The life cycle of angiosperm plant involves a regular alternation of generation between gametophytic phase and sporophytic phase. The dominant phase is the sporophytic phase and the gametophytic phase depends upon the sporophytic phase for its existence. The sporophytic plant body is formed from the seed. The plant produces flowers which bear two types of sporophylls; the microsporophyll (stamen) and megasporophyll (carpel).
The stamen bears two pollen sac or micro sporangium which will give rise to the microspore mother cell. The microspore mother cell is the last cell of sporophytic phase. Each microspore mother cell under goes meiotic division and produce four microspores (pollen grains). The pollen grains give rise to the male gametophyte.
The mega sporophyll consists of ovule; the ovule encloses a mass of tissue known as the nucellus. From the nucellus the megaspore mother cell develops which represent the last cell of sporophytic phase. The megaspore mother cell undergoes meiotic division and forms four megaspores. The embryo sac of angiosperm is non sporic and it develops from a megaspore. The entire three mega spore lying towards to the micropyle degenerate and only one megaspore becomes functional and forms the embryo sac. A mature embryo sac is seven celled and eight nucleated structure which consists of three antipodal cells at the chalazal end, one egg or female gamete and two synergids at the micropylar end and a central cell found at the centre of the embryo sac. The central cell is formed by the fusion of the two polar nuclei.
The pollen grains from the anther are carried to the stigma and this process is known as the pollination. Pollen grains give rise to the male gametophyte. Each pollen grain consists of two male nuclei and tubed nucleus. After pollination the pollen germinates on the stigma and the pollen tube moves through the style carrying the two nuclei. On reaching the ovule the tube nucleus disappears then the pollen tube enters the embryo sac through the micropyle and releases the two male gametes into it.
One of the male gamete fuses with the egg and this fusion of gametes is known as the syngamy. The syngamy results in the formation of the zygote; where the second male gamete fuses with the diploid central cell so known as the triple fusion forming the endosperm which is triploid. The zygote so formed as a result of fertilization is the first cell of sporophytic generation. The zygote develops to embryo later forms the seed and the ovary develops to fruit. The seeds on germination give rise to the sporophytic plant body.
