Types Of Embryo Sacs Angiosperms
Types of Embryo Sacs
The previously mentioned development of female gametophyte is that of a Normal Type 8 -nucleate monosporic embryo sac. However, there are several variations from this normal type and the embryo sacs are divided into categories on the basis of these variations.
The embryo sac of this type may be 4-nucleate or 8-nucleate.
Polygonum or Normal Type
Already described under the heading of development of female gametophyte.
It is the characteristics of the family Onagraceae. The megaspore nucleus divides twice and produces four nuclei at the micropylar end. Three of them act as Egg Apparatus while the fourth one is the Polar Nucleus.
The bisporic embryo-sac are 8-nucleate.
In this type, after the first division, one of the dyad degenerates, the other dyad forms eight nuclei by successive divisions. The mature sac has eight nuclei which are arranged in the same manner as in Polygonum Type. This type is usually found in Liliaceae, Amuryllidaceae and Orchidaceae, but is less common in dicots.
After the formation of dyad, the lower cell may disintegrate or its nucleus divides once or twice producing four nuclei.
Eight tetrasporic types are known. In these types, protoplasts of all four megaspores build together the embryo sac. The number of nuclei in the embryo sac varies from 4-16.
Each of four megaspores nuclei divides twice forming 16 nuclei which are uniformly distributed along the periphery of the embryo-sac. Two nuclei at the micropylar end form an egg and a synergid. Eight of them fuse to form Secondary Nucleus, while the remaining six remain at the periphery of the embryo sac.
In this type the 16 nuclei are arranged in quarters, one at each end of the embryo-sac and two at the sides. Three nuclei of each quarter become cells while the fourth one moves towards the center and act as Polar Nucleus. Therefore, there are four triads and four polar nuclei. One cell of the micropylar triad as the Egg, and is the only functional cell.
The nucleus of the megaspore mother cell divides meiotically producing four nuclei. One daughter nucleus moves towards the micropylar end while the other three moves towards chalazal end. Each nucleus divides twice to form four nuclei at the micropylar end and twelve nuclei at the chalazal end. The four micropylar nuclei give rise to Egg Apparatus and Upper Polar Nucleus, and the twelve chalazal nuclei give rise to one lower Polar Nucleus and Eleven Antipodal Cells.
Chrysanthemum cinerariafolium Type
The four megaspore nuclei show 1+2+1 arrangement, i.e., one nucleus lie at each pole and the two in the center. Megaspore nucleus at micropylar end divides twice forming four nuclei. The nucleus present at the chalazal end shows no regular division. The embryo-sac may have 6, 9 or 10 nuclei.
Fritllaria Type (8-Nucleate)
Three out of the four megaspore nuclei move towards the chalazal end and the fourth one towards the micropylar end. The three nuclei present at the chalazal end fuse to form a triploid (3n) nucleus which then divide mitotically to form two triploid nuclei at the chalazal end. At the end of the division there are two haploid nuclei at micropylar end and two triploid nuclei at the chalazal end. The nuclei divide once again, resulting in the formation of four haploid nuclei at micropylar end and four triploid nuclei at chalazal end. One nucleus from each end act as Polar Nucleus and fuse after migration to the center forming a 4n secondary nucleus while the remaining three nuclei at the micropylar end form the Egg Apparatus and three nuclei at the chalazal end form the Antipodal Cells.
Plumbagella type (8-Nucleate)
The four megaspore nuclei show 1+3 arrangement, i.e., one nucleus at the micropylar end and the three nuclei at the chalazal end. The nucleus of chalazal end fuse to form triploid (3n) nucleus. In this way, the embryo-sac become binucleate. Both the nuclei divide mitotically and form two haploid nuclei at micropylar end and two triploid nuclei at chalazal end. One nucleus from each end migrates to the center and fuse to form tetraploid Secondary Nucleus (4n). The nucleus at the micropylar end forms the Egg while the triploid nucleus at the chalazal end forms a single Antipodal Cell.
The four megaspore nuclei arrange themselves crosswise, i.e., one at the micropyle, one at the chalazal, one at the right side and one at the left side of the embryo sac. The nuclei diide and eight nuclei are formed arranged in four pairs. One nucleus from each pair migrate to the center and fuse to form tetraploid (4n) Secondary Nucleus. One nucleus at the micropylar end forms the Egg, while the remaining three nuclei degenerate.
Adoxa Type (8-Nucleate)
Four megaspore nuclei divide to form eight nuclei. Three arrange themselves at the micropylar end and form the Egg Apparatus. The other three are arranged at the chalazal end and form the antipodal cells while the remaining two are the polar nuclei.