Chapter 6 — Reproduction – The Generating System

Reproduction is the process by which living organisms produce new individuals of their own kind. It differs from every other life process in one respect: it is not needed for the survival of the individual. An animal that never reproduces lives just as long. Reproduction is needed for the survival of the species: it replaces the individuals that die, and it is the means by which the characters of the species are handed on, and by which variations arise that allow the species to change with a changing world. It is also the process that produces the enormous numbers of a species, so that some survive predators, disease and famine.

At the level of the cell, reproduction depends on the copying of DNA, the chemical of the chromosomes in which the instructions for building the organism are written. Before a cell divides it makes an exact copy of its DNA, and each daughter cell receives one copy; that is how the offspring come to resemble the parent. The copying is very accurate but not perfect, and the small errors, together with the mixing of DNA from two parents in sexual reproduction, are the source of the variations that make each individual slightly different. Variation is the raw material of evolution: in a changing environment, some variants survive better than others.

There are two broad modes. In asexual reproduction a single parent produces offspring without the formation or fusion of gametes (sex cells). The offspring are produced from ordinary body cells by mitosis and are genetically identical to the parent and to each other; a group of such identical individuals is a clone. Asexual reproduction is fast, needs only one individual, and is common in unicellular organisms, in many simple animals and in plants. Its weakness is that it produces no new combinations of characters, so a disease or change of climate that kills one is likely to kill all.

In sexual reproduction two parents, or two organs of one parent, produce special cells called gametes, a male gamete (sperm or pollen nucleus) and a female gamete (egg or ovum), which fuse together in fertilisation to form a single cell, the zygote, from which the new individual develops. Gametes are formed by a special kind of division, meiosis, that halves the number of chromosomes, so that when two gametes fuse the normal number is restored; the zygote therefore receives half its chromosomes from each parent and is a new combination. Sexual reproduction is slower and costlier, needs two individuals to meet, and produces fewer offspring, but every offspring is different, and this variation is what gives the species its ability to adapt and survive. Almost all animals and plants reproduce sexually, and many can also reproduce asexually.

The distinction can be summed up: asexual reproduction copies, sexual reproduction combines. This chapter takes the asexual modes first, then sexual reproduction in flowering plants and in humans.

Binary fission. The simplest form of reproduction is the splitting of a single-celled parent into two equal daughter cells. It occurs in bacteria, in Amoeba, Paramecium, Euglena and other protozoa, and in the malaria parasite at some stages. In Amoeba, when the cell has grown to full size, the nucleus divides first by mitosis into two nuclei; the cytoplasm then constricts in the middle and pinches into two, each part receiving one nucleus. The two daughter Amoebae are half the size of the parent, feed and grow, and divide again in a day or two. In favourable conditions there is no death: the parent simply becomes the two offspring. Amoeba divides in any plane; Paramecium divides transversely across its long axis; Euglena divides lengthwise. Bacteria divide by binary fission as often as every twenty minutes, which is why an infection or the spoiling of milk proceeds so fast.

Multiple fission. In unfavourable conditions, such as the drying of a pond, an Amoeba withdraws its pseudopodia, rounds up and secretes a hard protective wall, becoming a cyst. Inside the cyst the nucleus divides repeatedly into many nuclei, each of which gathers a little cytoplasm around it; when favourable conditions return, the cyst wall breaks and many tiny Amoebae emerge. This is multiple fission. The malaria parasite Plasmodium reproduces by multiple fission inside the red blood cells of a patient: each parasite divides into many, the cells burst all at once releasing them, and the toxins released cause the fever that returns every 48 or 72 hours.

Budding. In budding a small outgrowth, the bud, forms on the parent and grows into a new individual, which may separate or remain attached. Yeast, a single-celled fungus, buds when well fed: a small bulge appears on the cell wall, the nucleus divides and one nucleus passes into the bud, the bud enlarges and is then pinched off, or itself begins to bud before separating, so that chains of cells form. This is how yeast multiplies so quickly in dough or in fermenting juice. Hydra, a small fresh-water animal with a tubular body and tentacles, buds from its body wall: a group of cells on the side of the body divides repeatedly and pushes out as a bud, which develops a mouth and tentacles at its tip; it feeds as a miniature Hydra while still attached, and finally constricts at its base and detaches to live on its own. Sponges and some corals also bud, and in corals the buds remain attached, building the colony that becomes a reef.

Fission and budding are both forms of asexual reproduction by mitosis, producing offspring genetically identical to the parent. The difference is that fission divides the parent into equal parts, so the parent ceases to exist as such, whereas budding produces a small new individual from an outgrowth while the parent continues.

Fragmentation. Some simple multicellular organisms reproduce by breaking into pieces, each of which grows into a complete organism. Spirogyra, the green filamentous alga that forms slimy floating masses in ponds, is the standard example. Its filament is a chain of identical cylindrical cells, each with a spiral chloroplast. When the filament is mature, or when it is disturbed by water currents or by an animal, it breaks into two or more fragments, and each fragment continues to grow by cell division into a full-length filament. This is possible only because every cell of Spirogyra is alike and capable of division; no special organ is involved. Fragmentation also occurs in some fungi, in the moss Marchantia and in a few animals such as sponges and flatworms.

Regeneration. Regeneration is the ability of an organism to grow back lost or damaged parts, and in some animals it is so complete that a fragment regenerates a whole animal, making it a form of reproduction. Planaria, a flatworm found under stones in streams, is the classic example: if it is cut into two or more pieces, each piece grows the parts it lacks and becomes a complete worm, the head end growing a tail and the tail end growing a head. Hydra too can regenerate from a piece, and a starfish can regrow from a single arm attached to part of the central disc, which is why fishermen who cut up starfish to protect oyster beds only multiplied them. Regeneration is carried out by special unspecialised cells that multiply rapidly and then differentiate into the tissues required. In more complex animals regeneration is limited to parts: a lizard regrows its tail (though without vertebrae), a crab its claw, and in humans the liver regrows after part is removed, skin and blood are constantly renewed, but a lost finger or leg does not return. Regeneration is not the usual method of reproduction of any animal; Planaria reproduces normally by sexual means and by splitting, and regeneration is a capacity called on when injury occurs.

Spore formation. Fungi, bacteria, mosses, ferns and some algae reproduce by spores, tiny single-celled units, usually with a thick protective wall, produced in enormous numbers and dispersed by air or water, each capable of growing into a new organism when it lands on a suitable place. The bread mould Rhizopus is the example studied. Its body is a mass of fine threads, the hyphae, spreading over the bread; some hyphae grow upward as stalks, each ending in a round knob, the sporangium, in which the contents divide into hundreds of spores. When the sporangium is ripe it turns black, which gives old mould its colour, and bursts, releasing the spores into the air. A spore that settles on moist bread or fruit germinates into a new mycelium within a day. The thick wall lets the spore survive drying, heat and cold for a long time, and its tiny size lets it travel far; the air of every room contains mould spores, which is why bread left out always goes mouldy. Mushrooms produce spores on the gills under their caps, ferns in brown patches on the underside of their fronds, and mosses in a capsule on a stalk. Spores are asexual, but many of these organisms also reproduce sexually at another stage of their lives.

Fragmentation, regeneration and spore formation are all asexual and all produce genetically identical offspring; they differ in the size of the unit that starts the new organism, from a spore of one cell to a fragment of many.

In plants, asexual reproduction by which a new plant grows from a vegetative part of the parent, a root, stem or leaf, rather than from a seed, is called vegetative propagation. It is possible because plants keep groups of dividing cells (meristems) at their buds and growing points throughout life, and because many plant cells can return to a dividing state. Vegetative propagation occurs naturally in a great many plants, and gardeners and farmers exploit it artificially.

By stems. Many plants have modified stems that serve for storage and propagation. The potato is an underground stem, a tuber, whose eyes are buds; a piece of potato with an eye planted in soil sprouts into a new plant, and this is how the crop is grown. Ginger and turmeric are rhizomes, horizontal underground stems with nodes, which branch and sprout new shoots. The onion and garlic are bulbs, short stems surrounded by fleshy leaves, with buds between the leaves. The Colocasia (arbi) is a corm. Above ground, the strawberry and the grass called doob send out runners, horizontal stems that root at their nodes and produce new plants. The water hyacinth spreads over a whole tank in a season by offsets, short runners each of which becomes a new plant; it is one of the worst weeds of the world because of this. Mint spreads by suckers.

By roots. The sweet potato is a swollen root whose buds sprout; the dahlia tuberous roots do the same. The guava, the shisham and the Murraya (curry leaf) send up shoots from their spreading roots, so a single tree becomes a thicket.

By leaves. The leaf of Bryophyllum (patharchatta, the air plant) has notches along its margin in which small plantlets, with roots, develop; they drop off and grow. A leaf laid on moist soil produces a row of new plants along its edge. Begonia and the snake plant can also be propagated from leaves.

Advantages. Vegetative propagation is faster than growing from seed, and it is the only way to multiply plants that produce no viable seed, such as the banana, seedless grape, sugar cane, jasmine and rose. The new plants are exact copies of the parent, so a variety with a good fruit or flower is preserved unchanged; a seed of a mango may give a poor tree, but a graft from a good tree gives a good tree. Plants from vegetative parts flower and fruit earlier because they start from mature tissue. Disadvantages. Because there is no variation, the plants have no ability to adapt, and a disease can destroy all of them; a virus present in the parent is passed to every offspring; and vegetatively propagated plants generally lack the deep taproot of a seedling. Many plants therefore use both methods: the potato is grown from tubers but bred from seed.

Gardeners, nurserymen and farmers propagate plants vegetatively by four main artificial methods.

Cutting. A piece of stem, root or leaf, the cutting, is separated from the parent and planted in moist soil or sand, where it forms roots and grows into a new plant. Stem cuttings are the commonest: a length of stem with a few nodes is cut obliquely below a node, the lower leaves are removed, the cut end is dipped in rooting hormone (a synthetic auxin), and the cutting is planted with two or three nodes buried. Roots grow from the nodes within a few weeks. Rose, hibiscus (China rose), bougainvillea, croton, money plant, sugar cane, tapioca and grapes are grown from stem cuttings; sugar cane fields in Andhra are planted entirely with pieces of cane called setts, each with two or three buds.

Grafting. In grafting, parts of two different plants are joined so that they grow as one. The stock is a rooted plant, chosen for its strong root system, hardiness or resistance to disease; the scion is a shoot or bud from another plant, chosen for its fine fruit or flower. The scion is cut to fit against the cut surface of the stock so that the cambium layers of both are in contact, the join is bound with tape and sealed with wax, and within weeks the tissues unite; the scion then grows on the stock's roots and produces the scion's fruit. Grafting is used for mango, citrus, apple, guava, rose and sapota, where seedlings would be variable or slow. In bud grafting a single bud with a shield of bark is inserted under the bark of the stock. The stock and scion must be closely related, usually the same species or genus.

Layering. In layering a branch is made to form roots while still attached to the parent plant, and is cut off only after it has rooted, so it is never without a supply of food and water. In simple ground layering a low, flexible branch is bent down, a ring of bark is removed or a slanting cut made on its underside, the wounded part is pegged into the soil and covered, and the tip is left above ground; roots form at the wound, and the layer is then severed. Jasmine, lemon, bougainvillea and grapevine are layered thus. In air layering (gootee), used for plants whose branches cannot be bent to the ground such as litchi, guava, pomegranate, sapota and croton, a ring of bark is removed from a branch, the wound is packed with moist moss or soil, wrapped in polythene and tied; roots grow into the moss within a month or two and the branch is cut below them and planted.

Tissue culture (micropropagation). This is the growing of whole plants from small pieces of tissue, even single cells, in the laboratory. A small piece of tissue, the explant, usually from a growing tip, is sterilised and placed in a sterile nutrient jelly (medium) containing sugars, minerals, vitamins and plant hormones. The cells divide into an unorganised mass called a callus. The callus is transferred to a medium with a different balance of auxin and cytokinin that makes it form shoots and then roots; the tiny plantlets are hardened off and planted out. From one piece of tissue thousands of identical, disease-free plants can be produced in a few months, in a small space and at any season. Tissue culture is used for orchids, chrysanthemum, banana, sugar cane, potato and many forest trees, for producing virus-free planting material, and for rapidly multiplying a new variety or a rare plant. It is the technological form of vegetative propagation and the basis of much of modern plant biotechnology.

The flower is the reproductive organ of a flowering plant (angiosperm). It is a modified shoot with four whorls of parts arranged on a swollen tip of the stalk, the thalamus or receptacle; the flower stalk is the pedicel. From outside inward the whorls are the calyx, the corolla, the androecium and the gynoecium. The first two are accessory whorls that protect and attract; the last two are the essential whorls that produce the gametes.

Calyx. The outermost whorl consists of sepals, usually green, leaf-like and small, which enclose and protect the flower in the bud and may remain below the fruit, as in the brinjal and the tomato. When sepals are coloured like petals, as in the lily, they are called tepals.

Corolla. The petals, usually large and brightly coloured, and often scented and provided with glands secreting nectar, attract insects and other animals that carry pollen. In wind-pollinated flowers such as grasses the petals are small or absent. The sepals and petals together are called the perianth.

Androecium. The male reproductive whorl consists of the stamens. Each stamen has a stalk, the filament, carrying at its top a two-lobed anther. Each anther has four pollen sacs in which the pollen grains are formed by meiosis. A pollen grain is a tiny cell with a thick, often sculptured wall; it contains two nuclei, the tube nucleus and the generative nucleus, and the male gametes are formed from the generative nucleus. When the anther is ripe it splits open (dehisces) and the pollen, usually yellow, is exposed to be carried away.

Gynoecium (pistil). The female whorl at the centre consists of one or more carpels. A carpel has three parts: the swollen base, the ovary, which contains one or many ovules attached to its wall; a slender stalk, the style, rising from the ovary; and the stigma, the sticky or feathery tip of the style on which pollen grains are received. Each ovule is an oval body attached by a stalk to the ovary wall, enclosed in two coats (integuments) that leave a small opening, the micropyle, at one end, and containing the embryo sac, in which lies the female gamete, the egg cell, together with two polar nuclei in the centre and a few other cells. After fertilisation the ovule becomes the seed and the ovary becomes the fruit.

A flower with all four whorls is complete; with any whorl missing it is incomplete. A flower having both stamens and carpels is bisexual (hermaphrodite), as in hibiscus, mustard, pea, brinjal and datura; a flower with only stamens or only carpels is unisexual, as in the separate male and female flowers of papaya, cucumber, pumpkin, maize and coconut. When male and female flowers are on the same plant, as in maize and cucumber, the plant is monoecious; when on different plants, as in papaya and date palm, it is dioecious, and only the female plant bears fruit. In the hibiscus (China rose), the flower the board asks students to dissect, there are five green sepals with an epicalyx of bracts below, five large red petals, numerous stamens with their filaments joined into a tube around the style, and a pistil of five fused carpels whose style ends in five red stigmas.

For fertilisation the male gametes must reach the female gamete, but pollen grains cannot move on their own. The transfer of pollen from the anther to the stigma is pollination, and it is the first step of sexual reproduction in a flowering plant.

Self-pollination is the transfer of pollen from the anther to the stigma of the same flower, or of another flower on the same plant. It is possible only in bisexual flowers and in monoecious plants, and it is certain to happen if the anthers and stigma ripen at the same time and lie close together; in the pea, groundnut, wheat, rice and tomato the flower may pollinate itself before it even opens. It needs no external agent, no pollen is wasted, and the plant can reproduce in isolation, but because the gametes come from one parent the offspring are nearly uniform and the variety cannot improve. Cross-pollination is the transfer of pollen from the anther of one plant to the stigma of a flower on a different plant of the same species. It is the only kind possible in dioecious plants such as papaya, and it produces offspring with new combinations of characters, more vigorous and better able to adapt, which is why most plants have devices that favour it: unisexual flowers; anthers and stigma ripening at different times, as in sunflower; the stigma held above the anthers; or the pollen unable to grow on the stigma of its own flower (self-sterility), as in many apples. Its cost is dependence on an outside agent and the waste of much pollen.

Agents of cross-pollination. Wind pollinates grasses, maize, wheat, rice, sugar cane, bamboo, coconut, date palm, pine and many trees. Wind-pollinated flowers are small, unscented, without nectar and dull in colour; they produce enormous quantities of light, dry, smooth pollen (a single maize plant sheds 20 to 50 million grains); the anthers hang out on long filaments to be shaken by the wind, and the stigmas are large, feathery and sticky to catch pollen from the air; the silk of a maize cob is a bundle of stigmas. Insects, above all bees, but also butterflies, moths, beetles, flies and wasps, pollinate most garden and orchard plants: mustard, sunflower, hibiscus, apple, mango, cotton, pumpkin, brinjal. Insect-pollinated flowers are large or grouped into showy heads, brightly coloured, scented and produce nectar; the pollen is sticky or spiny and clings to the insect's body; the stigma is sticky. The insect visiting the flower for nectar or pollen brushes against the anthers and carries the pollen to the next flower. Some flowers are pollinated by birds such as sunbirds (silk cotton, coral tree, Erythrina), whose flowers are red and tubular with abundant nectar, and by bats (some cactus, kigelia, banana in the wild), whose flowers open at night. Water pollinates a few aquatic plants such as Vallisneria and Hydrilla, whose pollen floats to the female flowers on the surface.

Pollination is of practical importance. Beekeepers place hives in orchards and mustard fields to increase the fruit and seed set, and plant breeders pollinate by hand, transferring pollen with a brush from a chosen male parent to a female flower whose own anthers have been removed and which is then bagged, to produce hybrids of rice, maize, cotton and vegetables with higher yields. The disappearance of bees from the use of insecticides is a threat to crops the world over.

Germination of the pollen grain. A pollen grain landing on a stigma of the same species absorbs the sugary fluid the stigma secretes and swells. Its inner wall grows out through a pore in the outer wall as a pollen tube, which pushes down through the tissue of the style, digesting a path with enzymes and guided by chemicals secreted by the ovule (chemotropism). The tube nucleus leads the way at the tip of the tube; the generative nucleus follows and divides into two male gametes. The tube grows down the style, enters the ovary, and reaches an ovule, which it usually enters through the micropyle. The tip of the tube bursts and releases the two male gametes into the embryo sac.

Double fertilisation. In the embryo sac one male gamete fuses with the egg cell to form the diploid zygote; this is fertilisation proper, and the zygote will become the embryo. The second male gamete fuses with the two polar nuclei in the centre of the sac to form a triploid nucleus, the primary endosperm nucleus, which will develop into the endosperm, the food tissue that nourishes the embryo. Because two fusions take place, the process is called double fertilisation; it occurs only in flowering plants. Fertilisation is thus internal, takes place inside the ovule within the ovary, and needs no water, an adaptation that freed flowering plants from dependence on wet conditions for reproduction. Immediately after fertilisation the petals, stamens and often the sepals wither and fall, and the ovary begins to swell.

Formation of the seed. The zygote divides repeatedly and grows into the embryo, which has a tiny root (radicle), a tiny shoot (plumule), and one or two seed leaves, the cotyledons. The endosperm nucleus divides to form endosperm tissue that fills the ovule with stored starch, oil and protein. In wheat, rice, maize, castor and coconut the endosperm remains as the food store of the mature seed (endospermic seeds); in pea, bean, gram and groundnut it is absorbed by the cotyledons, which become thick and fleshy and are themselves the food store (non-endospermic seeds). The integuments of the ovule harden into the seed coat (testa), with the micropyle remaining as a tiny pore. The whole ovule has become the seed: an embryo with a food supply inside a protective coat, in a dormant state in which it can wait, sometimes for years, until conditions are right.

Formation of the fruit. While the ovules become seeds, the wall of the ovary grows and ripens into the fruit wall (pericarp), and the ovary becomes the fruit. The fruit protects the seeds and helps to disperse them. Fleshy fruits such as mango, tomato, guava and grape are eaten by animals, which drop or pass the seeds far from the parent; dry fruits such as the pod of pea and the capsule of cotton split open and scatter or release the seeds; fruits with wings (drumstick, shisham), hairs (cotton, madar) or parachutes (dandelion) are carried by wind; the coconut floats; and hooked fruits (Xanthium, the bur) cling to fur. A fruit formed from the ovary alone is a true fruit; when other parts join in, as the thalamus in the apple and cashew apple, it is a false fruit. Seedless fruits such as banana and some grapes develop without fertilisation (parthenocarpy), which is why they must be propagated vegetatively.

Germination. When a seed receives water, warmth and oxygen, it swells, the seed coat bursts, the radicle emerges first and grows down to form the root, and the plumule grows up to form the shoot, feeding at first on the stored food until the first green leaves begin to photosynthesise. Seeds do not need light to germinate, but they must not be buried too deep, and most need a period of dormancy after ripening.

Humans reproduce only sexually, and the sexes are separate. The male system produces the male gametes, the sperms, and delivers them into the female body; the female system produces the female gametes, the ova, receives the sperms, and shelters and nourishes the developing baby. Both systems become functional at puberty, between the ages of about 11 and 15, when the pituitary gland begins to secrete the gonadotrophic hormones that awaken the gonads.

Testes. The male gonads are a pair of oval testes (singular testis), each about 4 cm long, which lie outside the abdomen in a pouch of skin, the scrotum. This is because sperm formation requires a temperature about 2 to 3 °C lower than that of the body; the scrotum holds the testes away from the body in warm weather and draws them close in cold. Each testis is packed with about 250 metres of tightly coiled seminiferous tubules, in whose walls sperms are formed continuously from puberty to old age, by meiosis, at the rate of several hundred million a day. Between the tubules lie cells that secrete the male hormone testosterone, which at puberty causes the growth of the reproductive organs and the secondary sexual characters, the beard and body hair, deepening of the voice, growth of muscle and bone, and the sexual drive, and which maintains sperm production. A sperm is a tiny cell about 0.05 mm long, with a head containing the nucleus with 23 chromosomes and a cap of enzymes for penetrating the egg, a middle piece packed with mitochondria to supply energy, and a long whip-like tail by which it swims.

Ducts. From the tubules of each testis the sperms pass into a long coiled tube on the surface of the testis, the epididymis, where they mature and are stored for some weeks. From it the vas deferens (sperm duct), a muscular tube, rises into the abdomen, loops over the bladder and joins the duct of the seminal vesicle to form the ejaculatory duct, which opens into the urethra. In the male the urethra thus carries both urine and sperms, but never both at the same time. Vasectomy, the tying and cutting of the two sperm ducts, is the male method of sterilisation.

Glands. Three glands add their secretions to the sperms to form semen. The paired seminal vesicles secrete a fluid rich in fructose, which feeds the sperms and makes up most of the volume of the semen. The prostate gland, surrounding the urethra below the bladder, secretes a thin alkaline fluid that neutralises the acidity of the urethra and the vagina and activates the sperms; its enlargement in old men obstructs urination. The small Cowper's glands secrete a lubricating mucus. A single ejaculation delivers 2 to 4 mL of semen containing 200 to 400 million sperms.

Penis. The penis is the organ by which semen is introduced into the female. It contains spongy tissue that fills with blood and becomes erect during sexual excitement, and the urethra runs through it to open at its tip. Its function is to place the sperms deep in the vagina, near the entrance of the uterus, so that they can begin their journey to the egg.

Ovaries. The female gonads are a pair of almond-shaped ovaries, each about 3 cm long, lying in the lower abdomen one on each side of the uterus. Unlike the testes, the ovaries do not make gametes continuously; a baby girl is born with all the egg cells she will ever have, about a million immature ova each enclosed in a small sac, a follicle. From puberty to menopause (about 45 to 50 years), one follicle matures each month, swells to about 2 cm, and bursts to release one mature ovum, a spherical cell about 0.1 mm across, the largest cell of the body, with 23 chromosomes and a store of food. This release is ovulation. The ovaries also secrete the female hormones oestrogen, which at puberty causes the growth of the breasts, the widening of the hips, the growth of the uterus and the female pattern of fat and hair, and progesterone, which prepares the uterus for pregnancy.

Oviducts (fallopian tubes). Each ovary is partly surrounded by the funnel-shaped, fringed opening of an oviduct, a muscular tube about 10 cm long lined with cilia that leads to the uterus. The released ovum is swept into the funnel and carried along the tube by the cilia and by peristalsis; fertilisation, if it occurs, takes place in the upper part of the oviduct. Tying the oviducts (tubectomy) is the female method of sterilisation.

Uterus (womb). The two oviducts open into the uterus, a hollow pear-shaped organ with a thick muscular wall, about 7.5 cm long in a woman who has not been pregnant, lying behind the bladder. Its inner lining, the endometrium, is richly supplied with blood and is where the fertilised egg implants and develops; its muscular wall stretches enormously in pregnancy and contracts powerfully at birth. Its narrow lower end, the cervix, opens into the vagina.

Vagina. The vagina is a muscular tube about 8 to 10 cm long that receives the penis and semen during intercourse, serves as the birth canal, and carries the menstrual flow to the outside. Its opening lies between the urethral opening in front and the anus behind; in the female the urinary and reproductive passages are separate.

The menstrual cycle. From puberty (menarche, at 11 to 14 years) to menopause, the female system runs through a cycle of about 28 days, controlled by the hormones of the pituitary and the ovaries. Taking the first day of bleeding as day 1: during days 1 to 5, menstruation, the thickened lining of the uterus built up in the previous cycle breaks down and is shed with about 50 to 100 mL of blood through the vagina. From about day 5 to day 14, under the pituitary's follicle-stimulating hormone (FSH), a follicle matures in the ovary and secretes oestrogen, which makes the endometrium grow again. On about day 14 a surge of luteinising hormone (LH) causes ovulation. The empty follicle becomes a yellow body, the corpus luteum, which secretes progesterone; from day 14 to 28 progesterone makes the endometrium thick, soft and full of blood vessels and glands, ready to receive an embryo. If the ovum is not fertilised, the corpus luteum degenerates after about 10 days, progesterone falls, the lining can no longer be maintained, and it breaks down: menstruation begins and a new cycle starts. If the ovum is fertilised, the embryo produces a hormone that keeps the corpus luteum alive, progesterone continues, the lining is retained, and menstruation stops for the duration of the pregnancy. The absence of a period is therefore the first sign of pregnancy. The fertile period, when intercourse can lead to conception, is the few days around ovulation, since the ovum lives about 24 hours and sperms about 3 days.

Fertilisation. During intercourse 200 to 400 million sperms are deposited in the vagina. Swimming by their tails and helped by contractions of the uterus, they pass through the cervix and uterus into the oviducts; the journey of about 15 cm takes some hours and most sperms die on the way. If an ovum is present in the upper oviduct, the few hundred sperms that reach it surround it; one sperm penetrates the outer coat of the ovum with the enzymes of its head, its nucleus enters, and the membrane of the ovum instantly changes so that no other sperm can enter. The nucleus of the sperm (23 chromosomes) fuses with the nucleus of the ovum (23 chromosomes) to form the zygote with 46 chromosomes, the first cell of the new individual. Fertilisation in humans is internal and takes place in the oviduct. The sex of the child is decided at this moment by the sperm: an ovum always carries an X chromosome, a sperm carries either X or Y, so an X sperm gives a girl (XX) and a Y sperm a boy (XY). The mother contributes nothing to the sex of the child, a fact of importance where women are blamed for bearing daughters.

Implantation and the embryo. The zygote begins to divide by mitosis within a day, into 2, 4, 8, 16 cells and so on, while the cilia move it down the oviduct into the uterus, which it reaches in about 5 to 7 days as a hollow ball of cells. This ball sinks into the soft, thickened endometrium and becomes embedded: this is implantation, and from now the woman is pregnant. The embedded cells grow into the embryo, which by the end of the second month, about 3 cm long, has all its organs begun, a beating heart, limbs, eyes and ears, and is now called a foetus. It grows within a fluid-filled sac, the amnion, whose amniotic fluid cushions it against shocks and keeps its temperature steady.

Placenta. The embryo cannot eat or breathe, and it must be supplied through the mother's blood. Finger-like projections (villi) from the embryo's outer membrane grow into the endometrium and, with the surrounding uterine tissue, form a disc-shaped organ, the placenta, attached to the uterine wall and joined to the foetus by the umbilical cord, which carries two arteries and a vein. In the placenta the blood of the foetus in the villi and the blood of the mother in the surrounding spaces come very close but do not mix; across the thin barrier oxygen, glucose, amino acids, minerals, vitamins and antibodies diffuse from the mother into the foetus, and carbon dioxide, urea and other wastes diffuse from the foetus into the mother's blood to be excreted by her lungs and kidneys. The placenta is thus the foetus's lung, gut and kidney in one. It also secretes the hormones, including progesterone, that maintain the pregnancy. Unfortunately, alcohol, nicotine, many drugs and some viruses such as rubella and HIV also cross it, so a pregnant woman must avoid smoking, alcohol and unprescribed medicines.

Pregnancy (gestation). Development takes about 280 days, or 9 months, from the last menstrual period. The foetus grows from a single cell to about 3 kg and 50 cm, folded head-down in the uterus. The mother needs extra food, especially protein, iron, calcium and folic acid, regular check-ups, and vaccination against tetanus.

Birth (parturition). At the end of pregnancy, hormonal changes, including a fall in progesterone and a rise in oxytocin from the pituitary, start rhythmic contractions of the uterine muscle, the pains of labour. The contractions become stronger and more frequent, the cervix widens, the amniotic sac bursts, and the baby is pushed head-first through the cervix and vagina into the world. The umbilical cord is tied and cut; the baby takes its first breath and cries, its lungs expand and its own circulation takes over. Shortly afterwards the placenta detaches and is expelled as the afterbirth. The mother's breasts, prepared by the hormones of pregnancy, begin to secrete milk under the influence of prolactin; the first milk, colostrum, is rich in antibodies, and breast milk alone is the complete food of the baby for the first six months.

The power to reproduce brings responsibilities, and reproductive health is the state of physical, mental and social well-being in all matters relating to the reproductive system. Two of its concerns for young people are infection and the control of fertility.

Sexually transmitted diseases (STDs). Some infections are spread chiefly by sexual contact, because their germs live in the fluids of the reproductive organs and cannot survive outside the body. Gonorrhoea and syphilis are caused by bacteria: gonorrhoea causes a burning discharge and can block the oviducts or sperm ducts, causing sterility; syphilis begins with a painless sore and, untreated over years, damages the heart, brain and the unborn child. Both are curable with antibiotics if treated early. Genital herpes and genital warts are viral, and the human papilloma virus that causes warts is a cause of cancer of the cervix, now preventable by a vaccine given to girls. Hepatitis B spreads by sexual contact as well as by blood. HIV/AIDS: the human immunodeficiency virus destroys the helper lymphocytes of the immune system, so that after some years the person develops acquired immunodeficiency syndrome and dies of infections and cancers the body can no longer resist. HIV spreads by unprotected sexual intercourse, by transfusion of infected blood, by needles and syringes shared by drug users or reused in clinics, and from an infected mother to her baby during birth or through breast milk. It does not spread by touching, hugging, sharing food, utensils or toilets, by mosquito bites or by living and working together, and the fear that isolates HIV-positive people is groundless and cruel. There is no cure and no vaccine, but antiretroviral drugs now keep the virus suppressed for decades and prevent transmission to the baby. All STDs are prevented by the same measures: avoiding sexual contact before marriage and outside a faithful relationship, using a condom, never sharing needles, insisting on screened blood and sterile instruments, and getting early treatment for any symptoms. Because STDs are surrounded by shame, sufferers hide them and spread them; frankness and testing are the answer.

Contraception is the prevention of pregnancy, and it is the means by which couples plan the number and spacing of their children, protecting the health of mother and child and the welfare of the family. The methods are of four kinds. Barrier methods physically stop the sperms reaching the ovum: the condom, a thin rubber sheath worn on the penis, which also protects against STDs including HIV, and the diaphragm, a cap placed over the cervix. Hormonal methods: the oral contraceptive pill, taken daily by the woman, contains oestrogen and progesterone and prevents ovulation; injectable and implanted hormones work for months. Intra-uterine devices (IUDs) such as the copper-T are small objects placed inside the uterus by a doctor, which prevent implantation and last for years. Surgical methods are permanent: vasectomy, cutting and tying the vas deferens in the man, and tubectomy, cutting and tying the oviducts in the woman; both are simple operations that do not affect sexual life or hormone production. Natural methods, avoiding intercourse in the fertile days around ovulation, are unreliable. Abortion, the medical termination of a pregnancy, is legal in India under specified conditions but is not a method of contraception, and its use to eliminate female foetuses is a crime.

Population growth. The human population of the world took until 1800 to reach one billion, and now exceeds eight billion; India alone has over 1.4 billion people, the largest of any country, on 2.4 percent of the world's land. The growth results from a fall in the death rate, brought about by vaccination, antibiotics, clean water, better food and the control of famine, while the birth rate has fallen more slowly. Every year India adds more people than the population of Australia. The consequences are felt everywhere: pressure on land, water, forests and fuel; crowded cities and slums; overloaded schools and hospitals; unemployment; and the pollution and loss of wildlife described later in the course. A growing population also keeps families poor, since food, schooling and health care must be shared among more children. Reproduction, which is a biological process, thus has social and economic consequences, and its regulation is a matter of public policy.

Family welfare. India was the first country in the world to adopt a national family planning programme, in 1952, now called the family welfare programme. Its aim is not compulsion but the small family by choice: to make contraception freely available through primary health centres, to educate people about it, and to improve the health of mothers and children so that parents can be confident that their children will survive and so need not have many. The measures that work best are the education of girls, since educated women marry later and have fewer, healthier children; raising the legal age of marriage (18 for women, 21 for men) and enforcing it; spacing births by at least three years; and immunising and feeding children so that they live. The symbol of the red triangle and the slogan of the small happy family are familiar across the country. Where these measures have been taken, as in Kerala and Tamil Nadu, the birth rate has fallen to replacement level; where they lag, it remains high.

Sex ratio and female foeticide. Nature produces about 105 boys for every 100 girls at birth, and because girls are hardier the numbers even out. In India, however, the child sex ratio has fallen to about 900 girls per 1000 boys, and in some districts of the north-west to below 850. The reason is the deliberate elimination of girls: the abortion of female foetuses after their sex has been found by ultrasound, which is female foeticide, and the neglect and killing of baby girls, which is female infanticide. Behind it lie the dowry system, the preference for sons to carry on the family and support parents, and the view of a daughter as a burden. It is a biological absurdity, since the sex of a child is determined by the father's sperm and not by the mother; it is a moral crime; and it is a social disaster, since a society short of women suffers more violence, trafficking and forced marriage. The Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act of 1994, strengthened in 2003, forbids the determination of the sex of a foetus and the advertisement of such services, and punishes the doctor, the clinic and the family. Ultrasound clinics must be registered and keep records, and every scanning room must display the notice that sex determination is a crime. Campaigns such as Beti Bachao, Beti Padhao and the scholarships and incentives for girl children in many states are attempts to change the attitude behind the law. The examination often asks a student to argue against female foeticide; the arguments are these: the sex of a child is determined by the father, daughters are as capable as sons, the girl has the same right to life, and a society that kills its daughters will have no mothers.