In a life cycle, angiosperms switch their generation from a diploid stage of sporophyte to haploid and then back again to diploid. This haploid stage is known as gametophytes. In animals, gamete formation takes place soon after meiosis. But in plants, it happens after gametophyte formation.
Sexual reproduction in plants starts with spore formation. The sporophyte is a diploid generation in angiosperms where haploid spores are produced by meiosis which in turn undergoes mitosis to form multi-celled haploid gametophytes. These haploid gametophytes differentiate to produce gametes – sperm and egg cells. Gametophytes are made up of certain cells and are implanted in the reproductive organs of the flower.
Male Gametophyte: Pollen Grain
The pollen grain is the male gametophyte in angiosperms. Microsporangia (singular: microsporangium) are bilobed pollen sac structures. Microspores develop and mature into pollen grains in them.
Inside the microsporangium, Pollen mother cell (PMC) undergoes meiosis and results in four microspores which eventually mature into pollen grains. The inner layer called tapetum nourishes the developing microspores. The pollen grains consist of two cells out of which a generative cell within a pollen tube cell. Once the microsporangium is mature it bursts and releases the pollen from the anther as pollen grain consists of two layers. The outer thick layer is called exine and the inner thin layer is called inline which protects the pollen from damage.
Female Gametophytes: Embryo sac
The whole process of female gametophyte development occurs in two different phases. The first phase involves the megasporogenesis, where a single diploid mother cell undergoes meiosis to form haploid megaspore tetrad out of which only one will survive and other three disintegrate. The functional megaspore grows into an embryo sac.
Megagametogenesis is the second phase where the functional haploid megaspore undergoes mitosis to generate 7-celled, 8-nucleate gametophyte known as embryo sac. Out of the eight nuclei, polar nuclei move to the center and fuse to produce a single diploid cell at the center. This is the cell which fuses with the sperm to produce triploid endosperm. Three of the nuclei develop into antipodal cells and two will transform into synergid cells which eventually disintegrate.
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