Reproduction

There are two types of reproduction:
Asexual reproduction
Asexual reproduction produces offspring that are identical to the one parent. One example is the splitting of bacteria into two. Another is when gardeners make cuttings, by removing a small piece from a plant and placing them in the ground to grow into a new plant.

Sexual reproduction
Sexual reproduction involves two parents mixing their genes to produce offspring that are genetically different from them. This is carried out by a mobile gamete, (which contains half of the males genes), such as a sperm or a pollen grain, fusing with a stationary female gamete (containing half of the female?s genes), such as an egg or an ovule.

The fusion of the male and female gametes is called fertilization. Some species (eg fish) use external fertilization. In this case millions of eggs and sperm are released into the water and fertilization takes place in the open water. Usually the adults do not care for the young so the survival rate is low. This is why so many gametes are needed. Some species (eg humans) use internal fertilization, which means that fertilization takes place inside the woman?s body (the fallopian, or egg tubes). This means that less sperm is wasted and also means that the young can grow safely inside the woman?s body. Animals on land cannot use external fertilization, because the sperm needs to be able to swim to the egg.

Insect pollinated flowers (you need to know the structure) produce sweet nectar that the insects feed on. They have brightly coloured petals and smells to attract the insects. The pollen is found on the anther, which is situated at the end of a long filament. When an insect visits the flower the pollen grains stick to the hairs on its body and are carried away when it leaves. When the insect visits another flower, some of the pollen grains are rubbed off onto the stigma, which is positioned at the end of a long stalk called the style at the center of the flower. When the pollen lands on the stigma we say that pollination has taken place. The pollen grows a pollen tube down the length of the style until it reaches the ovary. The nucleus moves down this tube and fuses with the female nucleus (ovule) in the ovary to make a zygote. This fusion of the nuclei is called fertilization.

Wind pollinated flowers (egg the flowers of grass) do not need to attract insects, so they are not brightly coloured, do not produce nectar and have no smell. The anthers hang outside the flower so that the pollen is easily picked up by the wind. The stigma also hangs outside the flower and it has many side branches to act like a net to catch the pollen as it blows past.

The zygote (fertilized ovule) grows to form the seed. The ovary, which usually contains many seeds, will grow into the fruit. The flesh of an apple is a fruit and so is a pod containing beans (which are the seeds). There is little point in a seed growing directly under its parent, because it would be competing for water, sunlight and space. It is best if the seed is dispersed (transported somewhere else). There are many ways of doing this. Some species of plant produce berries that are designed to be eaten by birds. However, the bird cannot digest the seed that passes through the digestive system and out with the faeces (an excellent fertilizer). Some seeds have hooks on the side that stick to the fur of passing animals and only fall off when the animal later cleans itself, which may be many miles from where the seed originated. Other seeds are dispersed by the wind. Some have wings so that they spiral to the ground. Others are very light and have feathery ends so that the wind can carry them great distances. Coconuts have a store of nutrients to survive long journeys as they float across oceans.

You need to know the structure of a bean seed, including: the testa (outer coat), micropyle (small hole where the pollen tube first entered to fertilise the ovule and where water will enter when the seed germinates), cotyledons (the two halves of the bean where starch is stored to feed the growing embryo), plumule (the part of the embryo that will grow upwards to form the shoot) and radicle (the part of the embryo that grows downwards to form the root).

Usually, seeds do not grow as soon as they leave the mother plant. They usually need to wait until the conditions are ideal. For instance, there would be no point in a seed germinating (starting to grow) in freezing conditions or in the middle of the dry season. Such things as temperature, moisture and oxygen are often seed to start germination. Sometimes a seed will lie dormant (still alive, but not growing) for many years before it germinates.