Reproduction : How Life Continues — NCERT Solutions

If a couple uses oral contraceptive pills but not condoms, the risk of sexually transmitted infections (STIs) still remains, because pills mainly prevent pregnancy by altering hormone release and do not stop the transfer of infection from one person to another. Condoms are needed to reduce the spread of STIs. So, pregnancy risk is reduced, but STI risk is not fully prevented without condoms.

Advantages: Human babies depend on adults for a long time, so they can receive long-term care, protection, food, and learning. This helps them develop skills, language, and social behaviour, which are important for human life.

Disadvantages: Because babies are helpless for a long time, adults must spend more time, energy, and resources caring for them. If care is lacking, survival chances decrease.

So, this dependence helps humans develop better but also increases the burden on parents and caregivers.

Pollen from another plant of the same species is dusted onto the stigma, so this is cross-pollination. The removal of anthers before maturity also prevents self-pollination.

The correct sequence is:

1. Pollination
2. Pollen germination on stigma
3. Fertilisation
4. Formation of zygote

So the order is (iii) → (i) → (ii) → (iv). If the options are arranged exactly as given in the chapter, this is the correct biological sequence.

The assertion is false because the zygote does not attach to the uterus wall immediately after fertilisation. It first divides and then implants into the thickened uterine lining.

The reason is true because the uterus wall is prepared to receive the zygote.

So the correct option is (iv) A is false, but R is true.

Asexual reproduction produces offspring that are genetically identical to the parent because it involves only one parent and is based on mitosis. In mitosis, the daughter cells get the same number of chromosomes and the same genetic material as the parent cell. Since there is no mixing of genes from two parents, the offspring are called clones.

The menstrual cycle stops during pregnancy because after fertilisation the zygote implants in the uterus, and the uterus must keep its lining thick and rich in blood vessels to nourish the developing embryo/foetus. Since the inner lining is being used to support pregnancy, it does not shed as menstruation. Therefore, periods stop during pregnancy.

Flowers that bloom at night are often white or light-coloured because these colours are easier to see in the dark and help attract night pollinators. The chapter says flowers use colour, fragrance, and nectar to attract pollinators, so pale colours are useful at night when visibility is low.

Vegetatively propagated plants tend to be more vulnerable to diseases because they are genetically identical to the parent and to one another. This means all the plants have similar traits, so if one is susceptible to a disease, the others may also be susceptible. Sexual reproduction creates variation, which improves the chances that some plants will resist disease.

If all flowers in a plant species could only self-pollinate, genetic diversity would decrease over generations. Self-pollination combines genes from the same plant, so there is little new variation. Over many generations, the offspring would remain more similar to one another and to the parent, reducing the chance of adapting to changing environments.

To produce a large number of genetically identical plants quickly, the farmer should use asexual reproduction methods, especially cutting, grafting, layering, and tissue culture.

These are effective because they involve one parent and produce genetically identical offspring. The chapter says such methods are widely used to efficiently propagate plants, and tissue culture can produce mass-produced healthy plantlets very rapidly.

Using different sugar concentrations, the set-up can test these hypotheses:

• Pollen germination depends on sugar concentration.
• There is an optimum sugar concentration for maximum pollen germination.
• Very low or very high sugar concentration reduces pollen germination.

So the experiment checks how changing sugar concentration affects pollen germination.

To make the experiment fair, the following parameters should be kept the same:

• Type of pollen grains used
• Amount of pollen placed on each slide
• Volume of solution on each slide
• Temperature
• Time allowed for germination
• Method of slide preparation and observation

Only the sugar concentration should change.

From the clues:

• Tomato: the stamens cover the stigma, so self-pollination can occur.
• Wheat: flowers open after pollination, which fits self-pollination.
• Papaya: male and female flowers are often on different plants, so cross-pollination is needed.

So the types are: Tomato — self-pollination; Wheat — self-pollination; Papaya — cross-pollination.

The passage asks several things about the apple orchard investigation: the hypotheses, parameters, comparison of yields, and inference. So the correct response is that all four points are part of the investigation.

The claim is not fully correct. In humans, ovulation does not always happen exactly on day 14.

Reasons:

1. The chapter says the menstrual cycle usually lasts 21–35 days, often around 28 days. So ovulation time can vary.
2. Day 14 is only an approximate middle point of a typical 28-day cycle, but not every woman has exactly the same cycle length.

So ovulation may occur around day 14 in a typical cycle, but not always in every case.

The researcher-farmers group may have formed these hypotheses:

• Natural pollinators increase apple fruit set and yield.
• Beekeeping improves pollination and reduces fruit drop.
• Orchards with more pollinator activity will produce more fruits than orchards without such support.

The different parameters in the apple orchard experiment are:

• Pollination condition: natural pollinators in Place A and beekeeping in Place B
• Fruit setting: number of fruits / total number of fruit-bearing branches
• Fruit drop: premature falling of developing fruits

These are the main measurable parameters.

The orchard at Place B would have the higher yield if beekeeping improved pollination, because more pollination usually means more fruit set and less fruit drop.

So, in terms of high yields of apple fruits, Place B is expected to perform better than Place A.

From the data, we can infer that beekeeping supports better pollination and increases apple yield. It improves fruit set and helps reduce fruit drop. Therefore, maintaining pollinators like bees is important for high apple production.

The L.S. and T.S. of the fruit should be linked with the internal structure of the ovary of the same species because after fertilisation the ovary develops into the fruit and the ovules develop into seeds. So the arrangement of seeds and chambers seen in the fruit matches the arrangement of ovules inside the ovary.

Vegetative propagation is helpful in agriculture because it allows farmers to efficiently propagate plants that are genetically identical to the parent and to grow desirable crops on a large scale. Methods like cutting, grafting, layering, and tissue culture are widely used and have improved agricultural and horticultural practices.

Meiosis helps create variation in sexual reproduction because it reduces the chromosome number to half and separates each pair of chromosomes so that each gamete gets only one chromosome from each pair. This causes a random mixing of chromosomes, so every gamete has a different combination of characters. When gametes fuse, the offspring get a unique combination, creating variation among individuals.

In a unicellular organism like amoeba or yeast, the parent cell divides to form two daughter cells that are almost identical copies. The chapter says asexual reproduction produces clones through mitosis, so the new cells are genetically the same as the parent.

However, the chapter does not directly discuss whether such organisms “grow old” or how aging happens in them. From the chapter content, we can only say that because they reproduce by division into similar cells, each division gives rise to new individual cells, but the chapter does not explain aging in unicellular organisms.

The chapter’s bead activity shows that with three pairs of characters, the possible combinations are:

$2 \times 2 \times 2 = 8$

So, gametes can carry 8 combinations of characters in that example. The chapter also says that with 23 pairs of chromosomes, the number of combinations would be far greater.

If pollination does not occur, pollen grains will not reach the stigma. Then a pollen tube will not form, the male gamete will not reach the ovule, and fertilisation will not take place. As a result, zygote formation will not occur, and therefore fruits and seeds will not form.