Sexual Reproduction In Flowering Plants-Concept
1. Structure in plant’s male reproductive organ which develops into pollen sac
2. Each anther has fourmicrosporangia
3. Development of pollen takes place here
Structure of Microsporangium and Formation of Pollen
1. Surrounded by four wall layers – epidermis, endothecium, middle layer, tapetum
2. The outer three layers protect the pollens and also help in release of pollens from anther
3. The innermost layer, tapetum nourishes the pollen grains
4. A group of compactly arranged homogenous cells called sporogenous tissue are present at the centre of microsporongium in an young anther
5. Each sporogenous cell is known as pollen or microspore mother cell
6. The microspores are arranged in the form of a tetrad
7. The microspores separate from each other and develop into a pollen grains
1. It is meiotic formation of haploid megaspores from diploid megaspore mother cell.
2. The arrangement of megaspores in a tetrad is commonly linear.
3. Only one megaspore of a spore tetrad is functional.
4. It is found inside a megasporangium.
5. Generally, a single megaspore mother cell is functional in a megasporangium.
1. The transfer of pollen grains from the anther to the stigma of the flower is called as pollination.
2. This process is important for the reproduction in plants as the male gamete participates in the process.
1. In the process of pollination when the pollen of an anther falls on the stigma of the same flower, it is known as autogamy.
1. When the pollen of another flower from the same plant falls on the stigma of a flower it is called as geitonogamous type of pollination and the process is called as geitonogamy.
1. Allogamy is a type of pollination in which a pollen grain of a flower of another plant falls on the stigma of a flower.
Factors which favour cross pollination
1. Many number of species are pollinated through cross pollination.The factors which favour cross pollination are unisexuality, dichogamy, self sterility, mechanical and structural barrier and heterostyly.
1. In some plants flowers are unisexual and bloom on different plants .In this case cross pollination is the only possibility. For example, palms and papaya.
2. In some plants, the male and female flowers are different but bloom on the same plant. For example, cucumber.
1. In some plants, the maturation period of the androecium and gynoecium is different.
2. There are plants in which the androecium matures earlier than the gynoecium .This condition is known as protandry. For example, sweet pea.
3. In some plants the gynoecium matures earlier than the androecium.This condition is called as protogyny. For example, custard apple.
1. There are certain plants in which even if the stigma receives pollen grains from the anthers of same flower the pollen fail to fertilise the ovary.
2. Only the pollen grains of the different plant of the same species are able to fertilise the ovary e.g., Orchid.Mechanical and structural barrier
1. There are certain plants in which the pollen grains of the same flower fail to reach its stigma.
2. There is a hood covering the stigma acts as mechanical barrier in the Pansy flowers.
Heterostyly1. In some flowers, the growth of the style and stigma grow on different heights which eliminates the chances of self pollination. For example, prime rose.
1. Pollen grain germinates on the stigma to produce pollen tube
2. Pollen tube grows through the tissue of style and reaches ovary
3. Enters ovule through micropyle and then into one of the synergid
4. In synergid, pollen tube releases two male gametes
5. One male gamete fuses with the nucleus of the egg cell, forms zygote (diploid)
6. Other male gamete fuses with two polar nuclei, produces a triploid endosperm nucleus
1. Apomixis is a mode of reproduction which does not involve formation of zygote through gametic fusion.