Notes on Sexual Reproduction

Introduction

• The production of new organism from two parents by making use of their sex cells (called gametes) is called sexual reproduction.
• In sexual reproduction, a male gamete fuses with a female gamete to form a new cell called zygote. This zygote then grows and develops into a new organism in due course of time.
• (The cells involved in sexual reproduction are called sex cells or gametes or germ cells)

Sexual reproduction in Flowering plants:

• 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).
• The flowering plants are reproduced by sexual reproduction method.
• 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 plant.

Parts of a Flower:

The main parts of flower are:

1. Receptacle: The base of a flower to which all the parts of a flower are attached is called receptacle.
2. Sepals (group name calyx): Sepals are green outermost leaf-like floral organs which protect the flower in the bud stage.

      3.   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.

     4.   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.

     5.  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.

The flowers which contain only sex organ, either stamens or carpels are called unisexual flower. For example: papaya and watermelon plants

The flowers which contain both the sex organs, stamens as well as carpel, are called bisexual flowers. For example: Hibiscus and mustard plants.

A new seed of the plant is formed when the male gamete in a pollen grain unites with the femal gametes present in the ovule.

Pollination:

• The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination.
• If the transfer of pollen occurs in the same flower or another flower on the same plant, it is called self-pollination.
• 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.
• Pollination is done by insects, birds, winds and water.

Fertilization:

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Fertlisation occurs when the male gamete present in pollen grain joins with the female gametes present in ovule.

• 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.
• 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.
• 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 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.

A seed is reproductive unit of a plant. The seed contains a baby plant (or embryo) and food for the baby plant.

• The part of baby plant in see which develops into shoot with leaves is called plumule and part which develops into root is called radicle. The part of seed which contains stored food for the baby plant is called cotyledons.
• When the seed gets suitable conditions like water, air warmth etc., it germinates and a new plant grows out of the seed.