Flowers are one of the most fascinating sights to behold.
Incidentally, flowers also happen to be the site of sexual reproduction as they contain the androecium and gynoecium – the male and female reproductive organs of a plant. The megaspore is formed when one of the nucellus cells, towards its micropylar end, gets differentiated into megaspore mother cell which can be easily distinguished from other cells due to its large size, dense cytoplasm and prominent nucleus. The megaspore mother cell undergoes meiosis to form four megaspores which arrange themselves in a linear tetrad. This process of formation of megaspores from the megaspore mother cell is called megasporogenesis. Of the four megaspores, usually the one near the chalazal end becomes functional whereas the other three, near the micropylar end, degenerate. The functional megaspore enlarges and simultaneously undergoes mitotic division to form an embryo sac. This type of formation of the embryo sac from a single megaspore is known as monosporic development. While undergoing these mitotic divisions, the nucleus of the functional megaspore, first divides to form two nuclei, which move to the two opposite ends of the embryo sac. This is the two nucleate stage of the embryo sac. The mitotic divisions continue and result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. A mature embryo sac thus has 8 nuclei after 3 mitotic divisions which are arranged in a group of four at each end of the embryo sac. Interestingly, the mitotic divisions that occur in the megaspore’s nucleus are free nuclear, which means the division of the nuclei doesn’t immediately trigger cytoplasmic division and cell wall formation. Instead, we see the formation of cell walls only after the 8-nucleate stage, due to which six of the eight nuclei organise into cells. The two remaining nuclei, called polar nuclei, migrate to the centre of the embryo sac. Thus, we find the embryo sac to be in 7-celled and 8-nucleate stage, with six cells at the poles and a large central cell with two nuclei in the centre. Meanwhile, the six cells organise to form the typical structure of the embryo sac, which is now ready for fertilisation. The three cells present at the chalazal end, for instance, group together to form the antipodal cells which do not have any specific function.
whereas the three cells at the micropylar end group together to form the egg apparatus.
While one of the cells functions as an egg or the female gamete, the other two cells are called synergids. The synergids as well as other cells form the embryo sac, the female gametophyte borne inside the pistil.