Reproduction in Angiosperm
The process of production of new individuals similar in form to the parent organisms is called reproduction. There are two types of reproduction: Asexaul reproduction and Sexual reproduction.
It is produced by a single organism without production of gametes. It is also defined as the production of offspring from a single organism without the fusion of gametes. Asexual reproduction in angiospermic plants occurs by various modes of vegetative growth. So, it is populary known as vegetative reproduction. It is a form of asexual reproduction in which new individuals are formed from vegetative organs such as roots, stems and leaves. Vegetative propagation in angiospermic plants take place by two methods: Natural vegetative propagation and Artificial vegetative propagation.
The reproduction which involves formation of male and female gametes and their fusion is called sexual reproduction. An angiospermic plant produces two kinds of gametes: The male gametes and the female gametes.
Development of Male Gametophyte
Microspore represents the beginning of the male gametophyte.The germination of the pollen grain starts within the pollen sac( microsporangia). The nucleus of microspore divides mitotically to form a large vegetative cell and a small generative cell.The vegetative nucleus remains in the centre while the generative nucleus is attached to the wall of the pollen grain in the beginning. Usually, the dehiscence of the anther occurs at this two- celled stage whereas, in some cases they become 3 celled before the dehiscence. Further development of microspore takes place at the stigma after pollination. After pollination the pollen grains are deposited on the stigma, where they absorbed stigmatic fluid and well up. Due to swell, the outer exine ruptures along the germ pore and intine comes out in the form of a pollen tube. Usually, only one tube develops fom a pollen grain. The pollen tube grows down through stigma ans stylke and enters the ovarian cavity. The tube nucleus lies at the tuip of the pollen tube and regulates the growth of the pollen tube. The generative cell divides and forms two male gametes. Each male gamete consist of a nucleus surrounded by a thin sheath of cytoplasm. The pollen tube with two male gametes and a tube nucleus constitute the male gametophyte.
Formation of Female Gametophyte
Formation of Female Gametophyte( Megagametogenesis): Formation of a female gamete within the ovule is called megagametogenesis. It occurs in following steps: 1. Formation of ovule 2. Ovules develop from the inner wall of the ovary. They develop as a tiny knob. The tiny knob gets raised from the ovary wall by a short stalk called funicle. The nucellar is surrounded by one or two integuments, leaving a small gap at the apical end. This opening is called micropyle. The basal region of the ovule, where the funicle remains attached is called chalaza.
Development of Megaspore
In general, a single hypodermal cell of the nucleus differentiates to form the archesporium. The megaspore mother cell divides meitotically to form a tetrad of four haploid cells. The first division is always transverse to form dyad cells. The second division is also tyransverse to form a linear tetrad of four megaspores. Usually only one of the four megaspore is functional and forms the embryosac or female gametophyte.