Reproduction: How Life Continues

Why This Matters

Every living thing has a clock ticking. A mango tree grows old and one day it dies. A cow, a dog, a sparrow, a person — each of us is born, grows up, and in the end passes away. So here is a strange question: if every single living thing dies, why is the Earth still full of life?

The answer is one word — reproduction. Before they die, living things make new individuals of their own kind. The old mango tree drops seeds, and those seeds grow into young mango trees. The cow gives birth to a calf. In this way life is handed on, like a relay race where one runner passes the baton to the next before stopping. The runners change, but the race never ends.

Reproduction is not just about making copies. It is also the reason a child is not an exact copy of either parent. Some children are taller, some can run faster, some can digest milk as adults while others cannot. These small differences, built up over thousands of generations, are what let living things survive when the world changes around them.

So this chapter answers two big “how” questions. How do living things make new ones? And how do those new ones end up a little different, in a way that keeps a whole species alive? Let us find out.

The Big Idea

The Big Idea: Reproduction is how living things make new individuals of their own kind, so that life carries on after each individual dies. It happens in two main ways. In asexual reproduction a single parent makes offspring that are near-exact copies (clones). In sexual reproduction two parents each give half their genes, so the offspring is a fresh mix — and this mixing creates the variation that helps a species adapt and survive. Flowers are the reproduction machines of flowering plants, and in animals and humans the same core idea plays out with eggs, sperm and fertilisation.

Let’s Break It Down

Before we start, let us be clear about two words we will use again and again — gene and chromosome. They are at the heart of this whole chapter.

Living things reproduce in two main ways. Figure 11.1 below sorts the asexual methods you will meet, and shows the one thing they all share, so you have the whole map before we zoom into each one.

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Asexual Reproduction — One Parent, Exact Copies

In asexual reproduction there is only one parent. No second parent is needed. The single parent makes new individuals that carry the same genes as itself. We call these near-identical copies clones.

This way of reproducing is common in simple living things — bacteria, amoeba, yeast, hydra, sponges — and in many plants too. Let us look at the different forms it takes.

Vegetative propagation in plants

Many plants can grow a whole new plant from a part of themselves — a piece of stem, a leaf, or a root. No flower, no seed needed. Because the new plant comes from the ordinary growing (“vegetative”) parts of the parent, this is called vegetative propagation.

You have probably seen it without realising:

• A potato has tiny “eyes”. Plant the potato and each eye sprouts a new plant.
• A piece of ginger or sugarcane stem, pushed into soil, grows into a full plant.
• A money plant stem cutting placed in water grows roots and becomes a new plant.
• A Bryophyllum leaf grows tiny plantlets right along its edge, which drop off and grow.

The key point: there is just one parent, so every new plant is genetically identical to it.

Farmers and gardeners have turned this natural trick into useful methods. Three common ones are cutting, grafting and layering.

• Cutting — a piece of shoot is cut, its lower leaves removed, and it is pushed into compost-rich soil at a slant. It grows roots and becomes a new plant.
• Grafting — a stem piece from a plant with desirable flowers or fruit (Plant B) is fitted into a slit on a strong, well-rooted plant (Plant A). The two heal together, and Plant B’s branch grows on Plant A’s roots. This is how one tree can carry a chosen variety of rose or mango.
• Layering — a low, flexible twig is bent down and its middle is buried in soil while still attached to the parent. After a couple of weeks roots grow from the buried part. The rooted twig is then cut off to live as a new plant.

A more modern method is tissue culture: scientists take a tiny piece from the growing tip of a plant and grow thousands of identical healthy plantlets in the lab. This has transformed banana farming — farmers get mass-produced, virus-free plants that give high yields.

Here is why a farmer might want all this. Asexual methods give genetically identical plants. So if you find one mango tree with perfect, sweet fruit, you can copy it exactly, thousands of times, and every tree will give that same sweet fruit. Growing from seeds would shuffle the genes and you could not be sure. Asexual propagation is also faster — you skip the whole flower-pollination-seed journey.

Budding — a small copy grows out of the parent

In some organisms, cells divide again and again at one particular spot on the parent’s body. This makes a small bulge that grows into a small copy. This bulge is called a bud, and the process is budding.

You can see it in two very different living things — yeast (a single cell) and hydra (a small water animal made of many cells). Figure 11.2 walks through both, stage by stage.

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In yeast, the bud is a new cell that finally pinches off. In hydra, you can often see several buds growing on one parent at the same time — each will become a new hydra.

Spore formation — millions of tiny travellers

Have you ever left bread or a roti out for a few days in warm, damp weather? It grows fuzzy patches — that is mould, a kind of fungus. Where did it come from? The bread was clean when fresh.

The answer is spores. A spore is a tiny, light, usually single-celled package that a fungus makes to reproduce. Spores are produced in huge numbers — millions from one little mould colony. They are so light that they float in the air all around us. When a spore lands somewhere warm and moist, like wet bread, it germinates and grows into new mould. So the mould did not appear from nowhere; its spores were already in the air and just needed a place to grow.

Long ago, people believed living things could appear on their own from non-living matter — that maggots simply “arose” from rotting meat.

The French scientist Louis Pasteur proved this wrong. He showed that new life always comes from already-living things. This supported the idea that all cells come from earlier cells, and it led to practices like boiling and sterilising to keep food and instruments germ-free.

Fission — splitting in two

The simplest asexual method of all is fission. A single-celled organism like amoeba simply divides into two. The parent cell splits, and now there are two complete cells where there was one. Each is a full living individual.

The thread that ties them together: mitosis

Look back at all these methods — vegetative propagation, budding, spore formation, fission. They look different, but underneath they all use the same kind of cell division: mitosis.

Mitosis is cell division that produces two daughter cells, each with the same number of chromosomes, identical to the parent. Because the chromosomes (and so the genes) are copied exactly, the offspring are genetically identical to the parent. This is the answer to the “why” — the offspring are clones because mitosis copies the genes exactly, with no second parent to mix anything in.

This method is fast and lets organisms fill up a space quickly when conditions are good. But notice the trade-off: there is no variation. Every clone has the same strengths and the same weaknesses. If a new disease can kill one of them, it can kill them all. That weakness is exactly the problem the next method solves.

Sexual Reproduction — Two Parents, Endless Variety

Sexual reproduction needs two parents. Both contribute genetic material to the offspring. The child inherits a mix from the mother and the father. This mixing is the whole point — it creates variation.

But there is a puzzle hidden here. If each parent gave the child a full set of chromosomes, the child would have double. Humans would have 46 + 46 = 92, and the next generation 184, and so on. The number would explode. That cannot be right — every human has 46. So how is the number kept steady?

How meiosis keeps the chromosome number steady (and creates variation)

Nature solves this with a special cell division called meiosis. Meiosis makes the special reproductive cells called gametes — the sperm (male) and the egg (female) in animals; in plants the pollen grain carries the male gamete and the ovule holds the female gamete (egg).

Here is the clever part. Meiosis halves the chromosome number. A human body cell has 46 chromosomes (we call this diploid — a full double set). Meiosis makes gametes with only 23 (we call this haploid — a single half-set). During meiosis, the two chromosomes of each pair separate, and each gamete gets only one from each pair.

Then, when a sperm (23) fuses with an egg (23), the child gets 23 + 23 = 46 again — the right number. Problem solved.

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Now for the variation. When meiosis separates each pair, it picks one chromosome from each pair at random. Let us count how many different gametes that can make. Activity 11.4 in your book uses coloured beads to show this, and the worked example below makes the counting clear.

So why does this variation matter so much? Because the world keeps changing — climate shifts, new diseases appear, food sources move. In a population full of variety, some individuals will, by luck, have a feature that helps them cope. Maybe some people can tolerate low oxygen high in the mountains. Maybe some can digest milk as adults. Those individuals survive and pass on their genes. Over many, many generations, this slow process shapes living things to fit their world — we call it evolution. Variation is the raw material that lets a species adapt and survive. Clones cannot do this, because they are all the same.

Sexual Reproduction in Flowering Plants

Flowering plants, called angiosperms, are the most varied group of plants on Earth. Their reproductive organ is the flower. A flower is not just pretty — it is a machine built to make seeds.

The parts of a flower

A complete flower has four parts, arranged in rings (whorls) from outside to inside. Figure 11.4 shows a flower cut down the middle so you can see them all.

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Let us name them from outside in:

• Sepals — the green outer covering. In the bud stage they wrap around everything and protect it.
• Petals — the coloured, often fragrant part. Their job is to attract insects and birds.
• Stamen — the male part. Each stamen has a stalk (the filament) and a head (the anther). The anther makes pollen grains, which carry the male gametes.
• Pistil — the female part, in the centre. It has three sub-parts: the stigma (sticky tip on top), the style (a thin tube), and the ovary (the swollen base). Inside the ovary are ovules, and each ovule holds an egg cell (the female gamete).

Pollination — getting the pollen to the stigma

For a seed to form, pollen from the anther must reach the stigma. This transfer is called pollination.

How do we know pollination is really needed? An activity in your book proves it. Take pea flowers. Remove the stamens (the source of pollen) from one flower bud before it opens, and wrap it in cloth so no outside pollen can reach it. Compare it with normal flowers. Result: fruits form in every flower except the one whose stamens were removed. No pollen on the stigma means no fruit. So pollen transfer to the stigma is essential for fruit formation.

There are two kinds of pollination:

• Self-pollination — pollen lands on the stigma of the same flower, or another flower on the same plant.
• Cross-pollination — pollen is carried to the stigma of a flower on a different plant of the same kind.

Cross-pollination is more useful for variation, because it mixes genes from two different plants.

But flowers cannot walk. So how does pollen travel, especially across to another plant? Nature uses helpers called pollinators — wind, water, insects and birds. Figure 11.5 shows self- and cross-pollination, and what happens next.

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Different plants use different pollinators, and their flowers are shaped to suit:

• Wind carries pollen in wheat, maize and rice. Their pollen is light, small and made in huge numbers, and the stigma is long and feathery to catch passing grains.
• Water carries pollen in aquatic plants like Vallisneria and Hydrilla.
• Insects (bees, butterflies) pollinate sunflower, hibiscus and marigold. These flowers are brightly coloured, give off scent, and make nectar to lure insects. Their pollen is large, sticky or spiny, so it clings to the insect’s body, and the stigma is sticky too.
• Birds like sunbirds pollinate flowers such as the coral tree.

This explains a neat fact. Wind-pollinated plants release far more pollen per flower (5–10 lakh) than insect-pollinated ones (20–40 thousand), yet often form fewer seeds. Why make so much pollen? Because wind is random — most grains land in useless places. Producing a huge cloud of pollen makes it likely that at least some grains reach a stigma. Insects, by contrast, carry pollen straight from flower to flower, so less pollen is wasted and fewer grains are needed.

Fertilisation and seed formation

Once a pollen grain lands on a matching stigma, something remarkable happens. The pollen grain grows a pollen tube that pushes down through the style into the ovary. The male gamete slides down this tube and reaches an ovule, where it fuses with the egg cell. This fusion of the two gametes is called fertilisation — the start of a new life.

After fertilisation, three changes happen together:

• The fertilised egg (the zygote) develops into an embryo — the baby plant.
• Each ovule becomes a seed (with the embryo inside).
• The ovary grows into a fruit around the seeds.

So next time you eat a mango, remember: the juicy part was the flower’s ovary, and the stone inside holds the seed. The seed is later spread by wind, water or animals. When it lands somewhere with water, air and warmth, it germinates and grows into a new plant.

Sexual Reproduction in Animals

Animals reproduce too, and many use sexual reproduction. They all face the same basic challenge: the male and female gametes must meet, and the young must survive long enough to grow up and reproduce. Animals solve this in different ways.

Two ways the gametes meet: external and internal fertilisation

• External fertilisation — the gametes meet outside the body. In many water animals like frogs and most fish, the female releases eggs into the water and the male releases sperm over them. Fertilisation happens in the water. The trouble is that the eggs are exposed — water currents wash many away, and other animals eat them. So very few survive.
• Internal fertilisation — the gametes meet inside the female’s body. Reptiles, birds and mammals use this. The fertilised egg or embryo is protected inside, so the chances of survival are higher.

This explains a puzzle: why do fish and frogs lay thousands of eggs, while birds and mammals have only a few young? Because external fertilisation is so risky, fish and frogs play a numbers game — lay a huge number so that even if most are lost, a few survive. Birds and mammals protect each young one well, so they need only a few.

How young animals are nourished

How does a developing animal get food before it can feed itself? It depends on the animal.

• Fish, amphibians (frogs) and insects put a little food store, called yolk, in each egg. But the mother cannot pack enough yolk for thousands of eggs. So the yolk is just enough to make a larva — an in-between feeding stage that hatches and then eats organic waste (rotting food, manure) to grow. Later it transforms into the adult. A caterpillar turning into a butterfly is exactly this.
• Reptiles and birds lay fewer eggs, and each egg has enough yolk to feed the embryo until it hatches as a young one.
• Mammals keep the zygote inside the mother’s body, where it grows and develops, fed directly by her. After birth, mammal mothers feed their young on milk.

So across the animal world, the broad pattern is: lots of eggs with little care, or few young with lots of care.

Reproduction in Human Beings

Humans are mammals, and we reproduce sexually with internal fertilisation. As a child grows into an adult during puberty, the reproductive organs mature and start making gametes — sperm in males, eggs in females. Figure 11.6 shows both systems in a simple form.

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The male reproductive system

The male system makes sperm and delivers them. Sperm are made in two oval testes, held in a skin pouch called the scrotum. The scrotum hangs outside the body to keep the testes a little cooler than body temperature, which sperm need to form properly. The testes also make a hormone (a chemical messenger) that controls sperm production and causes the changes boys go through at puberty.

From the testes, sperm travel along a long tube, the vas deferens, which joins the urethra (the same passage that carries urine). Glands such as the seminal vesicles and prostate add fluids that nourish the sperm and help them move. Each sperm has a head holding its genetic material and a long tail for swimming towards the egg.

The female reproductive system

The female system has a pair of ovaries, two oviducts (fallopian tubes), a uterus, and a vagina. The ovaries make the eggs and release hormones that bring about the changes of puberty. The oviducts connect each ovary to the uterus. The uterus is a bag-like organ where a baby develops. It opens into the vagina through a narrow passage called the cervix.

How gametes are made, and how different they are

Making gametes is called gametogenesis, and it happens in the testes and ovaries. As you learnt earlier, gametes are made by meiosis, which halves the chromosome number. Human body cells have 46 chromosomes, but sperm and eggs have only 23. So when sperm and egg combine, the zygote has the full 46 again — the same as the parents.

Male and female gametes are very different in size and number.

This pattern — many small moving sperm, a few large still eggs — is seen across most animals.

What happens when sperm meets egg

At birth, a girl’s ovaries already hold millions of immature eggs. From puberty onward, usually one mature egg is released each month from an ovary. This release is called ovulation. Around the same time, the inner lining of the uterus thickens, getting ready to receive a baby.

The released egg travels into the oviduct. During reproduction, millions of sperm enter and swim up the tract. If a sperm reaches the egg in the oviduct and fuses with it, a zygote forms. The zygote divides by mitosis as it travels to the uterus and then implants into the thick uterus lining, which now feeds it. This implantation marks the start of pregnancy.

What happens when the egg is not fertilised — the menstrual cycle

If the egg is not fertilised, it survives about a day and then breaks down. The thick, blood-rich uterus lining, prepared to nourish a baby, is no longer needed. So the lining sheds and leaves the body through the vagina along with some blood. This is called menstruation (a “period”), and it usually lasts 3 to 7 days.

This whole sequence — egg ripens, uterus lining thickens, no fertilisation, lining sheds — repeats roughly every 21 to 35 days (often about 28). It begins at puberty (around ages 10–14 in girls) and continues until menopause (around age 50). Figure 11.7 shows the stages of a typical 28-day cycle.

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Menstruation is a normal, healthy sign — not something to be ashamed of. During a period, use clean menstrual products (pads, cups), change them every 4 to 6 hours, wash hands before and after, and dispose of used products wrapped in paper in a bin — never flush them.

Pregnancy and childbirth

Pregnancy in humans lasts about nine months, split into three stages called trimesters. In the first trimester the fertilised egg becomes an embryo and the main organs start forming; from about the ninth week the developing baby is called a foetus. In the second trimester it grows bigger and the mother can feel it move. In the third trimester it grows fast and gets ready for life outside. Throughout, the uterus protects and feeds the baby. At childbirth, strong contractions of the uterus muscles push the baby out through the birth canal. If a normal birth is not safe, doctors may use medical or surgical help to deliver the baby safely.

After birth, the baby needs care. Breastfeeding is important because the mother’s milk gives complete nutrition and protects the baby from many diseases. The mother’s health matters too — she needs good food, rest, and should avoid smoking, alcohol and medicines taken without advice.

Growing up: maturity and responsible choices

During adolescence the body becomes physically capable of reproduction. But emotional maturity — handling feelings, thinking clearly, making careful decisions — takes longer to develop. Being physically able is not the same as being ready for adult responsibilities.

Because close physical contact can pass on infections, some illnesses spread this way are called Sexually Transmitted Infections (STIs) — such as gonorrhoea, herpes, syphilis and HIV (which can lead to AIDS). Some are not yet curable. Using condoms helps prevent both STIs and pregnancy.

To prevent unwanted pregnancy, several contraceptive (pregnancy-preventing) methods exist: barrier methods like condoms that stop sperm reaching the egg; oral pills that change hormones to alter egg release; and devices like the copper-T placed in the uterus. Surgical methods can block the vas deferens in males or the fallopian tubes in females.

In India, finding out a baby’s sex before birth (prenatal sex determination) is strictly banned by law. This is because choosing to end a pregnancy based on the baby’s sex badly distorts the ratio of boys to girls in society. The law protects a healthy, balanced society.

Common Mistakes

These are the slip-ups students most often make. Read each one and fix it in your head now.

Quick Check

Try these before the practice problems. Read the explanation even if you get it right.

Practice Problems

Try each one yourself first, then reveal the full solution.

Easy

Medium

Challenge

Summary

After this chapter, you can now explain:

• Why living things reproduce — to make new individuals of their own kind so that life continues after each one dies.
• The two types of reproduction — asexual (one parent, clones, by mitosis) and sexual (two parents, variation, using meiosis to make gametes).
• The forms of asexual reproduction — vegetative propagation (cutting, grafting, layering, tissue culture), budding (yeast, hydra), spore formation (moulds), and fission (amoeba) — and why they all give identical clones.
• Why sexual reproduction creates variation, why meiosis halves the chromosome number, and why variation helps a species adapt and survive.
• Reproduction in flowering plants — the four parts of a flower, the difference between pollination and fertilisation, self- vs cross-pollination, and how the ovule becomes the seed and the ovary becomes the fruit.
• Reproduction in animals and humans — external vs internal fertilisation and why egg numbers differ, the male and female reproductive systems, how gametes are made by meiosis, the menstrual cycle, and the basics of pregnancy and responsible, healthy choices.

What’s Next

You have now seen the huge variety of ways living things reproduce — from a single splitting amoeba to a flowering mango tree to a human family. That variety raises a natural next question: with millions of different kinds of living things on Earth, how do scientists make sense of them all? In the next chapter, Chapter 12 — Patterns in Life: Diversity and Classification, you will learn how living things are grouped and named, so that this vast living world becomes something we can study, compare and understand.