Reproduction in Plants and Animals

One of the most important features of all living things is that they can produce new individuals of their own kind. The process by which living organisms produce young ones (offspring) similar to themselves is called reproduction. Reproduction is essential because individual organisms do not live forever; through reproduction, a species continues to exist generation after generation. Without reproduction, every kind of plant and animal would soon disappear from the Earth.

Reproduction is of two main types: asexual reproduction and sexual reproduction. In asexual reproduction, a new organism is produced from a single parent, without the joining of special sex cells. The offspring are usually exactly like the parent. This type is common in many plants and in simple animals, and it is quick and simple. In sexual reproduction, two parents are usually involved, and it requires the joining (fusion) of a male sex cell and a female sex cell. The offspring produced may resemble the parents but are not exactly identical to either of them.

The key difference between the two types lies in the number of parents and the involvement of sex cells. Asexual reproduction needs only one parent and no sex cells, and produces identical offspring. Sexual reproduction usually needs two parents and the fusion of male and female sex cells (a process called fertilisation), and produces offspring with a mixture of features. Because of this mixing, sexual reproduction creates variety among offspring, which is useful for a species over time.

Both plants and animals reproduce, and many can do so by either method depending on the organism. In this chapter we will look first at how plants reproduce — both asexually and sexually — and then at how animals reproduce. Understanding reproduction helps us see how new plants grow from seeds, cuttings, and spores, and how young animals are born, ensuring that life continues on our planet.

Many plants can reproduce without seeds and without flowers, producing new plants from a single parent. This is asexual reproduction in plants, and it is very common. Because only one parent is involved and no sex cells join, the new plants are exactly like the parent plant. There are several methods of asexual reproduction in plants, including vegetative propagation, budding, spore formation, and fragmentation.

The most common method is vegetative propagation, in which a new plant grows from a part of the parent plant — its root, stem, or leaf — rather than from a seed. For example, a piece of a potato with an "eye" (bud) can grow into a new potato plant; the stem of a rose or sugarcane can be cut and planted to grow a new plant (a cutting); the leaf of a Bryophyllum plant grows tiny new plants along its edges; and ginger grows from its underground stem. Gardeners and farmers use vegetative propagation to quickly grow many identical plants from a single good parent.

Some simpler organisms use other methods. In budding, a small outgrowth called a bud forms on the parent and grows into a new individual; yeast, a tiny fungus, reproduces by budding. In spore formation, the parent produces large numbers of tiny spores, each able to grow into a new organism when conditions are right; fungi like bread mould and plants like ferns and mosses reproduce by spores, which is why bread left in a warm, moist place soon grows mould. In fragmentation, an organism breaks into pieces and each piece grows into a new individual, as seen in the green alga Spirogyra.

Asexual reproduction in plants has clear advantages: it is fast, needs only one parent, and produces plants exactly like the parent, which is useful when we want to keep a good variety unchanged. This is why methods like cuttings and grafting are widely used in agriculture and gardening to grow flowers, fruits, and crops. Understanding these methods shows how a single plant or part of a plant can give rise to many new plants without seeds.

In flowering plants, sexual reproduction takes place in the flower, which is the reproductive part of the plant. A flower may contain both male and female parts, and through their working together, seeds are formed that can grow into new plants. To understand this, we first look at the parts of a flower and then at the processes of pollination and fertilisation.

A typical flower has four main sets of parts. The sepals are the small green leaf-like parts that protect the flower when it is a bud. The petals are usually brightly coloured to attract insects and birds. Inside the petals are the reproductive parts: the stamen, which is the male part, and the pistil (carpel), which is the female part. Each stamen has a stalk and a top part called the anther, which produces the pollen grains (containing the male sex cells). The pistil in the centre has a sticky top called the stigma, a stalk called the style, and a swollen base called the ovary, which contains ovules (containing the female sex cells).

The first key process is pollination, the transfer of pollen grains from the anther (male part) to the stigma (female part). When pollen is transferred within the same flower or another flower of the same plant, it is self-pollination; when it is transferred to a flower on a different plant of the same kind, it is cross-pollination. Pollination is carried out by agents such as insects, wind, water, and birds — for example, bees carry pollen as they move from flower to flower seeking nectar.

After pollination comes fertilisation, the fusion of the male sex cell (from the pollen) with the female sex cell (in the ovule). The pollen grain on the stigma grows a tube down the style to reach an ovule in the ovary, and the male and female sex cells join. This fertilised ovule then develops into a seed, and the ovary grows into a fruit that encloses and protects the seeds. Thus, through the flower's parts, pollination, and fertilisation, flowering plants reproduce sexually and form the seeds and fruits from which new plants — and much of our food — come.

After fertilisation, the ovules of a flower become seeds, and the ovary becomes a fruit around them. A seed is very important because it contains a tiny baby plant (embryo) together with a store of food, all protected inside a tough seed coat. When conditions are right, the seed can germinate — that is, the baby plant inside it begins to grow, using the stored food, until it can make its own food by photosynthesis. In this way, each seed has the potential to grow into a new plant.

For seeds to grow successfully, they must not all fall and grow in one crowded spot right beneath the parent plant. If they did, the young plants would be overcrowded and would have to compete with one another and with the parent for sunlight, water, and minerals. To avoid this, seeds are spread out and scattered to new places — a process called seed dispersal. Dispersal helps plants spread to new areas, reduces competition, and helps the species occupy more habitats.

Seeds are dispersed by several agents, and many seeds and fruits have special features suited to their method of dispersal. Wind carries light seeds and those with wings or hair-like structures, such as the seeds of the drumstick, maple, and dandelion, which float through the air. Water carries seeds that can float, such as the coconut, which is carried across water. Animals disperse seeds in two ways: some fruits are eaten by animals, and their seeds pass out unharmed in the droppings far away, while other seeds have hooks or spines that stick to an animal's fur or to clothes and are carried away. Some plants disperse their own seeds by an explosive bursting of the fruit, which flings the seeds outward, as in the balsam (touch-me-not).

Seed dispersal connects sexual reproduction in plants to the spread of plant life across the Earth. By forming seeds, protecting them in fruits, and dispersing them widely, flowering plants ensure that new plants grow in many places, far from the crowded base of the parent. This is also why fruits — which protect and help disperse seeds — are so common, and why many of them, being food for animals (and us), help carry seeds far and wide.

Animals reproduce to produce young ones and continue their species, and like plants they can reproduce sexually or, in some simple animals, asexually. Most animals reproduce sexually, which requires a male and a female parent. The male produces male sex cells called sperms, and the female produces female sex cells called eggs (ova). Reproduction begins with fertilisation — the fusion of a sperm with an egg — to form a single cell that grows into a new individual.

Fertilisation in animals can happen in two ways. In internal fertilisation, the sperm and egg join inside the body of the female; this occurs in humans, other mammals, birds, and reptiles. In external fertilisation, the sperms and eggs are released into water, where they meet and join outside the body; this occurs in animals like fish and frogs, which release large numbers of eggs and sperms into the water. The fertilised egg, called a zygote, then develops into a young one.

Animals also differ in how the young develop and are born. Animals that lay eggs, from which the young later hatch, are called oviparous animals — examples include birds, frogs, fish, reptiles, and insects. Animals that give birth to live young, which have developed inside the mother's body, are called viviparous animals — examples include humans and most other mammals like cows, dogs, and cats. In some animals, the young one looks quite different from the adult and changes form as it grows; this change is called metamorphosis, as when a tadpole becomes a frog or a caterpillar becomes a butterfly.

A few simple animals can reproduce asexually, from a single parent. Hydra, a small water animal, reproduces by budding: a bud grows out from its body and develops into a new Hydra, which then separates. The single-celled Amoeba reproduces by binary fission: the parent cell simply divides into two new individuals. These examples show that, just as in plants, animal reproduction can be sexual (needing two parents and producing varied offspring) or asexual (needing one parent and producing identical offspring), ensuring that each kind of animal continues on the Earth.