Reproduction: How Life Continues — Class 9 Science
Exploration · Chapter 11
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Chapter summary
Key ideas & terms
- Asexual reproduction
- Reproduction involving a single parent and no gametes, producing offspring that are genetically identical copies of the parent.
- Binary fission
- An asexual method in which a single parent cell, such as amoeba, divides into two equal daughter cells.
- Vegetative propagation
- Asexual reproduction in plants from vegetative parts (root, stem or leaf), as in potato eyes and Bryophyllum leaf buds.
- Sexual reproduction
- Reproduction involving two parents and the fusion of male and female gametes, producing offspring with mixed characters.
- Fertilisation
- The fusion of a male gamete with a female gamete to form a zygote, which develops into a new individual.
- Zygote
- The single cell formed by the fusion of two gametes during fertilisation, which divides and develops into the offspring.
Extra questions & answers
What is reproduction and why is it important?
Reproduction is the biological process by which living beings produce new individuals of their own kind; it is important because it ensures the continuation of a species across generations.
Differentiate asexual and sexual reproduction.
Asexual reproduction needs one parent and gives genetically identical offspring with no variation, while sexual reproduction needs two parents and gametes, giving offspring with a mix of both parents’ characters and hence variation.
Name four methods of asexual reproduction with an example of each.
Binary fission (amoeba), budding (yeast or hydra), fragmentation (spirogyra) and spore formation (bread mould) are four asexual methods.
What is vegetative propagation? Give two examples.
It is asexual reproduction in plants from a vegetative part such as a root, stem or leaf; examples are new plants from the eyes of a potato and from the leaf margins of Bryophyllum.
How do cuttings, layering and grafting differ as artificial propagation methods?
In cuttings a stem piece is planted to grow roots, in layering a stem is bent to the soil to root while still attached, and in grafting a desired shoot (scion) is joined to a rooted stock so both grow together.
Why does sexual reproduction produce variation?
Because the offspring receive a combination of genetic material from two different parents through the fusion of gametes, leading to small differences that help the species adapt to changing environments.
What is fertilisation?
Fertilisation is the fusion of a male gamete with a female gamete to form a single cell called a zygote, which then develops into a new individual.
Describe how a flower brings about sexual reproduction.
Pollen carrying male gametes lands on the stigma, grows a pollen tube down to the ovule, and the male gamete fuses with the egg; the fertilised ovule becomes a seed and the ovary becomes the fruit.
What changes occur in the human body at puberty?
Hormonal changes cause the reproductive organs to mature and produce gametes, and secondary sexual characters develop, preparing the body for the ability to reproduce.
What is the role of the menstrual cycle?
The menstrual cycle prepares the female reproductive system for a possible pregnancy each month; if the egg is not fertilised, the thickened uterine lining is shed as menstruation.
Think It Over
When does a farmer prefer asexual or sexual methods of reproduction for crops production?
A farmer prefers asexual reproduction mainly vegetative propagation like cutting, grafting, layering or tissue culture when he wants to produce many genetically identical plants quickly, to preserve desirable traits like taste, yield or disease resistance, and to get faster growth without waiting for seeds. He prefers sexual reproduction by seeds when he wants genetic variation, to develop new high-yielding or disease-resistant varieties, to produce seeds that can be stored and transported easily, and to grow crops where variation helps adaptation to changing climate or soil.
Why do you think most complex animals and flowering plants use sexual reproduction, while many simple organisms, like yeast and hydra mainly reproduce asexually?
Complex animals and flowering plants live in changing environments and have many different genes. Sexual reproduction mixes chromosomes from two parents by meiosis and fertilisation, creating variation which helps species adapt, survive diseases and evolve. Simple organisms like yeast and hydra often live in stable, favourable conditions where rapid, energy-efficient copying is more useful. Asexual reproduction by budding or mitosis is faster, needs only one parent and produces many clones quickly to increase population.
Activity 11.7: Let us find out
1. Compare and analyse the two strategies in terms of (Table 11.3)— Pollen to seed ratio, Efficiency of pollination and seed formation
From Table 11.3 wind-pollinated grasses release about
2. Explain why producing a very large number of pollen grains can still be an effective pollination strategy.
Wind is a random, non-directed agent. Most pollen grains never land on a compatible stigma and are lost. By producing millions of light, small pollen grains, the chance that at least some reach a receptive feathery stigma increases. Even with very low success rate per grain, the absolute number of seeds formed is enough to sustain the population, so huge production is evolutionarily effective for wind-pollinated plants.
Pause and Ponder
1. 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 next. The pollen tube carries the male gametes down the style to the ovary where one male gamete will fuse with the egg cell inside the ovule to form a zygote.
2. 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 calotropis and dandelion seeds have light, fluffy hair-like pappus. This adaptation is for wind dispersal. The hairs act like a parachute, allowing the seeds to float and be carried long distances by wind currents.
3. 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. Pollen is transferred from the anther of a flower on one plant to the stigma of a flower on another plant of the same species, here from one variety to the other, which is necessary for seed set in this case.
4. Why do animals with external fertilisation generally produce more eggs than animals with internal fertilisation?
In external fertilisation eggs and sperm are released in water where they are exposed to water currents, predators, and may not meet. Many eggs are lost or eaten, so survival rate is low. Producing thousands of eggs increases the probability that some will be fertilised and survive. In internal fertilisation, gametes and embryos are protected inside the female body, so fewer eggs are needed.
5. In animals, which fertilisation method the gametes are more protected?
Gametes and the resulting zygote are more protected in internal fertilisation. Inside the body of the female they are safe from drying, predators and environmental variation, whereas in external fertilisation they are exposed to water and other hazards.
6. 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 or adolescence. This is the stage of reproductive maturity when sex hormones from the testes cause secondary sexual changes in boys such as rapid height growth, broadening of shoulders, muscular development and deepening and cracking of voice.
7. 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?
With a regular
8. A human zygote has just formed. How many chromosomes does it have?
A human zygote has
9. What protective devices can be used during sexual activity to reduce the spread of STIs?
Barrier methods like male condoms and female condoms or vaginal covers can be used. When used correctly and consistently they prevent direct contact of body fluids and reduce transmission of sexually transmitted infections and also prevent unwanted pregnancy.
10. If a couple uses oral contraceptive pills but not condoms, which risks remain and why?
Oral pills prevent ovulation by changing hormones, so pregnancy risk is reduced, but they do not provide barrier protection. The risk of sexually transmitted infections like gonorrhoea, syphilis, genital warts, herpes and HIV remains, because STIs spread through close physical contact and body fluids which pills cannot block. Only condoms reduce STI transmission.
11. 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?
Advantage: Long dependence allows prolonged brain growth, learning, language and social skills with parental care, leading to high intelligence and complex culture. Strong family bonding also gives protection. Disadvantage: It requires heavy parental investment, especially from the mother, makes infants vulnerable for longer, reduces number of offspring a parent can support at one time, and limits mobility of the care-givers.
Revise, Reflect, Refine
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
The process ensured is (ii) Cross-pollination and subsequently (iii) Fertilisation, but the specific controlled transfer is cross-pollination. Removing anthers prevents self-pollination, and dusting pollen from another plant guarantees cross-pollination, which then leads to fertilisation and seed formation. The most direct answer is (ii) Cross-pollination.
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
Correct order is (iii) Pollination → (i) Pollen germination on stigma → (ii) Fertilisation → (iv) Formation of zygote. First pollen lands on stigma, then germinates and grows a tube, then male gamete fuses with egg, forming the zygote.
3. 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.
The correct option is (iv): Assertion (A) is false. The zygote does not attach to the uterus immediately — it undergoes repeated mitotic divisions as it travels down the fallopian tube and implants in the uterine wall as a blastocyst only after several days. (Strictly, Reason (R) is also not fully accurate, since the uterine lining is prepared cyclically after ovulation rather than ‘always’; but because A is false, option (iv) is the intended answer.)
4. Why does asexual reproduction produce offsprings that are genetically identical to the parent?
Asexual reproduction involves only one parent and is based on mitosis. Mitosis copies chromosomes exactly without reduction or mixing, so daughter cells get the same number and type of chromosomes as the parent cell. There is no fusion of two different gametes, so no new combinations arise and offspring are clones, except for rare mutations.
5. Explain why the menstrual cycle stops during pregnancy.
If fertilisation occurs, the zygote implants in the thick, blood-rich uterine lining. Hormones, mainly progesterone, maintain this lining for pregnancy and prevent it from shedding. Ovulation and menstruation are hormonally suppressed during this period so that the embryo can continue to develop safely in the uterus.
6. Why are flowers that bloom at night white or light in colour as compared to flowers that bloom during the day?
Night-blooming flowers depend on nocturnal pollinators like moths and bats that are active in dim light. White or light colours reflect moonlight and are more visible at night, and such flowers often have strong fragrance to attract pollinators. Day-blooming flowers can use bright colours like red, yellow and blue to attract bees and butterflies in daylight.
7. Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants?
Vegetatively propagated plants are genetically identical clones. They have no variation, so if one plant is susceptible to a pathogen, all are susceptible. Sexually reproduced plants have variation due to mixing of genes, so some individuals may have resistance genes and survive, protecting the population.
8. 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.
If only self-pollination occurs, pollen from the same flower or same plant fertilises its own ovules. Offspring receive genes from a single parent lineage, so very little mixing occurs. Over many generations genetic diversity would decrease, the population would become uniform, less able to adapt to new diseases or environmental changes, and harmful traits could accumulate.
9. A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.
Suitable methods are vegetative propagation methods like cutting, grafting, layering, and especially tissue culture. Tissue culture can produce thousands of healthy plantlets from a small shoot tip in a short time under sterile conditions. These are asexual methods based on mitosis, so they produce clones with the same desirable traits, are fast, require less land initially and ensure uniformity.
10. 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?
Hypotheses could be: pollen germination rate depends on sugar concentration, there is an optimum sugar concentration for maximum germination and pollen tube growth, very low or very high sugar inhibits germination due to lack of nutrients or osmotic stress, and that sugar provides energy for tube elongation.
10. Suresh prepares slides with pollen grains in different sugar concentrations (0%, 2.5%, 5%, 7.5%, 10%) to study the germination of pollen. (ii) What parameters should be kept the same in this set-up?
Controlled variables should be kept same: same plant species and pollen source, same amount of pollen per slide, same temperature, same incubation time, same light, same pH, same volume of solution, same microscope observation time, and same procedure for counting germinated grains, so that only sugar concentration varies.
11. 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. Flowers open after pollination., Wheat Flowers open after pollination., Papaya Male and female flowers are often borne on different papaya trees.
Tomato where stamens cover stigma and flowers open after pollination shows self-pollination, likely autogamy within the same flower. Wheat which opens after pollination also shows self-pollination. Papaya where male and female flowers are on different trees cannot self-pollinate and must undergo cross-pollination, requiring pollen from a male tree to reach the female tree via wind or insects.
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?
Hypotheses are: decline in apple yield is due to insufficient pollination from reduced natural pollinators, and introducing managed beekeeping will increase cross-pollination, improve fruit set and reduce fruit drop, thereby increasing yield and providing additional honey income.
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. (ii) What are the different parameters in the experiment?
Independent variable is pollination management: natural pollinators only at Place A versus natural plus managed beekeeping at Place B. Dependent variables are fruit setting per branch and fruit drop percentage. Controlled variables should be apple variety, tree age, orchard size, altitude, climate, soil and other farming practices kept similar to compare fairly.
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. (iii) Compare and analyse the data of two experimental orchards Places A and B, in terms of high yields of apple fruits.
Data in Fig. 11.24 shows Place B with beekeeping has higher fruit setting percentage and lower fruit drop compared to Place A with only natural pollinators. Higher fruit set means more flowers successfully fertilised and retained, and lower premature drop means developing fruits stay until maturity. Together this indicates greater overall yield at Place B.
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. (iv) Based on your analysis, what do you infer from the data?
Managed beekeeping significantly improves apple pollination and yield when natural pollinators are declining. Mixed farming with bees enhances fruit set, reduces drop, increases honey production and farmer income, and is a sustainable solution to pollinator scarcity under climate change.
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.
The claim is not correct. Ovulation around day 14 is an average for a