- In a population variations are useful for survival of a population.
- Sexual mode of reproduction allows individuals to have more variations to be generated.
- Generation of variations in DNA copying mechanism of asexual mode is fairly slow
- Each new variation is made in a DNA copy that already has variations accumulated from previous generations.
- Combining variations from two or more individuals would thus create new combinations of variants.
- Each combination would be novel, since it would involve two different individuals.
- For each new cell to not have twice the amount of DNA that previous generation have, special lineages of cells in specialised organs have only half the number of chromosomes and DNA as compared to other cells.
- Two individuals combining during sexual reproduction, result in re-establishment of the number of chromosomes and DNA content in new generation.
- Motile germ cell is called male gamete and germ-cell containing food is called female-gamete
Sexual reproduction in flowers
- Flower can be termed as the sexual organ of a plant.
- All the parts of a flower are arranged around an axis.
1. Sepals: Green leaf like structure.
2. Petals: Colourful structures, which add attraction to a flower. This attraction is not only having ornamental value, but a more important role in facilitating reproduction. Insects and birds, attracted by the colour, help transfer pollen grains or male gametes from male flower to female flower. This helps in pollination.
3. Male Reproductive Organ of Plant: Androecium
- The flower of a plant contains tube like structures called stamen.
- At the top of stamen is a chambered structure called Androecium.
- Androecium is responsible for the production of male gamete also called pollen grains.
4. Female Reproductive Organ of Plant:
- GynoeciumUsually at the centre of a flower you can notice Gynoecium.
- Gynoecium is pitcher shaped structure with a long tube protruding out of it.
- The gynoecium produces female gamete also called eggs.
Difference between Unisexual and Bisexual Plants
- Stamens and carpels are the reproductive parts of a flower.
- Carpel is present in the centre of the flower and is the female reproductive part.
- Carpel is made up of three parts , the swollen part is ovary , the middle elongated part is style and the terminal part is sticky and called stigma.
- The ovary contains ovule which has an egg cell, the male gamete fuses with the female gamete present in ovule.
Pollination In flower
- Pollination is the transfer of pollen from a male part of a plant to a female part of a plant, later enabling fertilisation and the production of seeds.
- The pollen needs to be transferred from stamen to the stigma.
- If the transfer of pollen occurs in the same flower it is self-pollination, if the transfers occurs in a different flower it is called cross-pollination
- Cross pollination can be facilitated by insects, birds, animals, air or water.
- A tube growing out of the pollen grain and travels through the style to reach the ovary helps the landed pollen grain to reach female-germ cell.
Zygote Formation in Plant Reproduction
- Once pollen grains enter the androecium, one of them enters the egg to fertilize it to form a zygote.
- Seeds are the zygote or embryo of the plant.
- To survive and to germinate, seeds need source of food.
- In all seeds there is abundance of food.
- That is why for our daily need we depend on so many seeds like rice, wheat, groundnut for food.
- During germination the food in the cotyledon is used to grow a new plant.
- Once green leaves come out, they take care of further food production
- After fertilization the zygote develops several times to form an embryo within the ovule.
- The ovule develops a tough coat and is gradually converted into a seed
- The seeds contain the embryo which develops into a seedling under proper conditions, this is known as germination