Chapter 16 — Reproduction

Every living thing is made by other living things of its own kind. Reproduction is the process by which an organism produces new individuals like itself. It does not keep an individual alive, because an animal can live its whole life without ever reproducing. What reproduction does is keep the species going from one generation to the next. Without it, a kind of organism would disappear when its last members died.

Reproduction also keeps copying a fixed plan. Inside every cell there is DNA, which carries the instructions for building a body. When an organism reproduces, it passes a copy of those instructions to its offspring, so a mango tree gives mango seedlings and a goat gives kids.

Biologists group reproduction into two broad kinds:

• Asexual reproduction needs only one parent. The offspring are almost exact copies of the parent.
• Sexual reproduction needs two parents, a male and a female. Each gives a special cell called a gamete, and the offspring receive features from both.

Some organisms use only one kind, and some, such as many flowering plants, can use both. A single plant may spread by runners (asexual) and also make flowers and seeds (sexual).

A key idea to keep in mind is variation. Children are similar to their parents but not identical, and brothers and sisters differ from each other. Variation comes mainly from sexual reproduction, because the new individual gets a mixture of features from two parents. Variation matters because a population whose members are slightly different is more likely to have some members that survive when conditions change, for example during a disease or a drought.

In this chapter we first study the ways in which organisms reproduce without a partner. Then we study flowering plants, whose flowers are their reproductive organs. Finally we study human reproduction and the care of reproductive health. Keep asking the same question in each part: how is a new individual formed, and how much does it resemble its parent?

In asexual reproduction a single parent produces offspring without gametes and without fertilisation. The offspring are almost exact copies, so there is very little variation. It is quick, and a single organism can produce many new ones, which is why it is common among simple organisms and among many plants. There are several types:

Binary fission. A single cell divides into two equal cells. Amoeba and bacteria reproduce this way. In some organisms, such as the malarial parasite Plasmodium, the cell divides into many daughter cells at once, which is called multiple fission.

Budding. A small bud grows out of the parent’s body, enlarges, and then separates as a new individual. Hydra and yeast reproduce by budding.

Fragmentation. The body breaks into pieces, and each piece grows into a new individual. The green alga Spirogyra does this.

Regeneration. If the body of some organisms is cut or broken into pieces, each piece can regrow the missing parts and become a complete individual. Planaria and Hydra can do this.

Spore formation. The parent produces tiny, light spores with protective coats. When a spore lands in a suitable place, it grows into a new individual. The bread mould Rhizopus reproduces this way.

Vegetative propagation. New plants grow from a vegetative part of a plant, that is, a root, stem or leaf, and not from a seed. Examples are the potato (from the buds, or “eyes”, on the tuber), ginger (from the underground stem), Bryophyllum (from buds on the leaf edges), and sugarcane and rose (from cuttings). Gardeners use cutting, layering and grafting to produce plants with the same good qualities as the parent, such as sweet mangoes.

An advantage of vegetative propagation is that the plant often flowers and fruits earlier than one grown from a seed, and the offspring keep the parent’s qualities. A weakness of all asexual methods is that, because the offspring are identical, one disease or change in conditions can harm all of them.

In sexual reproduction two parents are usually involved: a male and a female. Each produces a special reproductive cell called a gamete. The male gamete is small and mobile. It is the sperm in animals, and in plants it is carried inside the pollen grain. The female gamete is larger, does not move, and stores food. It is the egg (or ovum) in animals and the egg cell inside the ovule in plants.

The male and female gametes fuse in a process called fertilisation. The cell that results is called a zygote. The zygote divides again and again and develops into an embryo, which grows into a new individual. Because the zygote has features from both parents, the offspring differ from each parent and from each other. This is the main source of variation in a population.

Why does variation matter? In a population where everyone is identical, a single disease or a change in the environment can harm everyone. In a varied population some members will usually have features that help them to survive, and they pass these features on. Over many generations this helps the species to continue.

Compare the two kinds of reproduction:

In flowering plants the same plant can have both male and female organs in one flower. In humans and most animals the male and female organs are in different individuals. Either way, the steps are the same: gametes form, they meet, they fuse, and a zygote begins to develop. The next topics follow these steps first in a flower and then in the human body.

In flowering plants the flower is the organ of sexual reproduction. A typical flower has four whorls, or rings, of parts arranged on a small swollen base:

Sepals are usually green and leaf-like. Together they are called the calyx, and they protect the flower while it is still a bud.

Petals are often bright and scented. Together they are called the corolla, and they attract insects and other animals for pollination.

Stamens are the male reproductive parts. Each stamen has a thin stalk called the filament and, at its tip, an anther. The anther produces pollen grains, and each pollen grain carries the male gamete.

Carpels are the female reproductive parts, and together they form the pistil. Each carpel has three regions: the stigma, a sticky top that receives the pollen; the style, a narrow tube; and the ovary, a swollen base. The ovary contains one or more ovules, and each ovule contains an egg cell.

A flower that has both stamens and carpels is called bisexual, for example hibiscus and mustard. A flower that has only one of them is unisexual. In papaya and watermelon, for example, some flowers have only stamens (male) and others have only carpels (female).

To study a flower, hold it carefully, identify the whorls from the outside to the centre, and then use a blade to cut the ovary lengthwise so that the ovules are visible. Drawing and labelling it is a good way to remember the parts. Notice that everything in a flower has a job, and the colour and scent of petals are there for a reason: they are advertisements for pollinating animals.

Pollen made in the anther must reach the stigma of a flower before fertilisation can happen. The transfer of pollen from an anther to a stigma is called pollination. It is the first step of sexual reproduction in flowering plants.

There are two kinds:

• Self-pollination: pollen falls on the stigma of the same flower, or of another flower on the same plant.
• Cross-pollination: pollen is carried to the stigma of a flower on a different plant of the same kind. This mixes features from two plants and so increases variation.

Plants are not able to move, so they use outside helpers called pollinating agents. These can be abiotic (non-living), such as wind and water, or biotic (living), such as insects, birds and bats. Each agent shapes the flower:

An insect that visits a flower for nectar brushes against the anthers and picks up pollen on its body. When it goes to the next flower, some of that pollen rubs off on the stigma. The insect gains food, and the plant gains a pollinator, which is why such relationships are said to benefit both partners.

If a pollinator is lost, the plants that depend on it may fail to make seeds and fruits. For this reason, protecting bees and other pollinators matters for farmers as well as for nature. In some crops, farmers even transfer pollen by hand with a small brush when the natural pollinators are few, which shows how important this single step is for a good harvest.

Pollination only brings the pollen to the stigma. Fertilisation takes several more steps:

When a pollen grain lands on a suitable stigma, it absorbs nutrients and begins to grow a thin tube, the pollen tube. The tube grows down through the style, carrying the male gamete, until it reaches an ovule in the ovary. The male gamete is released into the ovule and fuses with the egg cell. This fusion is fertilisation, and it forms the zygote.

After fertilisation the flower changes quickly:

• The zygote divides repeatedly and develops into an embryo inside the ovule.
• The ovule grows a tough covering and becomes the seed.
• The ovary grows larger and becomes the fruit.
• The petals, sepals and stamens usually dry up and fall off.

So a seed contains a tiny embryo and stored food, wrapped in a seed coat, and a fruit is the ripened ovary that protects the seeds. A mango is a fruit, and the hard stone inside its flesh holds the seed.

Ovule and seed. Before fertilisation the ovule holds an egg cell. After fertilisation, it holds an embryo and has become a seed. Ovary and fruit. Before fertilisation the ovary is the swollen base of the carpel. After fertilisation, it has become the fruit.

When a seed finds water, warmth and air, the embryo uses the stored food, the seed germinates, and a new plant, the seedling, begins to grow. The cycle then continues. A bean seed soaked in water for a day can be split open to show the seed coat and the embryo with its food store, a simple activity that makes these ideas easy to see.

Notice one more point about variation. Because the zygote is made from gametes of two different plants (in cross-pollination), the seedling is not identical to either parent. This is why seeds from a good plant do not always give plants with exactly the same qualities, and gardeners use cuttings or grafting when they need an exact copy.

A plant cannot move, but its seeds can travel. Seed dispersal is the spreading of seeds away from the parent plant. It is important for several reasons. Seeds that fall right under the parent would compete with it, and with each other, for light, water and minerals. Dispersal reduces this competition, lets the plant colonise new places, and makes sure a species is not lost if one place is damaged.

Seeds are dispersed by different agents, and the seed or fruit usually has a feature suited to its agent:

Agents may be biotic (animals, including humans) or abiotic (wind and water), the same two groups that carry pollen. A bird that eats a berry and drops the seeds far away, or a squirrel that buries a nut and forgets it, helps the plant to spread without knowing it.

You can see these features in a simple activity. Collect different seeds and fruits from a garden or a path, and sort them by how they might travel. Which have wings or fluff? Which have hooks? Which would float? Predicting the agent from the shape is a good test of understanding.

Seed dispersal and pollination often depend on the same animals, such as birds and insects, so protecting these animals protects the plants as well. A forest that has lost its birds and bats may find that fewer of its seeds are carried to new places.

In humans, as in most animals, reproduction is sexual. The male reproductive system makes the male gametes, the sperms, and delivers them to the female body. Its main parts are:

• Testes (singular: testis): two oval organs that make sperms. Testes also make the male hormone testosterone, which brings about the changes of puberty. They lie outside the abdomen in a bag of skin called the scrotum, because sperm production needs a temperature a little lower than that of the body.
• Vas deferens: a tube from each testis that carries sperms upwards.
• Glands: the seminal vesicles and the prostate gland add fluids. The fluid gives the sperms nutrition and helps them to move. The sperms together with these fluids are called semen.
• Urethra: a tube that passes through the penis. It carries semen, and it also carries urine from the bladder, but not at the same time.
• Penis: the organ that delivers semen into the female body.

A sperm is a very small cell. It has a head that contains the nucleus, a middle part that supplies energy, and a long tail that lashes from side to side so that the sperm can swim. A male makes millions of sperms, because only a few of those released will survive the journey and only one will fertilise an egg.

Sperm production begins at puberty, which is the stage, usually in the early teens, when the body becomes able to reproduce. At puberty a boy’s voice deepens, hair grows on the face and body, and the body grows rapidly. These changes are controlled by testosterone. The changes are normal, and they happen at different ages for different people, so there is no need to worry if a friend changes sooner or later than you do.

Remember the path in order: testis, vas deferens, urethra, outside the body. Putting the structures in order of the path that a sperm follows is a good way to learn them for an examination.

The female reproductive system makes the female gametes, the eggs (ova), receives the sperms, and provides a place where a baby can develop. Its main parts are:

• Ovaries: two small organs, one on each side of the lower abdomen. They produce eggs and also the female hormones.
• Oviducts (fallopian tubes): two tubes that lead from the ovaries to the uterus. An egg travels along an oviduct, and fertilisation usually takes place here.
• Uterus (womb): a muscular, pear-shaped organ where the embryo is held and grows. Its lining is rich in blood vessels.
• Cervix and vagina: the narrow lower end of the uterus and the passage that connects it to the outside of the body. A baby passes through them at birth.

An egg is much larger than a sperm. It cannot move by itself, and it carries stored food. From puberty, usually in the early teens, one egg is normally released from an ovary about every month. This release is called ovulation.

The menstrual cycle. The cycle lasts about 28 days in many women, though it varies from person to person and between months. In each cycle:

• The lining of the uterus becomes thicker and richer in blood to prepare for a possible embryo.
• An egg is released from an ovary at about the middle of the cycle.
• If the egg is fertilised, the embryo settles in the thick lining and the cycle pauses during pregnancy.
• If the egg is not fertilised, the lining and some blood are not needed and are shed through the vagina. This is menstruation, or a period, and it usually lasts a few days.

The first period is called menarche, and the cycles stop at a stage of life called menopause. Menstruation is a healthy, natural process and not an illness or a sign of being unclean. Girls should be able to talk about it openly with parents, teachers and doctors, and should be taught to manage it hygienically, which is taken up in the last topic.

In a diagram, you can follow the path of an egg: ovary, oviduct, uterus, vagina. Use it to remember where fertilisation happens (the oviduct) and where the embryo grows (the uterus).

During sexual intercourse the male releases millions of sperms into the female body. The sperms swim up through the uterus into the oviducts. If an egg is present in an oviduct, one sperm fuses with it. This is fertilisation, and the result is a zygote. Fertilisation is internal in humans, because it takes place inside the female body.

The zygote starts to divide as it moves down the oviduct towards the uterus, forming a ball of cells. This ball settles into the thick, blood-rich lining of the uterus, a step called implantation. From now on the female is pregnant, and the ball of cells is called an embryo. After the first two months or so, when the main organs have formed, the embryo is called a foetus.

The embryo cannot feed itself, so a special organ called the placenta grows. It is a disc-like tissue that is attached to the wall of the uterus. The embryo is linked to it by a cord. Through the placenta, the mother’s blood provides the embryo with oxygen and nutrients, and wastes from the embryo, such as carbon dioxide, are taken away. The mother’s and the embryo’s blood do not mix; substances pass across thin walls.

The time from fertilisation to birth is called the gestation period, and in humans it is about nine months. During this time the mother needs a balanced diet, rest, regular medical check-ups and protection from infection and harmful substances, because what she eats, drinks and breathes may reach the baby.

At the end of this period, the muscles of the uterus contract strongly, which pushes the baby out through the vagina. This is birth. Afterwards the mother’s body produces milk, which gives the baby the food it needs in its first months and also helps to protect it from infection.

Note the contrast with a hen’s egg. In birds, the embryo develops outside the body in a shell with a store of food. In humans, it develops inside the uterus and is fed through the placenta, so humans are said to be viviparous (they give birth to young ones), while birds are oviparous (they lay eggs).

Reproductive health means a state of complete physical, mental and social well-being in everything to do with reproduction, not only the absence of disease. A person who is reproductively healthy has a body that works well, has the knowledge to make safe decisions, and is able to live without fear or pressure.

Hygiene. During and after puberty the body needs particular care:

• Bathe daily and keep the genital area clean, and wear clean, dry cotton undergarments.
• During menstruation, change sanitary pads or cloth often, use clean ones, and wash the hands before and after.
• Eat a balanced diet with iron-rich foods, take regular exercise, and sleep well.
• Visit a doctor for check-ups, and ask a trusted adult or a doctor when you have a question or a worry.

Infections. Some diseases, such as gonorrhoea, syphilis and HIV/AIDS, are passed on through sexual contact. They can be avoided, and the best protection is knowledge, avoiding risky behaviour, and early treatment. Using a condom also gives protection.

Birth control. Families and the nation have to plan for the number of children, so that each child can be fed, educated and cared for. Methods that prevent pregnancy are called contraceptives:

Contraceptives should be chosen with a doctor’s advice, because each has advantages and side effects. A condom is the only method in this list that also protects against many infections. Surgical methods are meant to be permanent. Abortion is not a method of birth control.

The population of India is large, and planning the family helps to improve the health of mothers and children and the use of resources. Society also needs to make sure that boys and girls are treated equally, and that the sex of an unborn child is never used as a reason for ending a pregnancy: such sex selection is illegal and harms the balance of the population.

Learning about reproduction has a long history in India. Long before microscopes, Indian physicians studied the human body with care. The Sushruta Samhita, an ancient Sanskrit text on medicine and surgery, describes how a student of surgery should learn the body by examining it part by part, and it gives detailed accounts of bones, muscles, vessels and organs. The Charaka Samhita describes the growth of the unborn child month by month and stresses the care, diet and rest that a pregnant mother needs. Some of their ideas were later corrected by modern science, but the habit of careful observation they show is the heart of biology.

Panchanan Maheshwari and plant embryology. Professor Panchanan Maheshwari (1904-1966) was a botanist at the University of Delhi. He studied how the embryo forms inside the ovule of flowering plants, the same steps you studied in pollination and fertilisation. His book on the embryology of flowering plants became a standard text across the world. His laboratory grew ovaries and ovules on a nutrient medium inside test tubes, and even placed pollen on such ovules so that fertilisation took place in the tube (test-tube fertilisation). This work laid the foundation of plant cell and tissue culture research in India, which is now used to raise thousands of identical, disease-free plants of crops such as banana and sugarcane from tiny pieces of a plant.

Assisted reproductive technologies (ART). Some couples are unable to have children because, for example, the oviducts are blocked or the number of healthy sperms is low. Modern medicine can help them. In in vitro fertilisation (IVF), eggs are taken from the mother's ovary and mixed with the father's sperms in a dish in the laboratory. When an embryo forms, it is placed in the mother's uterus, where it implants and grows in the normal way. A baby born in this way is popularly called a "test-tube baby", though the baby grows in the uterus, not in a tube. India's first IVF baby was born in Kolkata in 1978, through the work of Dr Subhash Mukhopadhyay. Such treatments are costly and are controlled by law, so that they are used safely and ethically.

Questions scientists are still asking. Science does not have every answer. Researchers study how heavy metals such as lead, mercury and cadmium, which can enter the body through polluted water, food or air, may harm the testes and ovaries. They also study whether working for long hours in extreme heat lowers the number of healthy sperms; recall that the testes lie in the scrotum because they need a slightly lower temperature. Asking such questions and testing them with careful evidence is how our understanding of reproductive health keeps improving.