Chapter 11: Reproduction: How Life Continues
Verified step-by-step solutions for your class notes.
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
- Correct Option: (ii) Cross-pollination
- Explanation: Removing the anthers prevents the flower from self-pollinating. Introducing pollen from a separate plant of the same species ensures cross-pollination.
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
- Correct Order: (iii) → (i) → (ii) → (iv)
- Explanation: First, pollen is transferred to the stigma (Pollination). The pollen then grows a tube down the style (Pollen germination on stigma). This tube delivers the male gamete to fuse with the egg cell (Fertilisation), which results in the initial single cell of the new generation (Formation of zygote).
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.
- Correct Option: (iv) A is false, but R is true.
- Explanation: Assertion (A) is false because the zygote divides into an embryo while traveling down the oviduct for a few days before it implants in the uterus wall. Reason (R) is true because the inner lining of the uterus thickens and enriches itself with blood vessels during every menstrual cycle to prepare for a potential zygote.
Why does asexual reproduction produce offsprings that are genetically identical to the parent?
- Core Reason: It involves only a single parent and uses mitosis, ensuring no mixing of genetic traits or gamete fusion.
- Explanation: Since there is only one parent, there is no second source of DNA to combine or mix traits. The cell division process responsible for this growth is mitosis, which replicates the parent's genetic material exactly, resulting in clones.
Explain why the menstrual cycle stops during pregnancy.
- Biological Mechanism: Persistent high levels of pregnancy hormones keep the uterine lining intact to support the developing embryo.
- Explanation: Once a zygote implants, the body releases specific pregnancy hormones. These hormones maintain the thick, nutrient-rich inner lining of the uterus to feed the growing baby. Because this lining is actively being preserved, it does not break down and shed, which pauses the regular monthly period.
Why are flowers that bloom at night white or light in colour as compared to flowers that bloom during the day?
- Evolutionary Adaptation: White or pale colors reflect moonlight and starlight efficiently, making them visible to night-active pollinators.
- Explanation: Bright pigments like red or yellow are difficult to see in complete darkness. White or light-colored petals reflect ambient night light, making them pop out against dark green foliage. These flowers also rely heavily on strong, aromatic fragrances to help nocturnal insects find them by smell.
Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?
- Genetic Reason: Vegetatively propagated plants are exact genetic clones of the single parent plant and lack variation.
- Explanation: Sexual reproduction mixes traits from two parents, creating unique gene combinations that might resist a disease. Because vegetative propagation uses pieces of the same plant, every offspring inherits the exact same vulnerabilities. If a pest or virus breaks through the defenses of one plant, it can effortlessly wipe out the entire crop of clones.
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.
- Long-term Trend: Genetic diversity would decrease significantly because the same genetic material is recycled repeatedly within the same plant line.
- Explanation: Self-pollination means the plant keeps fertilizing itself. Without incoming pollen from an outside source, there is no way to introduce fresh genetic traits. Over time, this locks in specific genes, eliminates variation, and makes it hard for the species to adapt to new environments.
A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.
- Recommended Methods: Stem cutting, grafting, layering, or using laboratory tissue culture techniques.
- Explanation: These vegetative propagation methods allow the farmer to bypass the slower process of growing from seeds. They utilize mitosis to regenerate a full plant from an existing shoot or stem. This guarantees the new plants mature quickly while retaining the high-yield or desirable qualities of the parent.
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?
- (i) Hypotheses to Test: "Pollen grains require an optimal sugar concentration to sprout pollen tubes successfully" or "An extreme sugar concentration can slow down or completely prevent germination."
- (ii) Constant Parameters: The plant species providing the pollen, the incubation temperature, the volume of liquid in each drop, and the total observation period.
- Explanation: Sugar provides nutrients and creates an osmotic pressure that signals the pollen grain to grow its tube. By testing different percentages, Suresh can find the exact value where germination peaks. Keeping variables like temperature constant ensures that sugar concentration is the only factor affecting the growth.
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: Stamens cover the stigma.
Wheat: Flowers open after pollination.
Papaya: Male and female flowers are often borne on different papaya trees.
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Plant Variety Structural Features / Prompts Pollination Method Determined Tomato Stamens physically cover the stigma. Self-pollination Wheat Flowers only open up after pollination concludes. Self-pollination Papaya Male and female flowers grow on completely separate trees. Cross-pollination - Tomato: Exhibits Self-pollination. Because the stamens build an enclosed shell directly surrounding the stigma, the pollen lands right on its own female organs.
- Wheat: Exhibits Self-pollination. Since the flowers remain physically closed until pollination concludes, outside pollen is blocked, forcing self-pollination.
- Papaya: Must undergo Cross-pollination. Because the male and female organs are located on completely separate trees, wind or insects are required to carry the pollen over.
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?
(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?
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Experimental Location Pollination Method Fruit Setting Rate Premature Fruit Drop Place A Natural Pollinators Only 26% 35% Place B Managed Beekeeping (Mixed Farming) 40% 8% - (i) Tested Hypotheses: Setting up managed beehives within an orchard can overcome the decline in wild pollinators, boosting successful fruit setting and reducing premature fruit dropping.
- (ii) Tracked Parameters:
• Independent Parameter: The choice of pollination management (wild local pollinators vs. introduced bee colonies).
• Dependent Parameters: The percentage of successful fruit setting and the percentage of premature fruit drop. - (iii) Data Comparison: As shown in the structured table above, Place B hit a fruit set of 40% compared to Place A's 26%. Furthermore, Place B reduced premature fruit dropping down to just 8%, whereas Place A lost 35% of its developing fruit early.
- (iv) Final Inference: Integrating beekeeping into orchards significantly improves fruit yield. It provides an effective way to protect crops from pollinator loss while giving farmers an extra source of income from honey.
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.
- Scientific Critique: The student's claim is incorrect because day 14 is just a biological average, not a rigid rule for every individual.
- Reason 1: Variable Cycle Lengths: The 14th-day milestone assumes a perfect 28-day model. Healthy cycles frequently vary between 21 to 35 days, which naturally shifts the actual day of egg release forward or backward.
- Reason 2: Hormonal Sensitivity: The delicate hormonal balance that triggers ovulation is sensitive to external disruptions. Common everyday variables like emotional stress, sudden illness, or changes in diet can delay or accelerate the cycle.