These questions appear inside the chapter itself, not at the end. The book page number is shown against each one so you can find it while reading.
In a china-rose (hibiscus or gudhal) plant, a pollen tube grows and continues through the style after pollen lands on the stigma. Which process is about to happen next?
Fertilisation is about to happen.
After pollination, the pollen grain on the stigma germinates and produces a pollen tube. This tube grows down through the style into the ovary. The male gamete moves through the pollen tube and reaches the ovule. There it fuses with the egg cell (the female gamete). This fusion of the male and female gametes is called fertilisation.
Fertilisation forms a zygote, which later develops into an embryo. After this, the ovule develops into a seed and the ovary enlarges into a fruit.
Look at the pictures (Fig. 11.16) of calotropis (madar) seeds and dandelion seeds given below. Can you guess what kind of seed dispersal these seeds are adapted for?
Both are adapted for dispersal by wind.
In the pictures, the madar seeds carry a tuft of fine, white, silky hairs, and each dandelion seed has a ring of feathery hairs on a thin stalk, like a tiny parachute. These features help in the following ways:
Carrying seeds away from the parent plant reduces crowding and competition for light, water and nutrients, and helps the plant spread to new places.
A farmer plants two varieties of maize side by side, but notices that seeds form only when pollen from one variety reaches the stigma of the other. What type of pollination is this?
This is cross-pollination.
When pollen is transferred from the anther of a flower of one plant to the stigma of a flower of another plant of the same type, it is called cross-pollination. Here, seeds form only when pollen of one maize variety reaches the stigma of the other variety — that is, pollen travels between two different plants.
(In maize this transfer is usually done by wind: maize produces a very large number of light, small pollen grains and has long, feathery stigmas to trap them.)
Why do animals with external fertilisation generally produce more eggs than animals with internal fertilisation?
In external fertilisation (for example, in most fish and frogs) the female releases eggs into the water and the male releases sperm over them. The eggs and the young that hatch from them are left unprotected in the open water:
By producing a very large number of eggs, these animals make sure that at least some of them get fertilised and survive to become adults.
In internal fertilisation (reptiles, birds and mammals) fertilisation takes place inside the body of the female, so the fertilised egg or embryo is much better protected. Since a larger share of the young survive, fewer eggs are enough. Table 11.4 of the book shows this clearly:
| Animal (fertilisation) | Eggs at a time · survival |
|---|---|
| Fish (external) | 100s–1000s · Low |
| Frog (external) | 5,000–50,000 · Low |
| Lizard (internal) | 2–20 · Moderate |
| Bird (internal) | 1–15 · Moderate to high |
In animals, which fertilisation method the gametes are more protected?
In internal fertilisation the gametes are more protected.
In internal fertilisation (as in reptiles, birds and mammals) the sperm are placed inside the body of the female and fertilisation takes place there. The gametes and the fertilised egg are therefore not exposed to water currents, changes in the surroundings or animals that would eat them. This is why the chances of survival of the young ones are generally higher than in external fertilisation, where eggs and sperm are released into open water.
Ravi suddenly notices that he is growing taller rapidly, his shoulders are broadening, and his voice cracks. What stage of life is he entering?
Ravi is entering puberty, the beginning of adolescence.
At puberty the reproductive organs start to mature and the body becomes capable of reproduction (reproductive maturity). In boys, the testes begin to produce sperm and also a hormone that causes the physical changes seen during puberty — a sudden increase in height, broadening of the shoulders and a deepening (‘cracking’) voice are all such changes.
These changes are normal and happen gradually. Emotional maturity usually takes longer to develop than the physical changes.
Rina’s period occurs every 28 days. Her last period was on the 5th of March. On which day is she most likely to get her next period?
Her next period is expected 28 days after the day her last period started, i.e. 28 days after 5 March. Counting step by step:
| Step | Working |
|---|---|
| Days of March left after 5 March | March has 31 days, so 31 − 5 = 26 days (6 March to 31 March) |
| Days still to be counted | 28 − 26 = 2 days |
| These 2 days fall in April | 1 April and 2 April |
Answer: she is most likely to get her next period on 2 April.
We say ‘most likely’ because the length of the cycle can vary a little from month to month (cycles of 21–35 days are normal), so the period may come a day or two earlier or later.
A human zygote has just formed. How many chromosomes does it have?
46 chromosomes.
Human body cells have 46 chromosomes. Gametes are formed by meiosis, which reduces the number of chromosomes to half, so each sperm has 23 and each egg has 23 chromosomes. When the sperm fuses with the egg during fertilisation, the two sets join together: 23 from the sperm plus 23 from the egg gives 46. This is how the zygote gets the same number of chromosomes as the parents.
What protective devices can be used during sexual activity to reduce the spread of STIs?
Condoms.
Sexually transmitted infections (STIs) such as gonorrhoea, herpes, syphilis, genital warts and HIV spread from an infected person to an uninfected person through close physical contact. A condom acts as a physical barrier — it prevents direct contact and stops the exchange of body fluids — so it reduces the spread of these infections. As a barrier it also stops sperm from reaching the egg, so it helps prevent pregnancy too.
If a couple uses oral contraceptive pills but not condoms, which risks remain and why?
The main risk that remains is the risk of sexually transmitted infections (STIs) such as gonorrhoea, herpes, syphilis, genital warts and HIV.
Why: Oral pills are medicines that work on the body’s hormones and change the release of eggs, so they help prevent pregnancy. But they do not form any barrier between the partners. STIs spread through close physical contact from an infected person to an uninfected person, and the pill does nothing to stop that. Only a barrier method like a condom reduces the spread of STIs.
In addition, the book notes that the pills may have some side effects on the body, because they act by altering hormones.
In many animals, the young ones can walk or find food soon after birth but human babies are completely dependent for a long time. What might be some advantages and disadvantages of this for humans as a species?
Advantages
Disadvantages
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?
Answer: (ii) Cross-pollination.
Removing the anthers before the flower matures means the flower can never receive its own pollen, so self-pollination is ruled out. The only pollen that reaches the stigma is the pollen dusted on from another plant of the same species. Transfer of pollen from a flower of one plant to the stigma of a flower of another plant of the same type is cross-pollination. This is exactly the method used in artificial hybridisation (remove the stamens, prevent self-pollination, transfer the desired pollen by hand).
Fertilisation did also take place (that is why seeds formed), but it follows any pollination. The step that the experiment deliberately ensured is cross-pollination. Self-pollination is impossible here, and tissue culture is an asexual method that does not involve pollen at all.
Arrange the following stages of sexual reproduction in plants in the correct order:
Correct order: (iii) → (i) → (ii) → (iv)
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.
Answer: (iv) A is false, but R is true (this is the intended answer — see the note on R below).
Why A is false: fertilisation takes place in the oviduct, not in the uterus. The zygote does not attach immediately. It first undergoes a series of mitotic divisions while travelling down the oviduct to the uterus, and only then implants into the inner lining of the uterus. Implantation marks the beginning of pregnancy.
About R: the idea behind R is correct — the uterus prepares itself to receive a zygote: before ovulation its inner lining becomes thick, and it becomes thicker and richer in blood vessels so it can nourish the zygote. Strictly speaking, however, the lining is not ready at every moment: if the egg is not fertilised, the lining is shed during menstruation and is rebuilt in the next cycle. So the word ‘always’ is not exactly right, and none of the four options says ‘both are false’. Of the given choices, (iv) is the one that matches the book.
Why does asexual reproduction produce offsprings that are genetically identical to the parent?
Asexual reproduction involves only one parent; there is no fusion of male and female gametes, so no genetic material from a second parent is mixed in.
The central process behind asexual reproduction (budding in yeast and hydra, spore formation in moulds, vegetative propagation in plants) is mitosis. In mitosis a cell divides into two daughter cells, each having the same number of chromosomes, identical to the parent cell. Since every cell of the offspring comes from the parent’s cells by mitosis, the offspring carry exactly the same genetic material as the parent.
That is why such offspring are genetically identical to the parent and are called clones.
Explain why the menstrual cycle stops during pregnancy.
Menstruation happens only when an egg is not fertilised. In that case the thick inner lining of the uterus, which was prepared to receive and nourish a zygote, is no longer needed, so it is shed along with some blood.
During pregnancy the situation is different:
Because the lining is not shed and the cycle of ovulation is paused, there are no periods during pregnancy. The body’s hormones keep this state; the cycle resumes some time after childbirth.
Why are flowers that bloom at night white or light in colour as compared to flowers that bloom during the day?
Flowers attract pollinators so that their pollen is carried to a stigma. Flowers that bloom during the day are often brightly coloured because they are pollinated by insects such as bees and butterflies, and by birds, which find flowers by sight in daylight.
At night there is very little light, and bright colours like red, orange or purple look dark and cannot be seen. Night-blooming flowers are pollinated by animals active at night, such as moths and bats. A white or light-coloured flower reflects the little light available (from the moon and stars), so it stands out against the dark background and can be found easily.
Many such flowers (for example, raat ki rani, the night-blooming jasmine) also have a strong fragrance, which helps night pollinators locate them even when they cannot be seen well.
Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?
Vegetative propagation is a form of asexual reproduction: the new plants grow from a part of a single parent and are genetically identical to it (clones). This makes them more vulnerable to diseases for these reasons:
In sexual reproduction, the offspring inherit a mix of characteristics from two parents, so each plant is slightly different. Some plants may be able to resist a disease and survive, so the population as a whole is less likely to be wiped out.
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.
The genetic diversity would decrease over the generations.
In self-pollination, the pollen comes from the same flower or another flower of the same plant, so both gametes come from the same parent. No new characteristics enter from another plant. As a result:
Low diversity is harmful: variations accumulated over generations help living beings adapt to changing environments. A uniform population may not have any individuals able to survive a new disease, pest or change in climate. In contrast, cross-pollination brings in characteristics from another plant and keeps the genetic diversity high.
A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.
The farmer should use vegetative propagation (asexual reproduction). Suitable methods are:
| Method | How it is done |
|---|---|
| Cutting | A piece of stem with nodes is planted in moist soil and grows roots and shoots (e.g. sugarcane, money plant, rose). |
| Grafting | A stem piece of a desired variety is fitted into a slit on a rooted plant and grows with it (e.g. rose, mango). |
| Layering | A flexible twig is bent and its middle part buried in soil; it roots and is then cut from the parent (e.g. lemon). |
| Tissue culture | Plantlets are grown in large numbers from a small piece of tissue such as the shoot tip (e.g. banana). |
Why these methods are effective:
Suresh prepares slides with pollen grains in different sugar concentrations (0%, 2.5%, 5%, 7.5%, 10%) to study the germination of pollen.
In this set-up the sugar concentration is the factor Suresh changes, and the germination of pollen (how many pollen grains form pollen tubes, and how long the tubes grow) is what he observes. The 0% slide acts as the control.
(i) Hypotheses that can be tested:
(ii) Parameters to be kept the same (so that only the sugar concentration affects the result):
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 —
| Flower | Prompt given in the book |
|---|---|
| Tomato | Stamens cover the stigma. |
| Wheat | Flowers open after pollination. |
| Papaya | Male and female flowers are often borne on different papaya trees. |
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.
Reading the graph (Fig. 11.24):
| Measure | A (natural) · B (bee colony) |
|---|---|
| Fruit set | 26% · 40% |
| Fruit drop | 35% · 8% |
(i) Hypotheses:
(ii) Parameters:
(iii) Comparison and analysis:
To see the combined effect, imagine each orchard has 1000 fruit-bearing branches, and take fruit drop as the percentage of the set fruits that fall early:
| Step | Place A · Place B |
|---|---|
| Fruits set (fruit set % × 1000 branches) | 26% of 1000 = 260 fruits · 40% of 1000 = 400 fruits |
| Fruits dropped early | 35% of 260 = 91 fruits · 8% of 400 = 32 fruits |
| Fruits that remain | 260 − 91 = 169 fruits · 400 − 32 = 368 fruits |
Place B keeps about 368 fruits for every 169 at Place A — a little more than twice the yield (368 ÷ 169 ≈ 2.2).
(iv) Inference: Pollination by bees greatly improves apple production — more flowers set fruit and far fewer young fruits fall off. The data support the idea that the decline of natural pollinators is an important reason for falling apple yields, and that beekeeping in orchards is a good solution: it raises the apple yield and also gives the farmer honey as an extra income. (To be fully sure, the experiment should be repeated at the same location, so that only the pollination method differs.)
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.
The claim is not correct. Ovulation happens around day 14 only in a typical 28-day cycle; it does not always happen on day 14.
So the correct statement is: “Ovulation takes place around the 14th day in a typical 28-day cycle” — which is how the book itself puts it.
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