![]() Both the cells thus undergo one mitotic division and again form a tetrasporic stage. The chalazal nuclei fused together and form 3n nucleus. Like Drusa type, out of four nuclei formed, one nucleus remains towards the micropyle, and the rest three at the chalazal end. In the mature embryo sac, egg and two synergids remain towards the micropyle, two (one from each pole) at the centre and the rest eleven at the chalazal end, e.g., Drusa oppositifolia of Apiaceae. 3.8):Īll the nuclei undergo two divisions and form 16 nuclei, out of which four nuclei remain towards the micropyle and the rest twelve at the chalazal end. Other types of embryo sac development (Fig. This type is also known as monosporic type, because, out of four megaspores, only one remains functional and forms the embryo sac. This type of embryo sac development is very common in angiosperms and is known as ordinary type or normal type or Polygonum type. The antipodal cells have viscous mass of cytoplasm, covered by cellulosic wall (Fig. In the egg apparatus, each nucleus is surrounded by viscous mass of cytoplasm without any wall, of which the middle one is the largest and called egg, ovum or oosphere and the rest two (one on each side of the egg) are the synergids or helping cells. The three nuclei of the micropylar end form the egg apparatus and the rest three at the chalazal end are called antipodal cells. It is also known as polar fusion nucleus or secondary nucleus. These nuclei fuse together and form 2n nucleus, the definitive nucleus. One nucleus from each pole then moves towards the centre and forms a pair of polar nuclei (Fig. Out of eight nuclei, initially four remain towards the micropyle end and the other four towards the chalazal end. Initially, the embryo sac is uninucleate and with further growth its nucleus divides by three successive divisions and forms eight nuclei (Fig. The functional megaspore becomes enlarged at the expense of tape tum and the nucellus and thus forms the female gametophyte i.e., the embryo sac. Megaspore (n) is the first cell of the female gametophyte (Fig. Megagametogenesis (Formation of female gametophyte i.e., Embryo sac): The functional megaspore forms the female gametophyte i.e., the embryo sac. Out of four megaspores, only one which remains towards the chalazal end behaves as functional megaspore and the other three which remain towards the micropylar end, gradually degenerate (Fig. The megaspores are then arranged in an axial row, called linear tetrad (Fig. Both the cells again divide transversely and thus four (4) haploid megaspores are formed. The first division of megaspore mother cell is transverse, forming two cells. The megaspore mother cell is diploid (2n), which undergoes meiosis and forms four haploid (n) megaspores (Fig. Megasporogenesis (Development of Megaspores): Sometimes, the archesporial cell does not divide and directly functions as megaspore mother cell. ![]() 3.6C) and the primary parietal cell undergoes repeated vertical divisions and forms layers of parietal cells (Fig. The primary sporogenous cell functions as megaspore mother cell (Fig.
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