Reproduction: How Life Continues: Class 9 Science · Chapter 11

A neem tree may stand for a hundred years or more, and a cat may live fifteen, yet sooner or later each of them dies. Neem trees and cats as kinds of living things, though, have been around for millions of years. That is possible only because, before their lives end, living things make offspring like themselves. This process is reproduction (जनन). A species outlives every one of its members because each generation produces the next.

Compare two families. A cat's litter of four kittens has one mother and one father, yet one kitten is ginger, one is striped and one is almost black. The tiny plantlets that sprout along the edge of a Bryophyllum leaf, on the other hand, all grow into plants just like the one the leaf came from. These two pictures show the two ways living things reproduce. In asexual reproduction there is only one parent, and its young are near-perfect copies of it. In sexual reproduction, two parents each contribute half of the genetic material, so every offspring is a new mixture.

In this chapter you will see how both ways work in microbes, plants and animals, and finally in human beings. You will also see why the small differences produced by sexual reproduction matter so much for the survival of a species.

11.1 Asexual Reproduction

Many organisms reproduce without a partner. A bacterium or an Amoeba simply splits in two. A yeast cell pushes out a bud. Simple many-celled animals, such as Hydra and sponges, can grow whole new animals from their own bodies. You met some of these in earlier classes. Plants are especially good at it.

Dig up a turmeric (हल्दी) plant and you find a thick, knobbly underground stem called a rhizome. Each knob can sprout leaves and roots, so farmers plant pieces of rhizome instead of seeds. An onion bulb forgotten in a dark corner of the kitchen sends up green leaves. A Bryophyllum leaf (पत्थरचट्टा, patharchatta) that falls on damp soil grows tiny plantlets from the notches along its edge (Fig. 11.1). Each time, a new plant grows from a root, stem or leaf of the parent. No flower or seed is involved. This is vegetative propagation.

Only one parent is involved, and the new plant is built from the parent's own cells. So the new plant is genetically identical to the parent: whatever features the parent had, the new plant has too.

11.1.1 How is vegetative propagation in plants helpful in agriculture?

Suppose a farmer near Ratnagiri owns one mango tree whose fruit is unusually sweet. If she sows its seed, the seedling comes from two parents, because the flower may have received pollen from any mango tree nearby. Its fruit may be quite different, and it may take seven or eight years to bear any. If she copies the tree itself, every new tree carries exactly the same genetic material, gives the same sweet fruit, and usually starts bearing within three to five years.

Gardeners, farmers and scientists have turned this natural ability into four deliberate methods (Fig. 11.2):

• Cutting: a piece of stem with a few buds is planted in moist soil and grows roots of its own. Roses, hibiscus and sugarcane are raised this way.
• Layering: a low, bendy branch is pinned into the soil while it is still attached to the parent. Once roots form on the buried part, the branch is cut free. Jasmine and bougainvillea are often layered.
• Grafting: a shoot of the variety you want (the scion) is joined to a rooted plant of a hardy variety (the stock). The two heal into one plant that bears the scion's fruit. Mangoes, guavas and roses are commonly grafted.
• Tissue culture: a few cells from a shoot tip are grown in a sterile jar on a jelly containing nutrients. They multiply into a mass that forms hundreds of tiny plantlets.

These methods let growers produce large numbers of plants of a proven variety, all of the same quality, quickly and without waiting for seeds. They also work for plants that make few or no seeds, such as the bananas we eat. That is why so many orchards, nurseries and sugarcane fields in India are planted with copies rather than seedlings.

Plants are not the only organisms that copy themselves. Two others are easy to watch under a school microscope: yeast and bread mould.

Under a hand lens, a well-fed Hydra, a tiny animal of ponds and lakes, often has a small side branch with its own little tentacles. Like the bulges on yeast, this is a bud, and making young this way is called budding. A bud starts where cells at one spot on the parent keep dividing. In Hydra it grows a mouth and a ring of tentacles, then snaps off and lives on its own; in yeast, the bulge receives a nucleus and pinches off as a new cell (Fig. 11.3).

Moulds are another group of organisms that reproduce asexually. A forgotten orange at the back of a fruit basket, a pickle jar that was not closed properly, a day-old piece of pav: moulds turn up wherever food is damp and left alone. They do not appear out of nothing. They grow from tiny cells floating in the air, as Activity 11.3 lets you check for yourself.

Look again at your drawing. The mould is a fungus whose body is a mat of fine threads, the hyphae. Some threads grow upwards and swell at the tip. In bread mould (Rhizopus), each swollen tip is a round sac packed with spores; in Aspergillus, a common green or black mould, chains of spores sit on a rounded head (Fig. 11.4). Why does one patch of mould make millions of spores? Because nearly all of them are wasted. A spore is a speck of a cell that the faintest breeze can lift, and wherever it lands, it can only wait. On a dry wall it stays inactive; on a damp slice of bread or a ripe guava it soon sends out new hyphae.

What do a yeast bud, a mould spore, a Hydra bud and a rooted hibiscus cutting have in common? Each is built entirely by mitosis. In this kind of cell division, a cell first copies its chromosomes and then shares them equally, so each of the two daughter cells gets exactly the same set as the parent cell. Offspring made only by mitosis are therefore genetically identical to their parent; they are clones.

Asexual reproduction is quick and needs no partner, so when food and warmth are plentiful a population can grow very fast: one yeast cell can become thousands in a day. Its weakness is that all the copies share the same strengths and also the same weaknesses.

11.2 Sexual Reproduction

In sexual reproduction, a new individual begins when two special cells, one from each parent, fuse. Each parent supplies half of the new individual's genetic material. That creates a puzzle. A human body cell has 46 chromosomes. If a child received all 46 from its mother and all 46 from its father, it would have 92. Its own children would have 184, and the number would keep doubling, generation after generation. That does not happen, because the fusing cells are made by a special division that halves the number first. This division is meiosis.

11.2.1 How does meiosis help create variations in sexual reproduction?

Your genetic information is stored in chromosomes, thread-like bodies in the nucleus of each cell. Count them in a dog's body cell and you find 78; in a garden pea, 14; in a human, 46. The human 46 are really 23 matched pairs: one member of each pair came from the mother's egg and the other from the father's sperm.

Biologists use two words here. A body cell, with both members of every pair, is diploid (double). A cell with only one member of each pair is haploid (single). Meiosis turns a diploid cell into haploid cells, and these are the gametes: sperm in male animals and eggs in female animals. Flowering plants make gametes too: the male gametes travel inside pollen grains, and the egg waits inside an ovule in the flower. A gamete has one job, to fuse with a gamete from the other parent. In humans, every sperm and every egg carries 23 chromosomes, and the cell they form has 46 again (Fig. 11.5).

Meiosis does more than halve the number. Which chromosome of a pair goes into a particular gamete is a matter of chance, and it is decided separately for every pair. One sperm may get the mother's copy of chromosome 1 and the father's copy of chromosome 2; the next may get the opposite. How many different gametes can one person make?

This shuffling is why two children of the same parents are never alike in every way, unless they are identical twins, who come from a single zygote. Differences between individuals of a species are called variations, and they are vital for its survival. When a drought, a new disease or a change in climate arrives, a few individuals may happen to carry a combination of features that helps them survive and reproduce. Their offspring inherit it, and over many generations the species changes. This slow change is evolution. A field of wheat grown from seed, for example, often holds a few plants that resist a new kind of rust fungus, and plant breeders collect exactly such plants.

11.2.2 Sexual reproduction in flowering plants

Most of the plants around you, from the grass on the playground to the banyan tree by the road, belong to one huge group: the flowering plants, or angiosperms, with nearly three lakh known species. For them, the flower is the organ of sexual reproduction. Cone-bearing plants such as pines reproduce sexually without flowers; the next chapter returns to them.

Pick a hibiscus bud and an open hibiscus flower. Wrapped tightly around the outside of the bud are green, leaf-like flaps, the sepals. They shield the delicate parts inside while the bud grows. When the flower opens, its large coloured petals show. Their colour, and in many flowers a scent and sweet nectar, draw insects and birds to the flower. Look further in and you find two more kinds of parts (Fig. 11.7).

A flower with all four whorls, like a hibiscus, is a complete flower; in some flowers parts are joined, as the petals of a sadabahar form a narrow tube. Start at the centre, with the pistil, the female part. Its swollen base, the ovary, holds one or more ovules, and each ovule contains an egg cell, the female gamete. A stalk, the style, rises from the ovary and ends in the stigma, which is often sticky, hairy or feathery so that pollen grains cling to it. Around the pistil stand the stamens, the male parts. Each is an anther, a small bag that makes pollen grains carrying male gametes, held up on a thin filament.

For a flower to turn into a fruit, pollen from an anther has to reach a stigma. Can you show that this transfer is really needed?

11.2.3 How does the process of pollination occur in flowers?

Pollen can reach a stigma in two ways (Fig. 11.8). In self-pollination, pollen lands on the stigma of the same flower or of another flower on the same plant, so both parents are really one plant. In cross-pollination, the pollen is carried to a flower on a different plant of the same kind. Cross-pollination mixes the genetic material of two plants, so it gives more varied seeds. Pollen from a different kind of plant usually does nothing: a mango stigma ignores pollen from a guava.

Without pollination there are no seeds, and usually no fruit. A pumpkin vine that no bee visits drops its female flowers without forming a single pumpkin.

11.2.4 Pollination strategies and reproductive success

Plants cannot walk to each other, so they depend on carriers of pollen, called pollinators: wind, water, insects and birds, and even bats. A flower's shape, colour and pollen suit its carrier, as Table 11.7 shows.

11.2.5 Fertilisation and seed formation

Landing on a stigma is only the start. What happens next is hidden inside the pistil (Fig. 11.9):

1. The pollen grain soaks up water and sugar from the stigma and sends out a thin pollen tube.
2. The tube tunnels down through the style, the way a root works through soil, and enters an ovule.
3. A male gamete slides down the tube and fuses with the egg cell in the ovule. This joining of a male and a female gamete is fertilisation.
4. The fused cell, the zygote, has complete chromosome pairs again. It divides many times to become the embryo, a miniature plant with a tiny root and shoot.

Look at a pea plant a week after its flowers fade (Fig. 11.10). The petals and stamens have dropped off, but the ovary has stretched into a green pod. Inside, each fertilised ovule has become a seed: an embryo and a food store wrapped in a seed coat. The pod itself, the ripened ovary, is the fruit. In brinjal and guava, the old sepals stay attached to the fruit.

A fruit's job is to get its seeds away from the parent plant, whether on the wind, on water or inside an animal that eats it. A seed that reaches damp soil at a suitable temperature sprouts (germinates) and becomes a seedling. Since no two seeds come from quite the same pair of gametes, the seedlings differ slightly, and that gives the species more chances when conditions change.

11.3 Sexual Reproduction in Animals

Some animals can reproduce asexually, as you saw with budding in Hydra. Most animals, though, reproduce sexually, and their first challenge is to bring sperm and egg together. There are two solutions (Fig. 11.11).

Frogs and most fish breed in water. As the female lays her eggs, the male sprays his sperm over them, and the gametes meet in the water itself: this is external fertilisation. It works only in water, where sperm can swim to the eggs, and it is wasteful. Currents sweep eggs away, fish and insects eat them, and many are never reached by a sperm. So these animals produce eggs in enormous numbers.

Reptiles, birds and mammals breed on land, where sperm could not swim from one animal to another. The male passes sperm into the female's body and the eggs are fertilised inside her: this is internal fertilisation. Afterwards the young are guarded, by a tough shell in reptiles and birds or by the mother's own body in mammals. Far more of them survive, so fewer eggs are needed.

11.4 Variations in Reproduction in Animals

Look at Table 11.9 and a trade-off appears: the animals that make the most eggs look after them least, and lose the most.

Making eggs costs food. A frog or fish laying thousands of eggs cannot pack each one with much yolk, the store of food in an egg. So its eggs hatch early into small, simple larvae whose main job is to eat. A tadpole grazes on algae, a caterpillar munches leaves, and a housefly maggot feeds on rotting waste. When a larva has built up enough food, its body is rebuilt into the adult form, a change called metamorphosis (Fig. 11.12).

Reptiles and birds lay fewer eggs, but each one holds enough yolk to feed the embryo until it hatches as a small version of the adult. In mammals, the embryo grows inside the mother's body and is fed by her.

Compare a hen's chick, which pecks for grain a few hours after hatching, with a pigeon chick, which hatches blind and bald and is fed by both parents for weeks. Mammals all nurse their young on milk, some for a few weeks and some, like elephants, for years. Broadly, young that do more of their developing inside the egg or the mother come out more ready to fend for themselves.

11.5 Reproduction in Human Beings

Humans reproduce sexually, with internal fertilisation, and the baby grows inside its mother, as in other mammals. Here you will meet the organs involved, follow a sperm and an egg from where they are made to where they join, and see how a baby develops. You will also see how to keep this part of life healthy and safe.

11.5.1 Reproductive maturity

Earlier classes covered the many ways adolescents change, in body and in feelings. Among those changes, the testes in boys and the ovaries in girls become active and start making gametes: sperm and eggs. The stage at which this begins is called puberty. It has no fixed date. It may begin anywhere between about 8 and 14 years of age, and each person's timing is normal for that person.

Inside the female body, a sperm and an egg can fuse to form a zygote. The zygote grows into an embryo in the uterus. Once its main organs have formed, the developing baby is called a foetus.

11.5.2 What are the parts of the male reproductive system?

A sperm is one of the smallest cells in the human body. Under a microscope it looks like a tiny tadpole: an oval head packed with the father's genetic material, and a whip-like tail that drives it forward. The male reproductive system makes such cells in huge numbers and delivers them into the female body (Fig. 11.13).

Sperm are made inside the two testes (singular: testis). Unusually for important organs, the testes hang outside the abdomen, inside a skin bag, the scrotum. The reason is temperature: sperm develop properly only at about 2–3 °C below normal body temperature. The testes are also glands. They release the hormone testosterone (a hormone is a chemical messenger carried by the blood), which keeps sperm production going and brings about the changes of puberty in boys, such as a deeper voice and facial hair.

Now follow a sperm out of the body. It leaves a testis through a long tube, the vas deferens, which loops up into the abdomen. Near the bladder, two kinds of gland, the seminal vesicles and the prostate gland, pour in a fluid that feeds the sperm and lets them swim. The vas deferens then joins the urethra, the tube that runs through the penis. The urethra carries urine at other times, but never urine and sperm together.

11.5.3 What are the parts of the female reproductive system?

Trace the path an egg takes and you meet every part of the female system (Fig. 11.14). It starts in one of the two ovaries, almond-shaped organs low in the abdomen that store eggs. The ovaries also release female hormones, such as oestrogen, which bring about the changes of puberty in girls. A released egg is drawn into the funnel-shaped end of an oviduct (fallopian tube) and moves along it to the uterus, a hollow, muscular organ about the size of a fist with a soft inner lining; this is where a baby develops. The narrow lower neck of the uterus, the cervix, opens into the vagina, a muscular passage to the outside of the body.

11.5.4 How are reproductive cells made?

Sperm and eggs are both gametes, yet they could hardly be more different (Table 11.10). A man's testes turn out millions of tiny, swimming sperm every day. A woman's ovaries usually release a single egg a month, but it is far larger and comes with a supply of food for the embryo's first days. This pattern, many small mobile male gametes and few large still female ones, runs through most of the animal kingdom.

Both kinds are made by meiosis, the halving division of Section 11.2.1, and making gametes is called gametogenesis. So every sperm and every egg carries 23 chromosomes, and the zygote they form has the 46 of a normal human body cell.

11.5.5 What happens when a sperm meets an egg?

A girl is born with all the eggs she will ever have, about 10 to 20 lakh immature ones, and makes no new eggs later. From puberty onwards, roughly once a month, one egg ripens in one ovary or the other and is set free. Follow that egg (Fig. 11.15):

1. Ovulation. The ripe egg bursts out of the ovary and is drawn into the oviduct. The uterus has already begun to thicken its lining, as if expecting a guest. This release of an egg is ovulation.
2. Meeting. After sexual intercourse, sperm placed in the vagina swim up through the uterus. Out of millions, only a few hundred reach the oviducts. If an egg is there, one sperm can enter it and their nuclei fuse: this is fertilisation, and in humans it happens in the oviduct.
3. Journey. The zygote divides by mitosis again and again while it drifts towards the uterus, becoming a tiny ball of cells.
4. Implantation. About a week after fertilisation, the ball of cells reaches the uterus, now lined with a thick, soft, blood-rich layer, and sinks into it. This is implantation, and pregnancy begins at this moment.

11.5.6 What happens when an egg is not fertilised?

In most months no sperm arrives, and the unfertilised egg survives only about a day before it breaks down. The thick, blood-rich lining of the uterus, built up to welcome an embryo, then has no use and breaks down too. Over the next 3 to 7 days the old lining leaves through the vagina together with some blood: this is menstruation, usually called a period. Meanwhile a new lining begins to grow, another egg starts to ripen, and the cycle starts over (Fig. 11.16).

This menstrual cycle usually repeats every 21 to 35 days; 28 days is a common average, and few people are exactly regular. Periods usually begin between about 10 and 14 years of age and stop at menopause, usually between about 45 and 55. Day 1 of a cycle is the first day of a period. Ovulation happens about 14 days before the next period begins, so it falls around day 14 in a 28-day cycle but around day 18 in a 32-day cycle.

11.5.7 Pregnancy and childbirth

A human pregnancy lasts about nine months, or about 40 weeks counted from the first day of the last period. Doctors divide it into three trimesters of about three months each (Fig. 11.18).

• First trimester (months 1–3). The heart starts beating within about six weeks, and the brain, spine and other major organs begin to form, mostly in the first two months. From about week 9, the growing baby is known as a foetus.
• Second trimester (months 4–6). The foetus grows quickly. Around the fifth month the mother usually starts to feel it move.
• Third trimester (months 7–9). The foetus gains weight fast, its lungs mature, and it usually turns head-down, ready for birth.

All this time the uterus shelters the foetus, and the mother's body supplies its food and oxygen. When birth begins, the muscular wall of the uterus squeezes hard, again and again, and pushes the baby out through the cervix and vagina, the birth canal. If a normal birth would put the mother or baby at risk, doctors can deliver the baby by an operation called a caesarean section.

• First milk. Breastfeeding should begin within an hour of birth. For the first six months, breast milk alone gives the baby all the food and water it needs, and antibodies that guard it against infections.
• Warmth and gentle care. Newborns lose heat quickly; skin-to-skin contact with the mother helps keep them warm.
• Vaccines on time. The first vaccines are given at birth, and the rest follow the dates on the vaccination card.
• Care for the mother. Recovering from birth takes weeks. She needs good food, rest and help at home, and should stay away from tobacco, alcohol and medicines not prescribed by a doctor, which can reach the baby through her milk.

11.5.8 Mother's health during pregnancy

Anaemia, too little iron in the blood, is common among pregnant women in India, and it can make a baby small and weak at birth. So a pregnant woman needs foods rich in iron, protein, calcium and folic acid, such as green leafy vegetables, pulses, milk and eggs, and doctors give her iron and folic-acid tablets as well. Several check-ups at a health centre (at least four are advised) catch problems early, and her doctor tells her how much rest and light exercise suit her. Worry and loneliness affect health too, so a family that shares the work and listens to her helps keep both mother and baby well.

11.5.9 What does it mean to be sexually mature?

A 14-year-old may be tall enough to reach the pedals of a car, but that does not make them ready to drive in city traffic. Sexual maturation is similar. During adolescence the body gradually becomes able to reproduce, as boys begin to produce sperm and girls begin their menstrual cycles. Readiness for what can follow, a relationship, a pregnancy, a child, depends on emotional maturity, and that grows more slowly, through the late teens and beyond. It shows in how a person handles strong feelings, says no to pressure, talks things through and weighs the consequences before acting.

11.5.10 How can unwanted pregnancies and infections be prevented?

Two risks come with sexual activity: infections and unplanned pregnancy. Knowledge and responsible choices can prevent both.

Some infections pass from person to person through sexual contact. These are sexually transmitted infections (STIs). They include gonorrhoea and syphilis, caused by bacteria, and genital herpes, genital warts and HIV, caused by viruses. HIV weakens the body's defences and, without treatment, leads to AIDS. Bacterial STIs can be cured with medicines, but some viral ones, including HIV, cannot yet be cured, only controlled. A condom forms a barrier that greatly lowers the risk of passing on an STI, and it also prevents pregnancy.

Methods that prevent pregnancy are called contraception. They work in different ways (Table 11.11).

Ending a pregnancy by a medical procedure is called an abortion (in law, a medical termination of pregnancy). It is safest early in pregnancy, usually within the first trimester, and in India it is allowed only under conditions set by law and only by trained doctors.

Some families have misused medical scans to learn whether an unborn baby is a girl, and then ended the pregnancy. Such sex-selective abortion has left fewer girls than boys in parts of India. To stop it, the law (the PC-PNDT Act of 1994) forbids revealing the sex of an unborn baby. A balanced sex ratio matters for a fair and healthy society, and every girl has an equal right to be born.