Sexual Reproduction in Plants

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The plants that sexually reproduce have the reproductive structures called the flowers. The flower is a condensed shoot with the nodes present very close to each other. The different parts of the plant are attached to the nodes. All the structures present at one node are collectively called the whorl. The first or the outermost two whorls are called the non-reproductive whorls. They are the calyx and corolla. The inner two whorls are androecium and gynoecium, the reproductive whorls.

Parts of a Flower

Calyx

is the outermost and most often green in colour. The individual units of calyx are called the sepals. It protects the inner whorls at bud stage.

Corolla

is the next inner whorl and is often coloured brightly. The individual units of corolla are called petals. They serve to attract bees, birds, etc which are the agents of pollination.

Androecium

is the male reproductive part of the flower. The individual units of androecium are called the stamens. Each stamen has a thread-like filament at the free end of which is attached the four-lobed anther.

The anther has four pollen-sacs, one in each lobe. The pollen-sacs contain cells called the microspore mother cells (MMC) or the pollen mother cells. MMCs undergo mieosis to produce four haploid microspores, also called the pollen grains. Each microspore divides once mitotically to produce two male gametes or sperm cells. Thus, each microspore mother cell produces 8 sperm cells.

Gynoecium

is the female reproductive part of the flower. The individual units are called the carpels or pistils.

A flower may have one to many carpels, either fused or free. Each carpel is made up of the basal ovary, middle style and the upper stigma. The ovary is the chamber where there are many ovules that are attached to an axis. Each ovule consists of a haploid egg and other associated cells. The stigma is a sticky structure that receives the pollen grains. The style is hollow and provides a passage for the male gametes to reach the female gametes, the eggs.

Pollination

Transfer of pollen grains to the stigma is called pollination. If the pollen grains are transferred to the stigma of the same flower, the pollination is called self-pollination or autogamy. If the pollen grains are transferred to the stigma of another flower of the same species, the pollination is called cross-pollination or allogamy.

Cross pollination is brought about by various agencies like wind, water, bees, birds, bats and other animals including man.

The anthers on maturity burst open with force and this is called dehiscence. This releases the pollen grains with force which are then carried by wind and water to other plants. In other plants, the flowers are brightly coloured and scented to attract the birds, bees, etc. The insect or the bird enters the flower to suck the nectar produced by glands at the base of the flower. The pollen grains present on the dehisced or open anther, stick to the legs or abdomen. When the same insect visits other flowers the pollen grains are transferred to the stigma of those flowers.

Fertilisation

On reaching the stigma, the pollen grains put out a tube. This is called germination of the pollen grain.

The tip of the tube contains the male nuclei. The tube grows and enters the ovule where it bursts at the tip releasing the male gametes. One of the male gametes fuses with the egg, the female gamete. The fusion of the male gamete with the female gamete is called fertilisation. This results in the formation of zygote that is diploid. The zygote develops into the embryo. The other male gamete fuses with the polar nuclei. This results in the formation of a triploid nucleus called the endosperm nucleus. Since the process of fertilisation involves two fusions, it is called double fertilisation.

The divisions of the endosperm nucleus result in the formation of the endosperm that nourishes the growing embryo. The ovule then becomes the seed and the ovary changes into fruit.