what happens to the megaspore in seed plants?

Read this essay to learn about the procedure of megasporogenesis and megagametogenesis in plants, explained with the help of suitable diagrams.

Evolution of Megaspore Female parent Cell:

The ovule develops as multicellular placen­tal outgrowth including the epidermal and a number of hypodermal cells. With further deve­lopment, this gives rising to nucellus and i or two integuments from its basal region. In ovules, with ii integuments, commonly the inner i is formed first than the outer i. The inner 1 is more than delicate and inconspicuously developed than the outer i.

One hypodermal cell of the nucellus becomes differentiated from the other past its bigger size, dense cytoplasm and conspicuous nucleus, called archesporial cell (Fig. 3.6A). The archesporial cell divides transversely and forms an inner primary sporogenous cell and an outer principal parietal jail cell (Fig. 3.6B).

The main sporogenous cell functions as mega­spore female parent prison cell (Fig. three.6C) and the master parietal prison cell undergoes repeated vertical divi­sions and forms layers of parietal cells (Fig. 3.6C). Sometimes, the archesporial jail cell does not divide and straight functions equally megaspore mother cell.

Megasporogenesis (Development of Megaspores):

The megaspore mother cell is diploid (2n), which undergoes meiosis and forms 4 haploid (due north) megaspores (Fig. 3.6D). The first divi­sion of megaspore mother jail cell is transverse, forming two cells. Both the cells again divide transversely and thus four (four) haploid mega­spores are formed.

The megaspores are and so arranged in an axial row, called linear tetrad (Fig. three.6D). Out of iv megaspores, only i which remains towards the chalazal end behaves as functional megaspore and the other three which remain towards the micropylar end, gradually degenerate (Fig. iii.6E). The func­tional megaspore forms the female person gameto­phyte i.e., the embryo sac.

Megagametogenesis (Formation of female gametophyte i.eastward., Embryo sac):

Megaspore (northward) is the beginning cell of the female person gametophyte (Fig. 3.7A). The functional mega­spore becomes enlarged at the expense of record tum and the nucellus and thus forms the female gametophyte i.e., the embryo sac. Initially, the embryo sac is uninucleate and with farther growth its nucleus divides by 3 successive divisions and forms viii nuclei (Fig. three.7B, C and D).

Out of eight nuclei, initially four remain towards the micropyle finish and the other four towards the chalazal end. One nucleus from each pole so moves towards the centre and forms a pair of polar nuclei (Fig. three.7E). These nuclei fuse together and class 2n nucleus, the definitive nucleus. It is also known every bit polar fusion nucleus or secondary nucleus.

The three nuclei of the micropylar cease grade the egg appa­ratus and the residual three at the chalazal end are chosen antipodal cells. In the egg appliance, each nucleus is surrounded past viscous mass of cyto­plasm without whatsoever wall, of which the centre one is the largest and chosen egg, ovum or oosphere and the residue two (one on each side of the egg) are the synergids or helping cells. The antipodal cells have viscous mass of cytoplasm, covered by cellulosic wall (Fig. iii.7F).

This type of embryo sac development is very common in angiosperms and is known equally ordi­nary type or normal type or Polygonum type. This type is also known equally monosporic blazon, because, out of four megaspores, merely one remains functional and forms the embryo sac.

Other types of embryo sac development (Fig. 3.8):

Monosporic type:

i. Oenothera type:

In this blazon (like Polygonum type), usual linear tetrad of megaspores are formed, but instead of the innermost i, the outermost megaspore (which is nowadays towards micropyle) remains functional and forms the embryo sac. The functional mega­spore undergoes two successive divisions and forms 4 nuclei.

All the nuclei remain towards the micropyle. Out of four nuclei, three nuclei form the egg apparatus (egg and two synergids) and the remaining i forms a sin­gle polar nucleus. 2nd polar nucleus and antipodal cells are absent-minded, due east.thou., Oenothera and other members of Onagraceae.

Bisporic blazon:

2. Allium type:

The megaspore mother cell divides to form two cells, the upper one quickly degenerates. The lower one then divides and forms two nuclei, distributed in the ii poles. Subsequently on, both the nuclei undergo two successive divisions and form usual octant type of embryo sac, i.eastward., poly­gonum blazon. Here 2 megaspore nuclei accept role in the development of embryo sac i.e., bisporic type, e.one thousand., Allium, Scilla, Trillium etc., of Liliaceae.

Tetrasporic type:

3. Peperomia type:

The megaspore female parent nucleus undergoes meiotic division and forms iv nuclei which remain crosswise in the embryo sac without any wall. All the nuclei undergo ii successive divisions and form 16 nuclei which remain dispersed inside the sac. Later on, out of 16 nuclei, egg and i synergid remain at the micropylar terminate, 6 antipodal cells towards the chalazal finish, and the rest eight at the centre forming polar nuclei, e.g., Peperomia of Piperaceae etc.

4. Penaea type:

Like Peperomia type, 16 nuclei are formed, those remain crosswise in the embryo sac. Later on, the nuclei are dis­tributed in a unlike way. The egg and two synergids remain at the micropylar finish, three nuclei at the chalazal terminate, and four at the centre and three each on the 2 side walls, eastward.g., Penaea of Penaeaceae.

5. Drusa type:

Like Peperomia type, initially four megaspores are formed, these are distri­buted in different ways. One megaspore remains towards the micropyle, and the rest iii at the chalazal terminate.

All 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 terminate. In the mature embryo sac, egg and two synergids remain towards the micropyle, two (one from each pole) at the centre and the residuum eleven at the chalazal cease, e.g., Drusa oppositifolia of Apiaceae.

vi. Fritillaria type:

Like Drusa type, out of four nuclei formed, i nucleus remains towards the micropyle, and the rest three at the chalazal end. The chalazal nuclei fused together and form 3n nucleus. Both the cells thus undergo i mitotic division and again form a tetrasporic stage. Out of four nuclei, two remain at each pole.

All the nuclei then undergo mitotic division and form eight nuclei. Out of four haploid nuclei at the micropyle, 1 egg and 2 synergids are formed, those remain at the micropylal end; iii triploid nuclei at the chalazal end and i from each pole remain at the centre (1 haploid and the other one triploid), eastward.thou., Fritillaria, Tulipa and some other mem­bers of Liliaceae.

7. Plumbagella type:

It is like Fritillaria type which forms 1st and 2nd tetrasporic phase with two haploid nuclei at the micropyle and two triploid nuclei at the chalazal finish of the embryo sac. Later on, the nuclei are distributed in such a style that the egg is at the micropyle, one triploid nucleus at the chalazal end and one triploid plus 1 hap­loid nuclei at the centre, e.g., Plumbagella of Plumbagellaceae.

eight. Plumbago type:

It is like Penaea type where firstly four nuclei are formed followed by eight nucleated embryo sac. The two nuclei at each side (four sides) remain crosswise. Subsequently, iv nuclei, one from each side, become aggregated in the eye. The nucleus at the micropylar end behaves as egg, e.thousand., Plumbago of Plumbaginaceae.

ix. Adoxa type:

In this type, the megaspore mother nucleus divides meiotically into four nuclei arranged 2 at each end. Both the nuclei — further undergo mitotic segmentation and thus eight nuclei are formed. Similar the normal type i.e., Polygonum type, one egg and 2 synergids remain at the micropylar region, three antipodal cells at the chalazal stop and ii nuclei remain in the center, e.one thousand., Adoxa, Sambucus of Caprifoliaceae.

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