Angiosperms are known for their fruits where seeds are encased within them. The case is totally different than in the gymnosperms where seeds are naked.Such a remarkable feature is due to the evolutionary advancement of the angiosperms. Their flowers miraculously turn into the fruits and seeds after the an extra-ordinary transfer of the pollen grains, followed by the double fertilisation. The pollination is not as simple as it looks.They can go either for the self-pollination or can go for the cross -pollination that is using the pollen grains of the same plant or taking from an another plant of the same species.
The phenomenon is not based on any fictitious tales but is supported by the concrete evidences in the biological science. That is the record that is set there by a series multi-disciplinary researches and the strong observations.
This is quite amazing that why some angiosperms are set to adopt the pollen grain of the same plant and are called to use self pollination or in-breeding techniques. While as the others use their out-breeding devices in order to avoid the self-pollination. Plants that go for the self pollination produce a progeny very close to their parent generations, with the negligible variations. This is how they maintain the purity of their race.They have existed from the past but they cannot be called as future plants as they are the living organisms with the minimum variations and therefore have a maximum threat of being damaged by the future environmental changes. Their genome is lacking a desired level of variation.
Those plants which adopt to go for the cross-pollination use multiple agents for the same. They can use wind ,water, insects, birds and many other carriers of pollen transfer. This is quite remarkable that those plants which go for the self-pollination don't show any attractive flowers or the flowers with extraordinary fragrance. This trait is more associated with the flowers that are set to go for the cross pollination. Especially the flowers that have insects and birds as their agents of pollination. This is because insects like bees and some birds are highly attracted towards such magnificent flowers. Insects also visit them for a specific type of food called as nectar. This can be called as an evolved feature of such flowering plants to survive in a competitive environment.
The journey starts with the transfer of a mature pollen grains to the receptive stigma, where it forms an amazing pollen tube after the germination. A pollen tube contains two cells that is a generative cell and a vegetative cell. This is quite mind boggling that how pollen tube enters an ovule. It can go either through the micropyle or chalaza or through the integument. The vegetative cell works for the formation of the pollen tube and by the complex mechanisms of the chemotropism it helps in the release of two male gametes in the embryo sac. And this is more like a surprising story that how pollen tube releases it generative cell in the embryo sac.A mature embryo sac contains a combination of eight cells with the two synergids, three antipodal cells,two polar nuclei and an egg cell.
After the pollination ,angiosperms go for the syngamy and the triple fusion. Syngamy leads to the embryo formation as one of the male gametes fuses with the egg cell and the triple fusion ,when one of the male gametes fuses with the two polar nuclei that consequently leads to the formation of a triploid endosperm. The endosperm acts a food reserve and the growing embryo acts as new plant after the seed germination. After this double fertilization the ovary turns into a fleshy fruit and the ovules into the seeds and finally the miraculous story ends with a great end. The extra-ordinary phenomenon of double fertilization is specific to the angiosperms only.
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