Revise, Reflect, Refine — Complete Solutions
🟦 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?
(i) Self-pollination
(ii) Cross-pollination
(iii) Fertilisation
(iv) Tissue culture
🟩 ANSWER
Correct option: (ii) Cross-pollination
The anthers were removed before maturity to prevent the flower’s own pollen from reaching its stigma.
Pollen from another plant of the same species was then transferred to the stigma. Therefore, cross-pollination was ensured.
After pollination, fertilisation occurred and seeds were formed.
🟦 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
The correct order is:(iii)→(i)→(ii)→(iv)
Therefore:
Pollination → Pollen germination on stigma → Fertilisation → Formation of zygote
- Pollen grains are transferred from the anther to the stigma.
- A compatible pollen grain germinates and forms a pollen tube.
- The male gamete reaches the ovule and fuses with the egg.
- This fusion forms a zygote.
🟦 QUESTION 3
Assertion (A): The zygote formed after fertilisation immediately attaches to the uterus wall.
Reason (R): The uterus is always prepared to receive the zygote.
Choose the correct option:
(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.
🟩 ANSWER
Scientifically, both the Assertion and the Reason are false. Therefore, none of the given options is completely correct.
The zygote does not immediately attach to the uterine wall. It first undergoes several mitotic divisions while moving through the oviduct towards the uterus. It then attaches to the thickened uterine lining through a process called implantation.
The uterus is also not always ready to receive a zygote. Its inner lining becomes thick and rich in blood vessels during a particular phase of the menstrual cycle.
Corrected statements:
- The dividing zygote attaches to the uterine lining after reaching the uterus.
- The uterus prepares itself during each menstrual cycle to receive a developing embryo.
🟦 QUESTION 4
Why does asexual reproduction produce offspring that are genetically identical to the parent?
🟩 ANSWER
Asexual reproduction involves:
- Only one parent
- No formation or fusion of male and female gametes
- Cell division mainly through mitosis
During mitosis, the genetic material of the parent cell is copied and distributed equally to the daughter cells.
Therefore, the offspring receive almost the same genetic material as the parent and are called clones.
Small variations may sometimes arise because of mutations or environmental effects.
🟦 QUESTION 5
Explain why the menstrual cycle stops during pregnancy.
🟩 ANSWER
After fertilisation, the zygote implants in the thickened lining of the uterus.
During pregnancy, hormones maintain this uterine lining so that it can:
- Nourish the developing embryo
- Provide a rich blood supply
- Support the growth of the foetus
Since the uterine lining is required for pregnancy, it is not shed. Ovulation is also usually stopped by hormonal changes.
Therefore, menstruation and the normal menstrual cycle stop during pregnancy.
🟦 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 often white or light-coloured because these colours are more easily visible in dim light.
This helps them attract nocturnal pollinators such as:
- Moths
- Bats
- Certain insects
Many night-blooming flowers also produce a strong fragrance and nectar to attract pollinators when visibility is low.
Day-blooming flowers can use bright colours because sunlight makes them clearly visible to 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 is a form of asexual reproduction. The new plants produced are genetically almost identical to the parent plant.
Therefore:
- There is very little genetic variation among the plants.
- Most plants have the same level of resistance or susceptibility to a disease.
- If one plant is affected by a pathogen, many other plants may also be affected.
Sexual reproduction produces genetic variation. Some sexually produced plants may possess characteristics that make them resistant to a particular disease.
Thus, genetically identical vegetatively propagated crops are more likely to be damaged together by the same disease.
🟦 QUESTION 8
If all flowers in a type of plant were only capable of self-pollination, how would it affect genetic diversity over several generations? Explain.
🟩 ANSWER
If a plant species reproduced only through self-pollination, its genetic diversity would gradually decrease.
In self-pollination:
- Pollen and egg usually come from the same plant.
- There is limited mixing of genetic material from different plants.
- Offspring remain genetically similar to the parent plant.
Over several generations, this may lead to:
- Reduced genetic variation
- Increased expression of harmful inherited traits
- Lower ability to adapt to environmental changes
- Greater vulnerability to pests and diseases
Therefore, cross-pollination generally produces greater genetic diversity than continuous self-pollination.
🟦 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 methods of vegetative propagation, especially tissue culture.
1. Tissue culture
A small piece of plant tissue is grown in a sterile nutrient medium under controlled conditions.
It is effective because:
- Thousands of plantlets can be produced quickly.
- The plantlets are genetically identical to the parent.
- Healthy and disease-free plants can be produced.
- It is useful for plants such as banana and other commercial crops.
2. Stem cuttings
Pieces of stems containing healthy nodes and buds are planted in suitable soil.
It is useful for plants such as:
- Rose
- Sugarcane
- Money plant
3. Layering
A branch is bent and covered with soil until it develops roots. It is then separated from the parent.
4. Grafting
A shoot of a desirable plant is joined to the rooted stem of another plant.
Tissue culture is the most suitable method when a very large number of identical plants is required within a short period.
🟦 QUESTION 10
Suresh prepares slides with pollen grains in different sugar concentrations—0%, 2.5%, 5%, 7.5% and 10%—to study the germination of pollen.
(i) What are the different hypotheses that can be tested using this set-up?
(ii) What parameters should be kept the same in this set-up?
🟩 ANSWER
(i) Hypotheses that can be tested
The experiment can test the following hypotheses:
- Sugar concentration affects the germination of pollen grains.
- Pollen grains germinate best at a particular optimum sugar concentration.
- Very low or very high sugar concentrations may reduce pollen germination.
- Sugar concentration affects the length or growth rate of the pollen tube.
- Pollen grains may not germinate properly in a solution containing no sugar.
The percentage of germinated pollen grains and the length of pollen tubes can be compared at each concentration.
(ii) Parameters to be kept the same
The following conditions should remain constant:
- Pollen grains from the same species and preferably the same flower
- Equal number of pollen grains
- Same age and maturity of pollen
- Equal volume of solution
- Same temperature
- Same duration of observation
- Same pH
- Same light and humidity conditions
- Same type and size of slides
- Same method of observation and magnification
Only the sugar concentration should be changed.
🟦 QUESTION 11
Look at the pictures of tomato, wheat and papaya flowers and use the given prompts to identify the type or types of pollination that may occur in them:
- Tomato: Stamens cover the stigma.
- Wheat: Flowers open after pollination.
- Papaya: Male and female flowers are often borne on different papaya trees.
🟩 ANSWER
| Plant | Likely type of pollination | Reason |
|---|---|---|
| Tomato | Mainly self-pollination | The stamens surround or cover the stigma, making it easy for pollen from the same flower to reach it. |
| Wheat | Self-pollination | Pollination usually occurs before the flower opens, reducing the chance of pollen arriving from another plant. |
| Papaya | Cross-pollination | Male and female flowers are often present on different plants, so pollen must be transferred from a male flower to a female flower on another plant. |
Papaya may depend on insects or wind to carry pollen between different plants.
🟦 QUESTION 12
In the lower Himalayan region of northern India, apple fruit yield is declining continuously because of climate change and a reduction in natural pollinators. Researchers studied two apple orchards at places A and B.
- Place A: Flowers were pollinated by naturally available pollinators.
- Place B: Managed bee colonies were introduced along with beekeeping.
The graph compares fruit setting and fruit drop in the two orchards. Answer the following:
(i) What hypotheses did the researcher-farmer group consider?
(ii) What are the different parameters in the experiment?
(iii) Compare and analyse the data from orchards A and B.
(iv) What can be inferred from the data?
🟩 ANSWER
(i) Hypotheses
The researchers may have proposed that:
- Introducing managed bee colonies increases pollination in apple flowers.
- Better pollination increases the percentage of fruit setting.
- Improved pollination reduces premature fruit drop.
- Beekeeping can improve the overall yield of apple orchards.
(ii) Parameters in the experiment
Independent variable:
- Presence or absence of managed bee colonies
Dependent variables:
- Percentage of fruit set
- Percentage of fruit drop
- Final yield of apples
Controlled parameters:
- Apple variety
- Age and health of trees
- Orchard size
- Weather conditions
- Soil and irrigation
- Fertiliser use
- Pest-control practices
- Time of observation
(iii) Comparison of data
According to the graph on textbook page 226:
| Condition | Fruit set | Fruit drop |
|---|---|---|
| Natural pollination | About 25–26% | About 35% |
| Managed bee colony | About 40% | About 8% |
The orchard with managed bee colonies had:
- A much higher fruit-setting percentage
- A much lower fruit-drop percentage
- A greater expected fruit yield
(iv) Inference
Managed bee colonies improve the transfer of pollen between flowers.
Therefore, beekeeping:
- Increases successful pollination
- Increases fruit formation
- Reduces premature fruit drop
- Improves apple production and farmers’ income
The data also show the importance of conserving natural pollinators.
🟦 QUESTION 13
A student claims, “In humans, ovulation always happens on day 14 of the menstrual cycle.” Critically examine this claim and state whether it is correct. Give at least two reasons for your answer.
🟩 ANSWER
The claim is not completely correct.
Ovulation occurs around day 14 only in a typical 28-day menstrual cycle. It does not always occur on exactly the 14th day.
Reasons
- Menstrual-cycle length varies
A menstrual cycle may normally range from about 21 to 35 days. Ovulation generally occurs approximately 14 days before the next menstrual period, not necessarily on day 14 after the previous period begins.
- Cycles vary among individuals
Different individuals may ovulate on different days because their cycle lengths are different.
- The same person’s cycle may also vary
Factors such as the following can delay or advance ovulation:
- Stress
- Illness
- Hormonal changes
- Nutrition
- Excessive exercise
- Changes in body weight
- Age
- Some cycles may occur without ovulation
Occasionally, an egg may not be released during a menstrual cycle.
Therefore, day 14 is only an approximate average for a 28-day cycle, not a fixed day for every person or every cycle.