Chapter 11: Reproduction: How Life Continues (Biology)

Removing the anthers before they mature means the flower cannot pollinate itself. Pollen from another plant of the same species is then placed on the stigma, so the set-up ensures cross-pollination (ii). The seeds that form show that the pollen grew a tube and fertilisation took place afterwards; the NCERT answer key gives (iii) Fertilisation, on the reasoning that seed formation proves fertilisation occurred.

• (i) is wrong: self-pollination is prevented by removing the anthers.
• (iv) is wrong: tissue culture grows plants from cells in a nutrient medium; it does not involve pollen or seeds.

The set-up ensures cross-pollination (ii), which then leads to fertilisation; the NCERT key marks (iii) Fertilisation, since seeds can form only after fertilisation.

(iii) → (i) → (ii) → (iv)

1. Pollination: pollen grains reach the stigma.
2. Pollen germination: the pollen grain grows a pollen tube down through the style to the ovule.
3. Fertilisation: the male gamete from the pollen tube fuses with the female gamete (egg) in the ovule.
4. Zygote formation: the fused cell (zygote) later develops into the embryo inside the seed.

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

Answer: (iv) (as in the NCERT answer key)

A is false: the zygote is formed in the oviduct. It does not attach at once; it undergoes a series of mitotic divisions while it travels to the uterus, and only then implants in the inner lining of the uterus.

R is taken as true: in every menstrual cycle the uterus prepares itself before and after ovulation, and its inner lining becomes thick and rich in blood vessels to receive and nourish a zygote. (If no fertilisation happens, the lining sheds during menstruation and is prepared again in the next cycle.)

(iv): A is false (the zygote first divides as it moves down the oviduct, then implants); R is true (the uterus prepares its lining every cycle).

In asexual reproduction there is only one parent, and no gametes fuse. New individuals are formed by mitotic cell division (as in fission, budding, fragmentation, spore formation or vegetative propagation). In mitosis, the DNA of the parent cell is copied and each daughter cell receives an exact copy. Since no genetic material from a second parent is mixed in, the offspring are genetically identical to the parent (clones), apart from rare copying errors (mutations).

There is a single parent and new cells form by mitosis, which copies the parent's DNA exactly; with no fusion of gametes from two parents, there is no mixing of genes.

Menstruation is the shedding of the thick uterine lining when no fertilisation has happened. During pregnancy, the embryo is implanted in this lining and needs it for nourishment, protection and the formation of the placenta. So:

• The thick, blood-rich lining of the uterus is maintained, not shed; so there is no menstruation.
• Hormones produced during pregnancy keep the lining in place and stop new eggs from maturing and being released, so ovulation also stops.

The cycle restarts some time after the baby is born.

Because the thick uterine lining is needed to nourish and protect the developing embryo, it is maintained instead of being shed, and pregnancy hormones stop ovulation.

Flowers need to attract pollinators. At night there is very little light, so bright colours like red or blue cannot be seen. White or pale flowers reflect even the faint moonlight or starlight, so they stand out in the dark and are easily noticed by night-time pollinators such as moths and bats. Many such flowers (e.g. raat ki rani, jasmine) also have a strong fragrance to guide pollinators. Day-blooming flowers can rely on bright colours, which are visible in sunlight, to attract bees and butterflies.

White or pale colours reflect faint moonlight and are visible in the dark, so they attract night pollinators like moths and bats (often helped by strong scent); bright colours would not be seen at night.

Vegetatively propagated plants are produced from a part of a single parent plant (stem, root, leaf), so they are all genetically identical (clones). There is no genetic variation among them:

• If a disease-causing organism can attack one plant, it can attack all of them in the same way; none of them may carry a gene for resistance.
• Diseases (e.g. viruses) present in the parent are also passed on directly through the cutting, tuber or bulb.

Sexually reproduced plants show variation because genes from two parents mix; some individuals may survive a disease, so the population is not wiped out.

They are clones with no genetic variation, so a disease that affects one affects all (and diseases pass on directly from the parent), whereas sexual reproduction gives variation and some plants may resist the disease.

Genetic diversity would decrease over generations. In self-pollination, the male and female gametes come from the same plant, so no new genes come in from other plants. Repeated self-pollination makes the plants more and more uniform (similar) genetically. As a result:

• The population would have less variation, so it would be less able to adapt to changes such as new diseases, pests or climate.
• Harmful traits could become more common, and the plants may become weaker over time.

Cross-pollination mixes genes from two different plants and keeps the diversity higher.

Diversity would gradually decrease, because genes are mixed only within the same plant; the uniform population would be less able to adapt to diseases or environmental change.

The farmer should use asexual (vegetative) methods:

• Tissue culture (micropropagation): a few cells or a small piece of tissue from a good plant are grown on a nutrient medium to form a mass of cells (callus), which develops into many plantlets. Thousands of identical, disease-free plants can be obtained in a short time and in a small space, in any season.
• Cuttings, layering, grafting, or growing from tubers, bulbs or runners (e.g. rose and sugarcane cuttings, potato tubers, grafted mango).

These methods are effective because:

1. Only one parent is involved and cells divide by mitosis, so all the plants are genetically identical to the parent and keep its desired qualities.
2. They are faster than growing from seed and do not depend on pollination or seed formation.
3. Plants that do not produce viable seeds (such as banana) can also be multiplied.

Vegetative propagation, especially tissue culture (also cuttings, grafting, tubers), because it gives many identical copies of the parent plant quickly, without needing pollination or seeds.

(i) Possible hypotheses:

• Pollen grains need sugar to germinate (the 0% slide acts as the control).
• The percentage of pollen grains that germinate depends on the sugar concentration.
• There is an optimum sugar concentration at which germination (or the growth of the pollen tube) is maximum; very high concentrations reduce germination.
• The length of the pollen tube changes with the sugar concentration.

(ii) Parameters to keep the same (controlled variables):

• The same type (species) of flower and fresh pollen of the same age.
• The same amount of pollen and the same volume of solution on each slide.
• The same temperature, light and humidity, and the same time of observation.
• The same water used to make the solutions, and clean slides; the same method of counting germinated grains under the microscope.

Only the sugar concentration should be changed (independent variable); the percentage of germinated pollen or the pollen tube length is measured (dependent variable).

(i) e.g. sugar is needed for pollen germination; germination varies with sugar concentration; there is an optimum concentration. (ii) Same pollen source and age, amount of pollen, volume of solution, temperature, light, humidity and observation time.

Tomato: self-pollination; wheat: self-pollination; papaya: cross-pollination.

Data from Fig. 11.24:

(i) Hypotheses:

• The fall in apple yield is due to the shortage of pollinators (poor pollination).
• Keeping honey bee colonies in the orchard (beekeeping) increases pollination, so it increases fruit setting and reduces fruit drop, giving a higher yield.

(ii) Parameters:

• Independent variable (what is changed): the type of pollination, natural pollinators only (A) or with bee colonies (B).
• Dependent variables (what is measured): fruit set (%) and fruit drop (%).
• Controlled variables (should be the same): apple variety, age and number of trees, soil, irrigation, fertilisers and pest control, and the same flowering season. (Since the two places differ, factors like climate and altitude should be as similar as possible.)

(iii) Comparison: at Place B, fruit set is about 14 percentage points higher (40% vs 26%), and fruit drop is much lower (8% vs 35%). So Place B both sets more fruits and keeps more of them on the tree till they mature, giving a much higher yield than Place A.

(iv) Inference: pollination by bees greatly improves apple yield. The shortage of natural pollinators is a major reason for the falling yield, and beekeeping in orchards is an effective way to increase fruit production. It also gives the farmer honey as an extra income. Protecting natural pollinators is equally important.

(i) Lack of pollinators lowers yield; bee colonies increase pollination and yield. (ii) Independent: type of pollination; dependent: fruit set and fruit drop; controlled: variety, tree age, soil, care, season. (iii) B has higher fruit set (40% vs 26%) and lower fruit drop (8% vs 35%). (iv) Beekeeping (better pollination) increases apple yield.

The claim is not correct. Day 14 is only an average for a typical 28-day cycle; it is not fixed for everyone.

1. Cycle length varies. The menstrual cycle typically lasts anywhere from 21 to 35 days. In a shorter cycle, ovulation happens earlier (e.g. around day 7–10 in a 21-day cycle); in a longer cycle, it happens later (e.g. around day 21 in a 35-day cycle). (Ovulation usually happens about 14 days before the next period, not 14 days after the last one.)
2. It varies in the same person. Stress, illness, diet, exercise, travel and hormonal changes can shift the day of ovulation from one month to another.
3. Cycles are often irregular, especially in the first few years after puberty and before menopause, and ovulation may sometimes not happen at all in a cycle.

Incorrect. Ovulation happens around day 14 only in a typical 28-day cycle; cycle length varies (21–35 days) between people and from month to month, and stress, illness or age can shift or skip ovulation.