SEXUAL REPRODUCTION – THE BASICS
1. Sexual Mode of Reproduction – In this process, the exchange of genetic materials takes place by a process called conjugation.
2. sexual reproduction involves the production of germ cells or gametes and their subsequent fusion to produce a new individual.
3. In sexual reproduction, a male gamete fuses with a female gamete to form a new cell called zygote.
4. This zygote then grows and develops into a new organism in due course of time.
5. One germ-cell is large and contains the food-stores while the other is smaller and likely to be motile. Conventionally, the motile germ cell is called the male gamete and the germ-cell containing the stored food is called the female gamete.
SEXUAL REPRODUCTION IN FLOWERING PLANTS
1. The plants in which the sex organs are carried within the flowers and the seeds are enclosed in a fruit are called angiosperms (flowering plants).
2. The flowering plants are reproduced by sexual reproduction method.
3. The function of a flower is to make male and female gametes and to ensure that fertilisation will take place to make new seeds for the reproduction of the plant.
4. The different reproductive parts of a flower are – sepals, petals, stamens, and carpels. Stamens and carpels are the reproductive parts of a flower which contain the germ-cells.
5. The flower may be unisexual (papaya, watermelon) when it contains either stamens or carpels or bisexual (Hibiscus, mustard) when it contains both stamens and carpels.
6. Parts of a Flower: The main parts of a flower are:
(i) Receptacle: The base of a flower to which all the parts of a flower are attached is called receptacle.
(ii) Sepals (group name calyx): Sepals are green outermost leaf-like floral organs which protect the flower in the bud stage.
(iii) Petals (group name corolla): The colourful parts of a flower are called petals. The petals lie inside the sepals. Petals attract insects to flowers for pollination. They also provide protection to centrally placed reproductive organs.
(iv) Stamen: Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour. The stalk of stamen is called filament and the swollen top of stamen is called anther. The anther of stamen makes the pollen grains and stores them.
(v) Carpel: Carpel is present in the centre of a flower and is the female reproductive part. It is made up of three parts. The swollen bottom part is ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma. The ovary contains ovules and each ovule has an egg cell.
7. Pollination: The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination.
(i) If the transfer of pollen occurs in the same flower or another flower on the same plant, it is called self-pollination.
(ii) If the pollen is transferred from the anther of a flower on one plant to the stigma of a flower on another similar plant, it is called cross-pollination.
(iii) Pollination is done by insects, birds, winds, and water.
(iv) After the pollen lands on a suitable stigma, a tube grows out of the pollen grain and travels through the style to reach the ovary leading to fertilisation.
8. Fertilization: The fusion of the male gamete with the female gamete is called fertilisation. Fertilization occurs when the male gamete present in pollen grain joins with the female gametes present in ovule.
(i) A male gamete moves down the pollen tube. The pollen tube enters the ovule in the ovary. The tip of pollen tube bursts open and male gamete comes out of pollen tube.
(ii) In ovary, the male gamete of pollen combines with the nucleus of female gamete or egg present in ovule to form a fertilized egg called zygote.
(iii) After fertilization, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is gradually converted into a seed. The seed contains the future plant or embryo which develops into a seedling under appropriate conditions. This process is known as germination.
(iv) The ovary grows rapidly and ripens to form a fruit. The other parts of flower like sepals, petals, stamen, stigma and style dry up and fall off.
9. The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule. This fusion of the germ-cells or fertilisation gives us the zygote which will growing into a new plant.