark place).
- Microscope Magnification: Observations should be made under the same magnification to ensure consistent counting.
Q11Revise, Reflect, Refine
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.
Solution
Based on the given prompts, the types of pollination are:
-
Tomato: The prompt states, "Stamens cover the stigma." This floral structure makes it highly likely for pollen from the anthers (part of the stamen) to fall directly onto the stigma of the same flower. This facilitates self-pollination.
-
Wheat: The prompt states, "Flowers open after pollination." This indicates that pollination occurs within the flower before it even opens. This is a clear mechanism for self-pollination. Although the chapter mentions wheat is also wind-pollinated, the specific prompt points towards self-pollination as a primary strategy.
-
Papaya: The prompt states, "Male and female flowers are often borne on different papaya trees." This condition, known as dioecy, makes self-pollination impossible. For reproduction to occur, pollen must be transferred from a male flower on one tree to a female flower on another tree. This is, by definition, cross-pollination.
Q12Revise, Reflect, Refine
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?
(ii)
What are the different parameters in the experiment?
(iii)
Compare and analyse the data of two experimental orchards Places A and B, in terms of high yields of apple fruits.
(iv)
Based on your analysis, what do you infer from the data?
Solution
(i)
Hypotheses:
The researcher-farmers group likely formulated hypotheses such as:
- Hypothesis 1: Introducing managed pollinators (beekeeping) in apple orchards will increase the overall fruit yield compared to relying solely on declining natural pollinator populations.
- Hypothesis 2: Beekeeping as a mixed farming technique will lead to a higher rate of fruit setting and a lower rate of fruit drop in apple trees.
(ii)
Parameters in the experiment:
- Independent Variable: The method of pollination (the factor being changed). This has two levels: Place A (natural pollination only) and Place B (natural pollination + managed beekeeping).
- Dependent Variables: The outcomes being measured to assess the effect. These are: fruit setting (number of fruits per branch) and fruit drop (rate of premature fruit fall).
- Controlled Variables: Factors kept constant to ensure a fair comparison, such as the variety of apple trees, soil conditions, irrigation, and other general farming practices.
(iii)
Comparison and Analysis of Data (based on expected results from Fig. 11.24):
To analyze the data, one would compare the values for Place A and Place B:
- Fruit Setting: The graph would likely show that the fruit setting rate is significantly higher in Place B (with beekeeping) than in Place A (natural pollination). This indicates that a greater proportion of flowers were successfully pollinated and developed into fruits in the orchard with bees.
- Fruit Drop: The graph would likely show that the fruit drop rate is lower in Place B compared to Place A. This suggests that the fruits formed through efficient bee pollination were healthier and more likely to develop to maturity without being dropped prematurely by the tree.
(iv)
Inference:
Based on the analysis, the inference is that the introduction of beekeeping is an effective strategy to combat the decline in apple yields caused by a lack of natural pollinators. The presence of a high density of bees ensures more efficient and successful pollination, leading to a higher number of fruits being set and a lower number of fruits being lost, ultimately resulting in a higher overall crop yield for the farmer. This supports the initial hypotheses.
Q13Revise, Reflect, Refine
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.
Solution
The student's claim is incorrect.
While day 14 is often cited as the day of ovulation, this is an average based on a typical 28-day cycle and not a fixed biological rule. Here are two reasons why the claim is not accurate:
-
Variable Cycle Length: As stated in the chapter, the human menstrual cycle length varies among individuals and even in the same individual from month to month. It can range from 21 to 35 days. Ovulation typically occurs about 14 days before the start of the next period. Therefore, in a 21-day cycle, ovulation would occur around day 7, and in a 35-day cycle, it would occur around day 21. It only occurs around day 14 in a 28-day cycle.
-
Influence of External and Internal Factors: The timing of ovulation can be influenced by various factors such as stress, diet, illness, and travel. These factors can cause the cycle to become shorter or longer, which in turn shifts the day of ovulation. The use of the words "typically" and "around" in the textbook itself indicates that this timing is an approximation, not a certainty.