Class 9 Science Chapter 11: Reproduction: How Life Continues

Reproduction involving only one parent, so the offspring are genetically identical to it. Seen in bacteria, amoeba, yeast, hydra, sponges and many plants.

New plants arising from the vegetative parts, that is, the growing parts of a plant, rather than from seeds. Potato, ginger, money plant, sugarcane and Bryophyllum are examples.

Repeated cell division at a specific site produces a small outgrowth called a bud, which enlarges and separates from the parent to live independently. Seen in yeast and in hydra.

A lightweight, usually single-celled body produced in huge numbers by a mould. Spores float on air currents and germinate quickly into a new individual given moisture and nutrients.

An offspring genetically identical to its parent, produced because asexual reproduction rests on mitosis, which gives daughter cells the same chromosomes as the parent cell.

It means that new plants arise from the vegetative parts, that is, from the growing parts of a plant. Potato and ginger sprout new plants from fleshy underground stems without producing seeds; money plant and sugarcane stem cuttings grow into new plants; and Bryophyllum leaves sprout tiny plantlets which eventually grow into new plants. Since only one parent is involved, the individuals produced are genetically identical.

Because asexual reproduction produces genetically identical individuals, so a desirable plant can be multiplied exactly. Scientists and horticulturists have developed cutting, grafting, layering and tissue culture from the natural process. These methods propagate plants efficiently, have significantly improved agricultural and horticultural practices, and enable farmers to cultivate desirable crops on a large scale.

Overgrown branches are cut at the end of the plant's growing season, and the cuttings prepared from them, their length and their nodes and internodes being noted. The shoot cuttings are collected in the morning, leaves are removed from the lower half of each. They are inserted about half their length into soil mixed with compost at an angle of about 45 to 60 degrees from the surface, then watered regularly.

In cutting, a piece of shoot is separated from the parent first and then planted in soil, where it must form roots of its own. In layering, the middle part of a flexible thin twig is buried under the soil while the twig is still attached to the parent. Only after roots develop there, in about 10 to 15 days, is the twig cut from the parent so that it can grow as a new plant.

A healthy rooted plant, Plant A, such as a wild rose variety, supplies the roots, while a healthy stem piece from another plant, Plant B, such as a yellow or pink rose, supplies the shoot. A wound or slit is made on a twig of Plant A, the Plant B cutting is inserted and fitted into it, and the slit is protected with cotton cloth or wrapping film against pests until it heals. The other branches of Plant A are cut, and the plant is watered regularly.

At one particular place on the parent's body the cells divide again and again, raising a little knob called a bud; the knob grows, breaks away, and thereafter lives on its own. It is seen in yeast, where small round outgrowths appear on the parent cells under a microscope. It is also seen in hydra, a multicellular animal, where many buds may often be growing on the parent's body at the same time.

It grows from spores already present in the air, which settle on the moist bread and begin reproducing rapidly. It was not present when the bread was fresh. This agrees with what Louis Pasteur's experiments proved: that new life always comes from pre-existing life, and not spontaneously from non-living matter.

Spores are made either inside a closed sac, or on a swelling carried at the end of a long strand of hypha. They come in enormous quantity — a single patch of mould releases them by the million — and since each is light and generally just one cell, the air carries them without difficulty; give one damp and something to feed on. It is a new individual in short order. Since mould spores need warmth as well as moisture, and lower temperatures slow or stop their reproduction, refrigerating perishable food prevents them from spoiling it.

Many plants sprout new shoots and roots from their existing parts. Potato and ginger, with fleshy underground stems, sprout new plants without producing seeds; money plant and sugarcane stem cuttings grow into new plants; and Bryophyllum leaves sprout tiny plantlets. Every one of these is vegetative propagation: the new plant has come out of the old one's vegetative structures, out of the parts that grow. Since only one parent is involved, the individuals produced are genetically identical.

Scientists and horticulturists have adapted the process into four methods.

In cutting, overgrown branches are cut at the end of the growing season. The cuttings are collected in the morning, leaves are removed from the lower half of each, and they are inserted about half their length into soil mixed with compost at an angle of about 45 to 60 degrees, then watered regularly.

In grafting, a healthy rooted plant supplies the roots and a stem piece from another plant the shoot. A slit is cut on a twig of the rooted plant, the other piece is fitted into it, and the wound is wrapped against pests until it heals.

In layering, the middle part of a flexible thin twig is buried while still attached to the parent. After 10 to 15 days roots develop there, and the twig is then cut away to grow as a new plant.

In tissue culture, healthy plantlets are mass-produced from the shoot tip, or apical meristem, which eliminates virus-infected plants and ensures high yields.

Budding is shown first in yeast. A pinch of yeast granules is added to 20 mL of sugar solution in a test tube, the mouth is plugged with cotton, and the tube is kept undisturbed in a warm place so the yeast becomes active. After one or two hours a drop is mounted on a slide and examined under a compound microscope, where small round outgrowths, buds, can be seen emerging from the parent yeast cells.

The same process occurs in hydra, a multicellular animal. At one particular place on the parent's body the cells divide again and again, raising a little knob called a bud; the knob grows, breaks away, and thereafter lives on its own. In hydra, many buds may often be seen growing on the parent's body at the same time.

Spores are shown by a second activity. A slice of bread or roti is moistened and placed on a wet cotton bed covered with tissue paper in a box, and kept in a warm dark place. After three days the surface shows thread-like structures with a round sac at the tip, and inside the sac are tiny round structures, spores.

The air had spores in it all along; some came down on the damp bread and multiplied there at speed. Spores form in such a sac or on a swollen vesicle on a fungal hypha, and are produced in huge numbers, millions from one colony. Being lightweight and usually single-celled, they float easily on air currents, waiting for moisture and nutrients to germinate quickly.

In every organism considered in this topic, from bacteria and amoeba to yeast, hydra, moulds and plants, the central process behind asexual reproduction is the same: mitosis.

Mitosis is the division that yields a pair of cells carrying the parent cell's chromosome count and matching it exactly. Nothing from a second parent enters, and nothing is mixed. So the offspring produced are genetically identical to the parent, and such offspring are called clones.

That single fact explains the definition given at the start, that asexual reproduction involves only one parent and hence produces genetically identical individuals.

Two consequences follow. The first is speed: it is quick, which lets numbers build up in a hurry whenever surroundings happen to be kind. A mould colony releasing millions of spores, or a hydra carrying several buds at once, both show how rapidly numbers can rise.

The second is usefulness to farmers. Because a desirable plant can be copied exactly rather than crossed, the methods of vegetative propagation let farmers cultivate desirable crops on a large scale. Tissue culture in banana farming is the clearest case: mass-produced plantlets from the apical meristem, free of virus infection, with high yields assured.

What is not gained is variation, since copying is precisely what mitosis does.

For long, people did think living things arose spontaneously from non-living matter, so the claim is an old one rather than a foolish one. Louis Pasteur settled the question by experiment: nothing alive arises except from something already alive. On that basis he founded the germ theory of disease, strengthened the cell theory that every cell arises from a cell already living, and led to practices such as food and instrument sterilisation.

Applied to the bread, the account runs as follows. The mould was not present when the bread was fresh, so something must have arrived. The air had spores in it all along; some came down on the damp bread and multiplied there at speed.

That spores should be available is no surprise. They are produced in huge numbers, millions from a single mould colony, and being lightweight and usually single-celled they float easily on air currents. What they are waiting for is moisture and nutrients, and a slice of damp bread offers both.

The activity's conditions are chosen accordingly. The chamber is kept moist, and warm at 25 to 35 °C, because mould spores need warmth and moisture to grow. Lower temperatures slow or stop their reproduction.

And that last point has a familiar use. It is precisely why we refrigerate perishable food. Before refrigerators became common about a hundred years ago, fresh food lasted only one or two days.

A special type of cell division that forms gametes, reducing the diploid chromosome number of the parent cell to half in the haploid daughter cells.

A thread-like structure in the nucleus of a cell, carrying genetic information. Humans have 23 pairs, making 46 in all.

A haploid cell used only for reproduction. In animals, sperm and eggs; in plants, the male gametes in pollen grains and the eggs in ovules.

Genetic differences among individuals, arising from the random mixing of chromosomes in meiosis. It is important for the survival of a species and contributes to evolution.

The passage of pollen from a flower's anther to its stigma. The pea experiment shows it is necessary for fruit formation.

Sexual reproduction involves two parents, both contributing to the genetic material of the offspring. Were the whole complement to come across from each parent every time, the count would go up twofold with every generation. The problem is solved by meiosis, a special type of cell division that reduces the chromosome number of a diploid parent cell to half in the haploid daughter cells, which are the gametes.

A chromosome is a thread lying in the cell's nucleus, and what it carries is genetic information; every species runs to its own particular count. A diploid cell has the full set, as a human body cell does with 23 pairs, making 46. A haploid cell has half that number. Gametes are the haploid cells formed by meiosis and used only for reproduction.

During meiosis the chromosomes of each pair separate, so each gamete receives only one chromosome from each pair, chosen independently. The bead activity models this with three pairs of coloured beads representing contrasting characters, one bead being picked at random from each pair. With just three pairs, eight combinations are possible. With 23 pairs of chromosomes, each carrying information for many characters, the number of possible combinations is enormous.

Dealt at random, no child comes out matching either parent or any brother or sister, the set each ends up with being that one's alone. Out of that comes variation from one individual to the next, and on variation a species' survival turns: where surroundings change, some will be better suited to them than others, and given time enough that is evolution. Two examples are the ability to tolerate low oxygen levels at high altitudes and the ability to digest milk in adulthood.

Sepals, petals, stamens and pistil, the sepals and petals being fused in some flowers. The sepal is the green covering forming the outermost whorl in the bud stage, and the petals are the coloured projections. The reproductive parts are the stamen, which is the male part, and the pistil, which is the female part.

The stamen is the male part and consists of a filament and an anther, the anther producing pollen grains that contain male gametes. The pistil is the female part and has three subparts. At its tip sits the stigma, which can be flat, sticky or both. Below that runs the style, a narrow tube that links stigma to ovary. The ovary at the base holds the ovules, and every ovule carries one egg cell, which is the female gamete.

Two juvenile flower buds and three freshly blossomed flowers are selected on the same plant, and stamens are removed from one bud and one flower. Muslin cloth bags are wrapped loosely around the untouched bud, the bud with stamens removed, the flower with stamens removed and one freshly blossomed flower, while one freshly blossomed flower is left uncovered. Fruit appears where the flowers were in every case but one: the flowers whose stamens had been taken out while still in bud.

That fruit forms only where pollen has passed from where it is made to where it must land. Carrying it across in that way is what the word pollination names.

Here two parents are needed to make one new individual, and the genetic material the offspring ends up with has come from both of them. A difficulty arises at once: were the whole complement to come across from each parent every time, the count would go up twofold with every generation.

Each species has a fixed number of chromosomes in its cells, and chromosomes are thread-like structures present in the nucleus, carrying genetic information. Humans have 23 pairs, each pair consisting of one chromosome from each of two different individuals, making 46 in all.

Meiosis is what gets round it: the division that makes gametes. What goes in is a diploid cell; what comes out carries half that count, and is haploid. Cells of that halved kind have no other use than reproduction, and are called gametes.

Among animals the male ones are sperm and the female ones eggs. In plants a pollen grain carries male gametes and hands them over at an ovule, inside which the eggs are.

The halving happens in a particular way. Meiosis parts the two members of every pair, which leaves each gamete holding one out of each pair and no more. Every human gamete therefore has 23 chromosomes, each carrying genetic information for many characters.

Meiosis parts the two members of every pair, which leaves each gamete holding one out of each pair and no more. Which one it receives is a matter of chance, and the choice is made independently for every pair.

The bead activity shows what that produces. Three pairs of beads of different colours are taken, each pair representing two contrasting characters on different chromosomes: light and dark green for blonde and black hair, light and dark blue for straight and curly hair, light and dark red for brown and black eye colour. One bead is picked at random from each pair and the combination written down.

With just three pairs of characters, eight combinations are possible. Now consider 23 pairs of chromosomes, each carrying genetic information for many characters, and the number of possible combinations becomes vast.

Dealt at random like that, no child comes out matching either parent, or any brother or sister. The set each of us ends up with is ours alone.

Out of that comes variation from one individual to the next, and on variation a species' survival turns. Where surroundings change, some individuals will be better suited to them than others; given time enough, that is evolution.

Examples are close at hand. Some people can tolerate low oxygen levels at high altitudes, and some can digest milk in their adulthood.

No other group of plants comes near the angiosperms, the flowering plants, for sheer variety; reproduction is the business of the flower, though plants without flowers, pines for one, reproduce sexually all the same.

Examining a bud shows the outermost part first. In the bud stage a green covering surrounds all the parts as the outermost whorl of the flower, and this structure is called a sepal. The coloured projections of the flower are called petals, and in some flowers sepals and petals are fused.

A complete flower carries four parts: sepals, petals, stamens and pistil.

The stamen is the male part. It consists of a filament, which is the stalk, and an anther, which produces the pollen grains containing male gametes.

The pistil is the female part, and it has three subparts. At the very top sits the stigma, flattened or tacky or both, which is exactly what is wanted for catching pollen. Below it the style runs down as a slender tube to the ovary. The ovary is the swelling at the bottom; inside are the ovules, and inside each ovule an egg cell — the female gamete.

So the two gametes begin the story at opposite ends of the flower: the male ones in pollen at the top of a stamen, the female ones in ovules at the base of the pistil.

On a single pea plant, two juvenile flower buds and three freshly blossomed flowers are selected. Stamens are removed from one of the two buds and from one of the three flowers. Muslin cloth bags are then wrapped loosely around four of them: the untouched bud, the bud with stamens removed, the flower with stamens removed, and one freshly blossomed flower. One freshly blossomed flower is left uncovered. All are observed regularly, and once pods are fully developed on the uncovered flower the cloth is removed from the rest.

Fruit appears where the flowers were in every case but one: the flowers whose stamens had been taken out while still in bud.

Each result follows from whether pollen could reach the stigma. The uncovered flower could receive pollen freely. The bagged untouched bud and the bagged untouched flower each still had their own stamens, so their own pollen could reach their own stigmas. The open flower whose stamens were removed had already been open before the treatment, so pollen had reached its stigma before the stamens went.

Only the bud whose stamens were removed had neither source. It had not yet opened, so no pollen from outside had reached it, and with the stamens gone it had none of its own.

It follows that unless pollen gets from anther to stigma no fruit will form, and that passage of pollen is what we call pollination.

Transfer of pollen to the stigma of the same flower, or of another flower on the same plant.

Transfer of pollen from the anther of a flower on one plant to the stigma of a flower on another plant of the same type.

A tube produced by a pollen grain on a compatible stigma, which grows down through the style into the ovary and carries the male gamete to the ovule.

The fusion of the male gamete with the egg cell in the ovule. It marks the beginning of a new life, and the fertilised egg is called a zygote.

The scattering of seeds away from the parent, by wind, by water or by animals. Where conditions are favourable a seed then germinates.

Self-pollination is the transfer of pollen to the stigma of the same flower, or of another flower on the same plant. Cross-pollination means pollen leaving one plant's anther and arriving at the stigma of a second plant of the same kind. Both are transfers of pollen to a stigma, and both matter for the formation of fruits and seeds.

The pollen grains are light and small, and produced in large numbers, while the stigma is long and feathery so as to trap them. Wheat, maize and rice are examples. No bright colour, nectar or fragrance is needed, since the wind requires no attracting.

Such blooms tend to be vividly coloured, to hold a sweet drink and to carry a scent, all of which bring visitors in. Their pollen is large and either tacky or covered in small spines, so as to cling to a body passing by, and the receiving tip is tacky in the same way. Sunflower, hibiscus and marigold are examples.

The grains put out tubes which push down the style and into the ovary. Along one such tube the male gamete travels until it reaches the ovule, and there it joins with the egg cell. That fusion of gametes is fertilisation, and it marks the beginning of a new life.

The joined cell is a zygote, and in time it becomes an embryo. While that goes on, the ovary around the ovules swells and turns into the fruit, and the ovules inside it turn into seeds. Wind, water or animals then carry the seed away, and wherever it finds damp, air and the right warmth it breaks open and a new plant comes up from it.

Wind-pollinated grasses such as maize and wheat release about 5,00,000 to 10,00,000 pollen grains per flower and form only about 50 to 200 seeds. Insect-pollinated plants such as sunflower release about 20,000 to 40,000 grains and form about 800 to 1,000 seeds. So the pollen-to-seed ratio is enormously higher for wind, which means insect pollination is far the more efficient.

Because the carrier is undirected. Wind cannot be aimed, so most grains land on soil, water, leaves or nothing at all, and a plant relying on it must compensate by sheer quantity. Even a tiny fraction arriving is then enough for a crop of seeds, and the plant is spared the cost of attracting, feeding and rewarding an animal pollinator.

It involves removing the stamens from the flowers, covering them with a bag to prevent self-pollination, and manually transferring the pollen with the desired characters. It is one of the methods of plant breeding, alongside selective breeding, in which farmers select plants of desirable characters for reproduction, and genetic engineering, which inserts the genetic material of desired characters into the DNA of selected varieties. Such methods have led to high yielding and disease resistant varieties and have revolutionised crop production.

Pollination depends on external agents called pollinators, and the chapter names four: wind, water, insects such as bees and butterflies, and birds. Each leaves its mark on the design of the flower.

Wind pollination is seen in plants such as wheat, maize and rice. Their pollen grains are light and small, so the wind can carry them, and produced in large numbers, since most will never arrive. The stigma is long and feathery, to trap those that do. Nothing showy is required at all.

Water pollination occurs in aquatic plants such as Vallisneria and Hydrilla, where currents carry the pollen from one flower to another.

Insect pollination is found in many plants, including sunflower, hibiscus and marigold. Such flowers tend to be strongly coloured, to hold nectar and to be scented, all of which brings pollinators in. The pollen itself is big, and either tacky or barbed, so that it fastens onto an insect; the stigma is tacky in its turn, to take it off again.

Bird pollination occurs in some flowers, such as those of the coral tree and the hibiscus plant, which are visited by the Indian white-eye and sunbirds.

The pattern is worth stating plainly. Where the carrier is an animal, the flower must attract and reward it, and the pollen must cling. Where the carrier is wind or water, none of that is needed.

It begins with pollination, the transfer of pollen to the stigma of a flower, whether from the same plant, which is self-pollination, or from another plant of the same type, which is cross-pollination.

Once pollen lands on a compatible stigma, a remarkable process begins. The grains put out tubes which push down the style and into the ovary. Along one such tube the male gamete travels until it reaches the ovule, and there it joins with the egg cell.

Two gametes joining like that is fertilisation, and a new life dates from it. The joined cell is a zygote, and in time it becomes an embryo.

Meanwhile the rest of the flower is changing too. The ovary around the ovules swells and turns into the fruit, and the ovules inside it turn into seeds.

Seed dispersal then takes place by wind, by water and by animals, carrying the seeds away from the parent. Madar and dandelion seeds, for instance, are light and tufted and go by wind.

Finally, given damp, air and the right warmth, the seed breaks open and a new plant comes up from it.

One further point stands over the whole sequence: sexual reproduction does two things at once, making new plants and throwing up differences among them, which is what lets a species hold its ground and fit itself to where it lives.

The figures make the contrast sharp. Wind-pollinated grasses such as maize and wheat release about 5,00,000 to 10,00,000 pollen grains per flower and form an estimated 50 to 200 seeds. Insect-pollinated plants such as sunflower release about 20,000 to 40,000 grains and form an estimated 800 to 1,000 seeds.

So insect pollination uses roughly a twentieth of the pollen and produces several times as many seeds. Measured by the pollen-to-seed ratio, it is enormously the more efficient of the two.

The reason is that the carrier is directed. An insect moving from flower to flower of the same kind delivers pollen where it is wanted, which is precisely why such flowers are brightly coloured, produce nectar and give off fragrance, and why their pollen is sticky or spiny.

Wind is undirected. It cannot be aimed, so most grains land on soil, water, leaves or nothing at all.

Yet wind pollination persists, and effectively. Producing a very large number of pollen grains compensates for the wastage: even a tiny fraction arriving is enough for a crop of seeds. And the plant pays nothing for its carrier, needing neither colour nor nectar nor scent to attract, feed or reward anyone.

So the two strategies are not better and worse but different bargains: precision bought with advertisement, or waste offset by quantity.

Sexual reproduction in plants matters practically in plant breeding, and three methods are named. In selective breeding, farmers pick plants with the characters they want and breed from them, so that those characters are carried forward. Artificial hybridisation goes further, controlling which pollen arrives. It involves removing the stamens from the flowers, covering them with a bag to prevent self-pollination, and then transferring by hand the pollen with the desired characters. The steps are exactly those of the pea experiment earlier in the chapter, but turned to a purpose: strip the flower of its own pollen, exclude any other, and then supply the one chosen. Genetic engineering works at a different level, inserting the genetic material of desired characters directly into the DNA of selected varieties. Together these have led to new varieties being developed, such as high yielding varieties and disease resistant varieties, and have revolutionised crop production in agriculture. The laboratory has contributed too. P. Maheshwari, called the founder of embryology in India, worked out how to carry out fertilisation outside the plant in flowering species, bringing an egg. A male gamete together in glass to raise new hybrids, and was one of the earliest to keep plant embryos growing on made-up nutrient media. The book he published in 1950, An Introduction to the Embryology of Angiosperms, is a classic still in use.

Fertilisation outside the body, as in frogs and most fish. Eggs go into the water from the female and sperm goes in over them from the male.

Fertilisation inside the body of the female, as in reptiles, birds and mammals. Survival chances are generally higher, the embryo being better protected.

Where a mother cannot supply yolk for thousands of eggs, each carries just enough to produce a larva, which feeds and grows before transforming into the adult.

The formation of gametes, taking place in the testes and the ovaries. It works by meiosis, halving the chromosome number from 46 to 23.

Sperm are small, numerous, motile and without stored nutrients; eggs are large, few, non-motile and provisioned. The pattern holds across most animals.

In external fertilisation the gametes meet outside the body, as in frogs and most fish, the female releasing eggs into the water and the male releasing sperm over them. For all the great number laid, the current destroys many and other animals eat many more. In internal fertilisation the gametes come together within the female — reptiles, birds and mammals all do this — and the young stand a better chance, the fertilised egg and then the embryo being far better sheltered.

All animals must ensure that the male and female gametes meet, and that the young survive long enough to grow and reproduce. The table shows two answers. Fish and frogs fertilise externally, produce hundreds to fifty thousand eggs at a time, and have low survival of young. Lizards and birds fertilise internally, produce only two to twenty or one to fifteen eggs, and have moderate to high survival. One strategy compensates by numbers, the other by protection.

A fish, a frog or an insect turns out eggs by the hundred or the thousand at a go, each provisioned with yolk, which is the food the embryo grows on. But no mother's body can furnish yolk on that scale for that many eggs. So the yolk in each egg is just enough to produce a larva, which hatches and then gets nutrition by eating organic wastes such as rotten food and manures, and grows. The stage is a way-station in development and its business is eating; when the animal has taken in enough, it remakes itself and the adult form appears, as with a butterfly or a frog.

What reptiles and birds lay is stocked well enough to see the embryo right through to its hatching. Mammals instead carry the zygote within the female and let it grow there, and once born the young are usually fed for a while on milk.

Its organs make the male germ cells, the sperm, and carry them across to the female. The making is done in a pair of oval organs, the testes, which hang in a skin pouch, the scrotum, and hang there because sperm will only form a little below the body's own temperature. From the testes sperm travel through a long tube, the vas deferens, which opens into the urethra, a common passage for urine and sperm. Along the way the seminal vesicles and the prostate contribute fluid, which feeds the sperm and keeps them lively and moving.

Hormones are chemicals that regulate different functions. The hormone the testes make governs how sperm are produced, and brings on the bodily changes of puberty in boys. So the testes do two jobs, producing gametes and producing a hormone.

There are two ovaries, a pair of oviducts (the fallopian tubes), the uterus and the vagina. The ovaries turn out the female cells and put out regulating chemicals besides, which bring on the changes of puberty. Each ovary is joined to the uterus by an oviduct. The uterus is the bag-shaped chamber in which a foetus is carried, and below it a narrow way called the cervix leads down into the vagina.

Gametogenesis is the process of the formation of gametes, taking place in the testes and the ovaries by meiosis, in which the chromosome number is reduced to half, so that human cells with 46 chromosomes give sperm and eggs with 23. In males it results in numerous tiny motile active sperm; in females in a single large egg. A sperm is very small, produced in millions, without stored nutrients and actively motile, while an egg is large, few in number, with stored nutrients and non-motile.

Two things every animal must manage: getting the two kinds of gamete into contact, and keeping the young alive long enough to grow up and breed in their turn. The strategies differ in how they answer it.

A fish lives in water, fertilises externally, produces hundreds to thousands of eggs at a time, and has low survival of young. A frog lives in water and on land, also fertilises externally, produces 5,000 to 50,000 eggs at a time, and also has low survival.

A lizard lives on land, fertilises internally, produces only 2 to 20 eggs at a time, and has moderate survival. A bird lives in water and on land, fertilises internally, produces 1 to 15 eggs, and has moderate to high survival.

The pattern runs straight down the table. In external fertilisation the eggs go into the water from the female and the sperm goes in over them from the male; and for all the great number laid, the current destroys many and other animals eat many more. Numbers must compensate for those losses.

In internal fertilisation the gametes come together within the female, and the young stand a better chance, the fertilised egg and then the embryo being far better sheltered. Far fewer eggs are then needed.

So the two are not better and worse but different bargains: quantity against protection.

The problem is one of supply. An egg contains yolk, which holds nutritive substances to nourish the developing embryo, but a mother's body cannot provide such large quantities of yolk for the several hundred to several thousand eggs that fish, amphibians and insects lay at a time.

The first strategy solves this by dividing the task. What yolk there is runs to a larva and no further, and that larva is what comes out of the egg. Thereafter it feeds itself on organic refuse — spoiled food, dung — and grows on that. The stage is a way-station in development, and its business is eating. When the animal has taken in enough, it remakes itself and the adult form appears, as with a butterfly or a frog.

The second belongs to reptiles and birds. They lay eggs in which the yolk is sufficient to feed the embryo all the way to hatching. Fewer eggs are laid, but each is fully provisioned.

The third belongs to mammals, in which the zygote grows and develops inside the female body, so that provisioning continues throughout rather than being packed in beforehand.

After that, care varies. In some species the young are ready to find their own food when born, while others need an extended period of feeding and care, depending broadly on how long the embryo developed. Mammals typically feed their young on breast milk.

The organs of the male reproductive system make the male germ cells, the sperm, and carry them across to the female.

The making is done in a pair of oval organs, the testes, which hang in a skin pouch called the scrotum. Position is the point: the pouch holds them a little below the body's own temperature, and sperm will only form there.

The testes do a second job besides. They make a hormone — a hormone being a chemical that keeps some function in order — and it is this that governs how sperm are made and that brings on the bodily changes of puberty in boys.

Leaving the testes, sperm go along a long duct, the vas deferens, which in the end joins the urethra, a passage that serves for urine as well. Along the way, glands such as the seminal vesicles and the prostate add fluids, which nourish the sperm and help them remain active and move.

The sperm itself is built for one task. Each is a head carrying the genetic material and a long tail with which it drives itself towards the egg. It carries no stored nutrients at all, which is part of why it can be so very small, and why millions of them can be produced.

That asymmetry with the large, provisioned, non-motile egg is seen across most animals.

On the female side there are two ovaries, two oviducts — the fallopian tubes — the uterus and the vagina.

An ovary makes the female germ cells, the eggs, and puts out hormones besides, which are what bring on the changes of puberty. From each ovary an oviduct runs to the uterus. The uterus is the bag in which a foetus develops, and it opens into the vagina by way of a narrow passage, the cervix. Sperm and egg meeting within the female make a zygote; that becomes an embryo, and the embryo in due course a foetus, all of it within the uterus.

The process of the formation of gametes is called gametogenesis, and it takes place in the testes and the ovaries. Meiosis is the means, and what it does is halve the chromosome count. An ordinary human cell holds 46, a sperm or an egg only 23 — the point of the halving being that the zygote the two make together comes out at the parents' own count.

The two outcomes differ sharply. In males, gametogenesis produces numerous tiny motile, active sperm. In females, it produces a single large egg.

The contrast runs across four features. A sperm is very small, produced in millions, has no stored nutrients and is actively motile. An egg is large, few in number, carries stored nutrients and is non-motile. This asymmetry is seen across most of the animals.

The monthly release of one ripe egg from an ovary, from puberty onward. It falls around day 14 of a typical 28-day cycle.

The settling of the dividing zygote into the thickened uterine lining, from which it then draws nourishment. This is the point at which pregnancy begins.

Where no fertilisation occurs, the lining built to receive a zygote comes away with some blood through the vagina, ordinarily over three to seven days.

The three stretches of a nine-month pregnancy: organs forming, the foetus growing and moving, and the baby readying itself for life outside.

Barriers, pills, intra-uterine devices and surgical blocks, all of which work by keeping sperm and egg from meeting or by altering the release of eggs.

From puberty onward one ovary usually releases a ripe egg each month, and the egg travels into the oviduct. Before that release the inner lining of the uterus has already begun to thicken. Millions of sperm enter through the vagina and swim up the tract, and if one reaches the egg in the oviduct and fuses with it, a zygote is formed. The lining grows thicker still and richer in blood vessels. The zygote divides repeatedly by mitosis on its way to the uterus and implants into that lining, from which it draws nourishment. That implantation marks the beginning of pregnancy.

In-vitro fertilisation brings egg and sperm together away from the body, generally in a dish in the laboratory, after which the fertilised egg is placed in the uterus in the hope that a pregnancy follows. The usual name for a child so conceived is a test tube baby, though the vessel is a culture dish and never a test tube. In 1978 Subhash Mukhopadhyay of Kolkata pioneered India's first such birth, Kanupriya Agarwal, nicknamed Durga, through experimental IVF work.

The egg remains viable for about a day and then degenerates. The inner lining of the uterus, which had become thick and rich with blood vessels to receive and nourish a zygote, is no longer needed, so it sheds and leaves the body through the vagina along with some blood. This is menstruation, or a period, and it usually lasts three to seven days.

Days 1 to 5 are menstruation, when the uterine lining sheds. Days 6 to 14 see that lining gradually rebuilt while an egg matures in the ovary. Day 14 is ovulation, when the ovary releases a mature egg. From day 15 to day 28 the lining becomes thicker and rich with blood vessels, and if no fertilisation happens it begins breaking down around the 28th day, whereupon the cycle repeats.

Every person has two sex chromosomes, females XX and males XY. The mother can contribute only an X. The father contributes either an X, which with the mother's X gives XX and a girl, or a Y, which gives XY and a boy. So it is the father's contribution that settles the matter.

Pregnancy lasts about nine months. In the first trimester the fertilised egg becomes an embryo over the first two months and the major organs start forming; from about the ninth week it is called a foetus. The second brings size and strength, and the mother generally begins to feel it move. In the third the growth is quick and the child prepares for the world outside. The uterus protects and nourishes it throughout.

Sexual maturation means the body becoming capable of reproduction, and it comes on gradually alongside the body's overall growth, as with sperm production in boys and menstrual cycles in girls. Emotional maturity takes longer, and means handling feelings, communicating clearly and making thoughtful decisions. The body arrives at readiness before the judgement does.

Barrier methods such as condoms or vaginal covers stop sperm from reaching the egg, and condoms also prevent the transmission of sexually transmitted infections. Oral pills are medicines that alter hormones and so change the release of eggs, though they may carry side effects. Intra-uterine devices such as the copper-T are placed in the uterus to prevent pregnancy, though they may sometimes irritate it. The surgical methods close off the vas deferens in a man, or the fallopian tubes in a woman, so that the two gametes never reach each other.

Both stories begin the same way. From puberty onward one of the ovaries usually releases a single ripe egg each month, and that release is ovulation. Before it happens, the inner lining of the uterus has already begun to thicken. The egg travels from the ovary into the oviduct.

If a sperm arrives, the story goes one way. Millions of them enter through the vagina and swim up the reproductive tract, and some may reach the egg in the oviduct. Should one fuse with it, a zygote is formed. The lining now grows thicker still and richer in blood vessels. The zygote divides again and again by mitosis while travelling to the uterus, and then implants into that lining to draw nourishment for its development. This implantation marks the beginning of pregnancy.

If no sperm arrives, the story goes the other way. The egg remains viable for about a day and then degenerates. The lining, built to receive and nourish a zygote, is no longer needed, so it sheds and leaves the body through the vagina along with some blood. That is menstruation, and it usually lasts three to seven days.

The whole round then repeats, every 21 to 35 days, from puberty until menopause.

The cycle is one round of three things: ovulation, the preparation of the uterus and menstruation. It repeats every 21 to 35 days, often around 28, usually beginning at puberty between the ages of 10 and 14 in girls, and continuing until menopause at around the age of 50.

Across a typical 28-day round, days 1 to 5 are menstruation, when the uterine lining sheds. Days 6 to 14 see that lining gradually rebuilt while an egg matures in the ovary. Day 14 is ovulation, when the ovary releases a mature egg. From day 15 to day 28 the lining becomes thicker and rich with blood vessels, and failing fertilisation it begins breaking down around the 28th day, whereupon the round begins again.

Certain practices keep this healthy. Menstrual products such as sanitary pads, preferably biodegradable, tampons or menstrual cups should be used. The genital area is washed with water, soap being avoided because it disrupts the natural bacterial balance. Hands are washed with soap before and after a pad is changed. Used products are wrapped in newspaper or their wrapper and binned, never flushed. Reusable pads are cleaned as directed and must be completely dry before reuse, and pads are changed every four to six hours, more often if the flow is heavy.

Menstruation is a sign of a healthy reproductive system.

A human pregnancy runs to nine months or so, and is counted in three trimesters. Through the first, two months see the fertilised egg become an embryo and the main organs begin to take shape; from the ninth week on it is spoken of as a foetus. The second brings size and strength, and the mother generally begins to feel it move. In the third the growth is quick and the child prepares for the world outside. Throughout, the uterus protects and nourishes it.

At birth the uterus tightens powerfully, again and again, and it is this that drives the child out along the birth canal. Should an ordinary vaginal delivery be out of the question, or unsafe for either of them, there are medical and surgical means of bringing the child out in safety.

A baby wants particular care afterwards. Feed it at the breast: a mother's milk is a whole diet in itself and carries protection against a good many diseases. Keep a newborn warm, get its vaccinations done to time, handle it gently.

The mother's own condition matters no less. She needs nutritious food and adequate rest, and is advised to avoid smoking, alcohol and any medicine not medically advised. While she is carrying she wants food that is balanced and well supplied with protein, vitamins and minerals; she should keep her appointments, and do as her doctor says about gentle exercise and about resting. Her emotional well-being matters too, and family support helps.

Because sexual activity involves close physical contact, some infections can pass from an infected person to an uninfected one. Among these sexually transmitted infections are gonorrhoea, herpes, syphilis and genital warts, and HIV, which may in the end bring on AIDS. For several there is still no cure. Using condoms can prevent their transmission, and helps prevent pregnancy besides.

For pregnancy alone there are several methods. Barriers such as condoms or vaginal covers stop sperm reaching the egg. Oral pills are medicines that alter hormones and so change the release of eggs, though they may have side effects. Intra-uterine devices such as the copper-T are placed in the uterus, though they may sometimes irritate it. The surgical methods close off the vas deferens in a man, or the fallopian tubes in a woman, so that the two gametes never reach each other.

An unwanted pregnancy may in some cases be removed by surgery, generally only within the first trimester, when the embryo is very small. This is called abortion.

The practice of self-selective abortion, often driven by a preference for a specific gender, is a significant concern, because it can throw a society's sex ratio badly out of balance. For that reason Indian law strictly forbids prenatal sex determination, so that the sex ratio is kept sound for a healthy, forward-looking society.

It means that new plants arise from the vegetative parts, that is, from the growing parts of a plant. Potato and ginger sprout new plants from fleshy underground stems without producing seeds; money plant and sugarcane stem cuttings grow into new plants; and Bryophyllum leaves sprout tiny plantlets which eventually grow into new plants. Since only one parent is involved, the individuals produced are genetically identical.

Because asexual reproduction produces genetically identical individuals, so a desirable plant can be multiplied exactly. Scientists and horticulturists have developed cutting, grafting, layering and tissue culture from the natural process. These methods propagate plants efficiently, have significantly improved agricultural and horticultural practices, and enable farmers to cultivate desirable crops on a large scale.

Overgrown branches are cut at the end of the plant's growing season, and the cuttings prepared from them, their length and their nodes and internodes being noted. The shoot cuttings are collected in the morning, leaves are removed from the lower half of each. They are inserted about half their length into soil mixed with compost at an angle of about 45 to 60 degrees from the surface, then watered regularly.

Sexual reproduction involves two parents, both contributing to the genetic material of the offspring. Were the whole complement to come across from each parent every time, the count would go up twofold with every generation. The problem is solved by meiosis, a special type of cell division that reduces the chromosome number of a diploid parent cell to half in the haploid daughter cells, which are the gametes.

A chromosome is a thread lying in the cell's nucleus, and what it carries is genetic information; every species runs to its own particular count. A diploid cell has the full set, as a human body cell does with 23 pairs, making 46. A haploid cell has half that number. Gametes are the haploid cells formed by meiosis and used only for reproduction.

During meiosis the chromosomes of each pair separate, so each gamete receives only one chromosome from each pair, chosen independently. The bead activity models this with three pairs of coloured beads representing contrasting characters, one bead being picked at random from each pair. With just three pairs, eight combinations are possible. With 23 pairs of chromosomes, each carrying information for many characters, the number of possible combinations is enormous.

Self-pollination is the transfer of pollen to the stigma of the same flower, or of another flower on the same plant. Cross-pollination means pollen leaving one plant's anther and arriving at the stigma of a second plant of the same kind. Both are transfers of pollen to a stigma, and both matter for the formation of fruits and seeds.

The pollen grains are light and small, and produced in large numbers, while the stigma is long and feathery so as to trap them. Wheat, maize and rice are examples. No bright colour, nectar or fragrance is needed, since the wind requires no attracting.

Such blooms tend to be vividly coloured, to hold a sweet drink and to carry a scent, all of which bring visitors in. Their pollen is large and either tacky or covered in small spines, so as to cling to a body passing by, and the receiving tip is tacky in the same way. Sunflower, hibiscus and marigold are examples.

In external fertilisation the gametes meet outside the body, as in frogs and most fish, the female releasing eggs into the water and the male releasing sperm over them. For all the great number laid, the current destroys many and other animals eat many more. In internal fertilisation the gametes come together within the female — reptiles, birds and mammals all do this — and the young stand a better chance, the fertilised egg and then the embryo being far better sheltered.