EMBRYO SACS

          Depending upon a number of megaspore nuclei taking part in the development, the embryo sacs (female gametophyte) of angiosperms may be classified into three main categories – monosporic, bisporic and tetrasporic embryo sacs.

            1. Monosporic Type:- In monosporic type only one of the four megaspores takes part in the development of female gametophyte (embryo sac). The female gametophyte of this type may be 8-nucleate and 4-nucleate.

            (a) Polygonum type – Polygonum type is the normal type of development of embryo sac. In this type the nucleus of functional megaspore divides to form two nuclei and moves apart: one towards the micropylar end and the other towards the chalazal end. These two nuclei again divide so that the number of nuclei increases to four. Each of these four nuclei divides again to form eight nuclei, four at each end. Now one nucleus from each end or pole moves forward to the centre and fuse together to form secondary nucleus (2n). The remaining three cells of the micropylar end surrounded by a thin wall forms the egg apparatus. The other three nuclei at the chalazal end surrounded by a thin wall forms the andipodal cells.

            (b) Onothera Type – In this type the megaspore nucleus divides twice and thus produces four nuclei at the micropylar end. These nuclei give rise to a normal egg apparatus and a single polar nucleus. This type of embryo sac development is seen only in the family Onagraceae.

            2. Bisporic Type:- Bisporic type of embryo sac development is also known as allium type. In this type two of the four megaspores take part in the development of female gametophyte. Since there is no cell wall formation during Meiosis II of megaspore formation, a megaspore dyad is formed and both the functional megaspore takes part in the development of embryo sac. The female gametophyte of this type is generally 8-nucleate.

            (a)  Endymion Type – In this type, after the formation of the dyad, the lower cell may disintegrate or its nucleus may divide once or twice producing 4-nuclei. The 8-nucleate embryo sac is formed by the nuclear divisions of the upper cell of the dyad. This type of embryo sac development was first observed in Endymion hispanicus (Battaglia, 1958).

            3. Tetrasporic Type:- In this type of embryo sac there are several variations. In several cases 16 nuclei are formed with the result of two divisions after megasporogenesis.

            (a) Paperomia Type – In this type each of the four megaspores nuclei divides twice, forming 16-nuclei which are uniformly distributed at the periphery of the embryo sac. Two nuclei at the micropylar end form an egg and a synergid; 8 are used to form the secondary nucleus and the remaining 6 nuclei are cut off at the periphery of the embryo sac.

            (b) Panaea Type – In this type 16 nuclei lie in four distinct quarters which are arranged crosswise, once at each end of the embryo sac and two at the sides. Now three nuclei at each quarter become cut off as cells, while the fourth one remains free and moves towards the centre. Thus there are 4 triads and 4 polar nuclei. Here the egg cell of the micropylar triad alone is functional. Such embryo sacs have been found in many members of Malpighiaceae.

            (c) Drusa Type – In this type 16-nucleate embryo sac was recorded (Hakanson, 1923) in Drusa oppositifolia. Here, when the meiotic divisions are over, three of the megaspore nuclei pass down to the basal end of the embryo sac, and only one remains at the micropylar end. This is followed by two successive divisions, forming four micropylar nuclei and twelve antipodals. The four mycropylar nuclei give rise to the egg apparatus and the upper polar nucleus, and the twelve chalazal nuclei to a lower polar nucleus and eleven antipodal cells. This type has been recorded in Mallotus, Mainthemum, Crucianella, Rubia, Ulmus, etc.

            (d) Crysanthemum cinerariafoluim Type – In this type the four megaspore nuclei show 1+2+1 arrangement, i.e., one nuclei lie at each pole and two in the centre. The two nuclei of the centre remain quite close to each other but do not fuse together. Now the megaspore nucleus of the micropylar end divides twice forming 4 nuclei, but there is no regularity in the division of the nucleus of the chalazal end. Thus, the embryo sac may have six, nine or ten nuclei.

            Sometimes the two central nuclei out of the four megaspore nuclei are fused together forming a single diploid (2n) nucleus; the next division produces six nuclei, one haploid (n) pair at micropylar end; one haploid pair at chalazal end and one diploid pair in the centre. The next division of these six nuclei produces three groups of four nuclei each. At the micropylar end the three nuclei make the egg apparatus, and one migrates in the centre forming upper haploid polar nucleus. The four haploid antipodal cells are formed at chalazal end. One of the diploid nuclei of the central quarter behaves as the lower polar nucleus and the remaining three organize them as additional antipodal cells. Thus there are 12 nuclei in the embryo sac. Sometimes less than 12 nuclei (i.e., ten or seven) are developed, because of the failure of certain divisions.

            (f) Fritillaria Type – This type of development of embryo sac has been reported in many angiosperms including Lilium. In Fritillaria and Lilium the behavior of the four megaspores nuclei is peculiar. Here three of the megaspore nuclei go to the chalazal end and the fourth one goes to the micropylar end. The three chalazal nuclei come to lie very close to each other. During the next stage, the micropylar nucleus divides normally but three chalazal nuclei fuse together forming a triploid (3n) nucleus which then divides mitotically forming two triploid nuclei at the chalazal end; so that at the close of the division there are two haploid nuclei at the micropylar end and two triploid nuclei at the chalazal end. One more division takes place, resulting in the formation of eight nuclei, of which the four chalazal nuclei are triploid and the four micropylar nuclei are haploid. Thus, the mature embryo sac consists of haploid cells of egg apparatus, three triploid antipodal cells and a tetraploid (4n) secondary nucleus formed by the fusion of two polar nuclei, one haploid and the other triploid. The endosperm nucleus, when formed after fertilization will be pentaploid (5n) in such type of embryo sac.

            (g) Plumbagella Type – This type of development of embryo sac has been reported only in Plumbagella micrantha. In this type the 4 megaspore nuclei take up a 1+3 arrangement, i.e., one nucleus at the micropylar end and three nuclei at the chalazal end. The three nuclei of the chalazal end fuse together forming a single triploid nucleus. Now the developing embryo sac is in binucleate stage. Both the nuclei divide once mitotically forming a second 4-nucleate stage, two micropylar haploid nuclei and two chalazal triploid nuclei. There are no further divisions. The nucleus situated near the micropylar end organizes into the egg; the triploid nucleus of the chalazal end forms the single antipodal cell, and the remaining two nuclei, one haploid and the other triploid, fuse to form a tetraploid secondary nucleus in the centre.

            (h) Plumbago Type – This type of development of embryo sac has been reported in Plumbago capensis. Here the four megaspore nuclei arrange crosswise and divide once to form eight nuclei in four pairs. One nucleus of the micropylar pair is now cut off to form the lenticular egg cell and one nucleus from each of the four pairs approach each other in the centre and fuse to form a tetraploid secondary nucleus. The remaining three nuclei degenerate.

            (i) Adoxa Type – This type of development of embryo sac was described for the first time by Johnsson (1879) in Adoxa moschatellina. In this type, the four megaspore nuclei divide to form eight nuclei which form a normal type of 8-nucleate embryo sac, i.e., a normal egg apparatus, three antipodal cells and two polar nuclei. This type of embryo sac is regular feature in Adoxa and Sambucus.

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