Chapter 11: Reproduction: How Life Continues

A farmer prefers asexual reproduction when he wants to produce a large number of plants with the same desirable qualities as the parent plant. For example, if a plant gives good fruits, high yield, disease resistance, or beautiful flowers, the farmer may use cutting, grafting, layering or tissue culture to produce identical plants quickly.

A farmer prefers sexual reproduction when he wants to grow crops from seeds or when he wants variation in plants. Variation is useful for developing new varieties with better yield, disease resistance, or better adaptation to changing environmental conditions.

So, asexual reproduction is preferred for quickly producing genetically identical plants, while sexual reproduction is preferred for producing variation and new improved varieties.

Most complex animals and flowering plants use sexual reproduction because it produces variation among offspring. These variations help organisms adapt better to changing environments and support the survival of the species.

Simple organisms like yeast and hydra mainly reproduce asexually because their body structure is simple, and they can reproduce quickly by methods like budding. Asexual reproduction helps them increase their population rapidly when conditions are favourable.

Thus, sexual reproduction is useful for variation and adaptation, while asexual reproduction is useful for fast multiplication.

Yes, some fungi have been found to degrade certain types of plastic. One example is Aspergillus tubingensis, which can help break down plastic-like materials under suitable conditions. Some other fungi, such as Pestalotiopsis microspora, have also been studied for their ability to degrade polyurethane, a type of plastic.

This shows that fungi are not only useful as decomposers in nature but may also help in solving environmental problems like plastic pollution. However, the use of fungi for large-scale plastic degradation still needs more research and proper scientific methods.

The process that is about to happen next is fertilisation.

After the pollen tube grows through the style, the male gamete travels through the pollen tube and reaches the ovule in the ovary. There, the male gamete fuses with the egg cell. This fusion is called fertilisation, and it forms a zygote.

Madar seeds and dandelion seeds are adapted for seed dispersal by wind.

These seeds have light, hairy or feathery structures that help them float in the air. Because of this, wind can carry them away from the parent plant to new places where they can germinate and grow.

This is cross-pollination.

In cross-pollination, pollen grains are transferred from the flower of one plant to the stigma of a flower of another plant of the same type. Here, pollen from one variety of maize reaches the stigma of another variety, so it is an example of cross-pollination.

Animals with external fertilisation generally produce more eggs because fertilisation and development take place outside the body, usually in water. Many eggs may be destroyed by water currents, eaten by other animals, or fail to get fertilised.

To increase the chances of survival, animals like frogs and fish produce a large number of eggs.

In internal fertilisation, the gametes and embryo are more protected inside the body, so fewer eggs or young ones are produced.

The gametes are more protected in internal fertilisation.

In internal fertilisation, the fusion of sperm and egg takes place inside the female body. This protects the gametes and the developing embryo from external dangers such as water currents, predators and drying.

The father determines the biological sex of a baby.

The mother always contributes an X chromosome. The father can contribute either an X chromosome or a Y chromosome.

If the father contributes an X chromosome, the baby will have XX chromosomes and will be biologically female.

If the father contributes a Y chromosome, the baby will have XY chromosomes and will be biologically male.

Therefore, the sex of the baby depends on whether the sperm from the father carries an X chromosome or a Y chromosome.

Ravi is entering the stage of puberty or adolescence.

During puberty, the body undergoes many physical and emotional changes. In boys, common changes include rapid increase in height, broadening of shoulders, growth of body hair and cracking or deepening of voice.

Rina’s menstrual cycle is 28 days long.

Her last period was on 5th March. So, her next period is most likely to occur after 28 days.

Counting 28 days from 5th March, Rina is most likely to get her next period around 2nd April.

A human zygote has 46 chromosomes.

A sperm has 23 chromosomes, and an egg also has 23 chromosomes. When they fuse during fertilisation, the zygote gets:

23 + 23 = 46 chromosomes

So, a newly formed human zygote has 46 chromosomes.

Condoms can be used during sexual activity to reduce the spread of Sexually Transmitted Infections, or STIs.

Condoms act as a barrier and reduce the chance of infection passing from one person to another. They also help in preventing unwanted pregnancy.

If a couple uses oral contraceptive pills but not condoms, the risk of sexually transmitted infections, or STIs, still remains.

Oral contraceptive pills help prevent pregnancy by affecting ovulation and hormones, but they do not act as a barrier between partners. Therefore, infections such as HIV, gonorrhoea, herpes, syphilis and genital warts can still spread.

So, pills may help prevent pregnancy, but condoms are needed to reduce the risk of STIs.

Human babies are dependent for a long time because their bodies and brains continue to develop after birth.

Advantages

Human babies get more care, protection and learning from parents and family. This long period of care helps them learn language, behaviour, emotions, social skills and problem-solving. It also helps build strong family bonds.

Disadvantages

The disadvantage is that human babies cannot survive on their own for a long time. They need continuous care, food, protection and attention. This requires a lot of time, energy and responsibility from parents and family members.

Thus, long dependency helps humans develop advanced learning and social behaviour, but it also increases the need for parental care and protection.

The correct option is (ii) Cross-pollination.

Explanation: The flower’s anthers were removed before maturity, so it could not pollinate itself. Later, pollen from another plant of the same species was dusted onto its stigma. This ensures cross-pollination.

Why other options are incorrect:

(i) Self-pollination is incorrect because pollen did not come from the same flower or same plant.

(iii) Fertilisation may happen after pollination, but the process ensured here is cross-pollination.

(iv) Tissue culture is a laboratory method of growing plants from plant tissues, not pollination.

The correct order is:

(iii) Pollination → (i) Pollen germination on stigma → (ii) Fertilisation → (iv) Formation of zygote

Explanation: First, pollen grains are transferred from the anther to the stigma. This is called pollination. Then the pollen grain germinates on the stigma and forms a pollen tube. The male gamete travels through the pollen tube and fuses with the egg cell. This fusion is called fertilisation. After fertilisation, a zygote is formed.

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

Explanation: The assertion is false because the zygote does not immediately attach to the uterus wall. After fertilisation, the zygote undergoes repeated divisions while travelling through the oviduct towards the uterus. Then it implants into the uterine lining.

The reason is true in the sense that, before ovulation and possible fertilisation, the uterus lining becomes thick and rich in blood vessels to receive and nourish the developing embryo. However, it is not “always” prepared throughout life; it prepares during the menstrual cycle. Therefore, a more accurate answer can also be understood as: A is false, and R is not fully correct because the uterus wall is not always prepared.

For school-level answer, write: A is false because implantation does not happen immediately.

Asexual reproduction produces offspring that are genetically identical to the parent because only one parent is involved and there is no fusion of male and female gametes.

In asexual reproduction, new individuals are formed mainly by mitotic cell division. In mitosis, daughter cells receive the same number and same type of chromosomes as the parent cell. Therefore, the offspring have the same genetic information as the parent.

Such genetically identical offspring are called clones.

The menstrual cycle stops during pregnancy because the fertilised egg develops into an embryo and implants in the uterus lining.

During the menstrual cycle, the uterus lining becomes thick and rich in blood vessels. If fertilisation does not occur, this lining breaks down and menstruation takes place. But during pregnancy, the embryo needs this thick uterine lining for nourishment and development. Therefore, the lining is not shed.

Hence, menstruation stops during pregnancy.

Flowers that bloom at night are usually white or light-coloured because these colours are more visible in dim light or moonlight.

Night-blooming flowers need to attract night-time pollinators such as moths and some insects. Light-coloured petals reflect more light, making the flowers easier to locate at night. Many night-blooming flowers may also produce a strong fragrance to attract pollinators.

Thus, white or light colour helps night-blooming flowers in pollination.

Vegetatively propagated plants tend to be more vulnerable to diseases because they are genetically identical to the parent plant.

Since there is very little or no genetic variation among such plants, if one plant is susceptible to a disease, most of the plants produced from it may also be susceptible to the same disease. A disease can therefore spread quickly through the whole crop.

In sexually reproduced plants, genetic variation is produced due to meiosis and fusion of gametes. Some plants may have better resistance against diseases.

If all flowers of a plant type were only capable of self-pollination, the genetic diversity would gradually decrease over several generations.

In self-pollination, pollen from the same flower or same plant fertilises the egg. This means the offspring receive genetic material from the same plant. As a result, there is less mixing of characters and fewer variations are produced.

Low genetic diversity can make the plant population more vulnerable to diseases, pests and environmental changes. However, self-pollination can help preserve useful traits of a plant.

The farmer should use vegetative propagation methods such as:

• Cutting

• Grafting

• Layering

• Tissue culture

These methods are effective because they are forms of asexual reproduction. They involve only one parent and produce plants that are genetically identical to the parent plant.

Cutting is useful for plants like rose, sugarcane and money plant.

Grafting is useful for combining desirable qualities of two plants.

Layering is useful for plants with flexible branches.

Tissue culture is very useful for producing a large number of healthy, disease-free plantlets quickly in a laboratory.

Thus, these methods help farmers produce many plants with the same desirable qualities, such as high yield, good fruit quality or disease resistance.

The following hypotheses can be tested:

• Sugar concentration affects pollen germination.

• Pollen grains may not germinate properly in 0% sugar solution.

• A certain sugar concentration may be most suitable for pollen germination.

• Very high sugar concentration may reduce or stop pollen germination.

• The length of pollen tube may vary with sugar concentration.

The following parameters should be kept the same:

• Type of pollen grains

• Number of pollen grains used

• Amount of solution on each slide

• Temperature

• Time allowed for germination

• Light conditions

• Type of slide and coverslip

• Microscope magnification

• Humidity or moisture conditions

Only the sugar concentration should be changed so that its effect on pollen germination can be studied correctly.

Thus, tomato and wheat mainly show self-pollination, while papaya mainly shows cross-pollination.

The researcher-farmers group may have thought of the following hypotheses:

• Beekeeping increases pollination in apple orchards.

• Apple orchards with bee colonies show higher fruit setting than orchards with only natural pollination.

• Bee pollination reduces fruit drop in apple orchards.

• Mixed farming with beekeeping can increase apple yield and also provide honey.

The different parameters are:

• Independent variable:

Type of pollination method

• Natural pollination

• Pollination with bee colony

• Dependent variables:

• Percentage of fruit set

• Percentage of fruit drop

• Apple yield

• Controlled parameters:

• Same crop, that is apple

• Similar orchard conditions as far as possible

• Similar time of observation

• Similar method of measuring fruit set and fruit drop

• Similar climate and soil conditions as far as possible

In Place A, with natural pollination, the fruit set was only 26%, while fruit drop was 35%. This means fewer fruits were formed and many developing fruits dropped early.

In Place B, with bee colonies, the fruit set increased to 40%, and fruit drop decreased to only 8%. This means more fruits were formed and fewer fruits dropped.

Thus, the orchard with bee colonies showed better apple yield.

From the data, we can infer that beekeeping improves pollination and increases apple yield.

Bee colonies help transfer pollen more effectively from one flower to another. This increases fruit setting and reduces fruit drop. Therefore, mixed farming with beekeeping is useful for apple farmers because it gives two benefits:

• Higher apple production

• Honey production

So, beekeeping can be an effective solution where natural pollinators are declining.

The claim is not fully correct.

In a typical 28-day menstrual cycle, ovulation often happens around day 14. However, it does not always happen exactly on day 14 in all humans.

Reasons:

• Menstrual cycle length varies:

The menstrual cycle can vary from person to person. It may be around 21 to 35 days, not always 28 days.

• Ovulation timing can vary:

Ovulation depends on hormonal changes. Stress, illness, nutrition, age and other body conditions can affect the timing of ovulation.

• Day 14 is only an approximate idea:

Day 14 is generally used for a typical 28-day cycle, but it is not a fixed rule for everyone.

Therefore, the correct statement is: Ovulation usually occurs around the middle of the menstrual cycle, but not always exactly on day 14.

Unicellular organisms like amoeba and yeast reproduce mainly by asexual methods. Amoeba divides by binary fission, and yeast reproduces by budding.

In amoeba, when one cell divides into two almost identical daughter cells, it may seem that the parent does not grow old in the same way as complex animals do. The original cell becomes part of the two new cells.

However, aging can still happen in some unicellular organisms in a different way. For example, in yeast, repeated budding may leave marks or changes in the parent cell. Over time, the older yeast cell may show reduced ability to divide.

So, aging in unicellular organisms is not the same as aging in humans or animals, but some signs of cellular aging can still occur. In many cases, asexual division helps unicellular organisms continue life by producing new cells quickly.