EMBRYO SAC OR FEMALE GAMETOPHYTE

Types of embryo Sac Development

There we types of embryo sac development. The classification is based on:

The number of 9 ses or spore nuclei entering into the formation of embryo sac Thus embryo sac may be monosporic, bisporic or tetrasporic tyr
The number, arrangement, and chromosome number of the nuclei in the mature embryo sac.
The total number of nuclear divisions occurring during megasporogenesis and development of female gametophyte.
Monosporic, Normal or Polygonum Type
It is commonly found in plant. It is commonly called normal type. However, it was first clearly described in Polygonum. Therefore, it is also called as Polygonum type.

This embryo sac has four well-defined megaspores. One of which gives rise to the embryo sac. The functional megaspore enlarges. Its nucleus divides. A large vacuole is formed between the nuclei. Thus the daughter nuclei move to the micropylar and chalazal poles of the embryo sac. Each nucleus divides twice. Thus four nuclei are formed at each pole. One nucleus from each pole migrates to the centre of the embryo sac. The two nuclei fuse to form a diploid secondary nucleus. Three nuclei at micropylar end are surrounded by membranes. They form egg apparatus. The central cell enlarged arid become egg cell. The other two cells becomes synergid. Thus embryo sac is formed containing 8-nucleoli and later 7-celled during its development.

Bisperic or Allium Type
This type of embryo sac is found in Allium. It is found in many monocot and dicot families. Two dyad cells are formed during first meiotic division duri-j. megasporogenesis. One of two dyad cell is abiyied The    of the surviving dyad cell towards the chalazal
end &lies to ft TM two haploid nuclei. These are called megaspore nuclei. These nuclei move towards opposite ends. These nuclei divide tw ice to form eight nuclei. One nucleus from each pole migrates to the centre of the embryo sac. Three nuclei at the upker end produce egg apparatus. The nuclei present at lower end form

antipodal cells. In this way 8- nucleate bisporic embryo sac develops.

types of embryo sacs

Tetrasporie Type

In this type of embryo sac wall is not formed after the meiotic nuclear division. All four haploid megapsore nuclei take part in the formation of the embryo sac. The resultant embryo sac• may be 8- nuceleate or 16-nucleate. Thus it has two types:

a)     Plunrnbago Type (8-Nucleate): In this case, the megaspore nuclei arrange themselves in a cross-like manner. One lies at the micropylar ends and the other lies at the chalazal end. The other two are present at each side of the embryo sac. Each nucleus divides once. Thus pairs of four nuclei are formed. One nucleus from each pair migrates to the centre. They fuse to form tetraploid secondary nucleus. The nucleus at micropylar and form the egg cell. The rest three nuclei degenerate. There are no antipodal cells and synergids.

b)     Fritillaria Type (8-Nucleate): This type of embryo sac occurs in a large number of genera. In this case, Three out of four megaspore nuclei are arranged in 3 + 1 fashion. Three nuclei migrate to the chalazal end. The remaining nucleus comes at the micropylar pole. The micropylar nucleus divides to form two haploid nuclei. The three chalazal nuclei fuse. The fusion nucleus ‘divides to form two triploid nuclei. Now the embryo sac contains four nuclei, two haploid micropylar nuclei and two triploid chalazal nuclei. Later each nucleus divides. Thus they produce four haploid nuclei at micropylar end and four triploid nuclei at chalazal end. One nucleus from each pole migrates to the centre. These fuse to forms a tetraploid secondary nucleus. The nuclei at micropylar end form egg apparatus. The nucleus at the chalazal end gives rise to antipodal cells.

c)      Pen.tea Type (16 Nucleate): In this case, 16 nuclei are arranged in quarters. One is present at each end of the embryo-sac and two are present at the sides. Three nuclei of each quarter become cells. The fourth nuclei of each quarter moves towards the center and act as polar nucleus. Therefore, there are four triads and four polar nuclei. One cell of the micropylar triad is the egg. It is the only functional cell.

d)     Drusa Type (16 Nucleate): In this case, one megaspore nucleus moves towards the micropylar. The remaining three megaspore nuclei move towards chalazal end. Each nucleus divides twice. Thus four nuclei are produced at micropylar end and twelve at chalazal end. One nucleus from each migrates towards the centre of the embryo sac. They fuse to form secondary nucleus. The three nuclei at micropylar end form egg apparatus. The eleven nuclei at chalazal end form antipodal cells.

e)     Adoxa Type (8-Nucleate): The four haploid megaspore nuclei

present in the cytoplasm undergo a mitotic division. They produce eight nuclei. These nuclei are arranged in typical manner. Three of them come at the micropylar end. Three comes at the chalazal end. And two come in the centre (fusion nucleus). Thus normal 8.nucleate seven celled embryo sac is formed.

0 Paperoma tye (16 Nucleate): In this case, each of four megaspores nuclei divides twice. They form 16 nuclei. These are uniformly distributed at the periphery of the embryo sac. Two nuclei at micropylar end form an egg and a Synergid. Eight of them fuse to form secondary nucleus. The remaining three stay at the periphery of the embryo sac.

DEVELOPMENT OF ENDOSPERM

The primary endosperm nucleus divides repeatedly. It forms polyploidy nutritive tissue called endosperm. There are two types of seeds for storage of food:

a)   Endospermic or albuminous seed: The endosperm supply food to the developing embryo. Such ..e,xls are called endospennic seeds. In plants like corn, wheat, the . idosperm tissue is present at the time of seed germination. So the .e are endospermic seeds.

b)   Non-endospermic or ex-albuminous sc. :ds: In some casts, the

endosperm is completely utilized by de eloping embryo. Such seeds are known as non-endosperrnic seeds. In beans and peas the endosperm tissue is completely digested by the developing embryo and stored in the cotyledons.

Formation of Endosperm

Endosperm is formed from the primary endosperm nucleus. Its formation starts before the formation of embryo. Primary endosperm nucleus is produced by fusion of monoploid polar nuclei (secondary nucleus) and a monoploid second male gamete. The endosnerm is thus triploid (3n). However in some case, it may be pentaploid (Penaea). It may be even 9n (Pepromia).

Structure of Endosperm

The cells of the endosperm are isodiametric. They store large quantity of food materials. The storage food is present in the form of starch granules, granules of proteins, or oils. In certain plants. the endosperm cells develop very thick hard walls of hemicelluloses. The parietal layer of the endosperm of grass functions like a cambium. This layer produces on its inside layers of thin-walled cells. These cells are packed with starch. The cells of outermost layer stops dividing. It is filled with aleurone grains. This layer is called aleurone layer. The cells of this layer secrete diastase and other enzymes. These enzymes digest the food stored in endosperm for developing embryo.

Structure of maize sad

Structure of maize sad

Types of Endosperm There are three types of endosperms on the basis of mode of development. These are nucelar type, cellular type and Helobial type.

Nuclear Type: In this case, the primary endosperm nucleus divides by free nuclear divisions. Wall is not formed between them. A vacuole appears in the centre of the embryo sac. It increases in size and. Therefore, the nuclei are pushed to the periphery along the wall of the embryo sac. Later, walls develop between the nuclei. Thus cellular tissues are formed.
Cellular Type: In this case, the primary endosperm nucleus divides and walls are formed between the daughter nuclei. These walls may be either transverse or longitudinal. It divides the embryo sac into two cells. Later, these cells divide by repeated divisions. It produces a tissue of irregularly arranged cells.
Helobial Type: This type of endosperm occurs in the order Helobiales (Monocotyledons). In this case, first division of primary endosperm nucleus is followed by a transverse wall. This wall divides the embryo sac into a small chalazal chamber and a large micropylar chamber. Then the nuclei in each chamber divide by free nuclear divisions. But, there are few nuclear divisions in the calazal chamber. The endosperm in this
chamber degenerate. Walls develop between nuclei in micropylar chamber. It produces cellular endosperm.

Development of Embryo SAC

Mosaic Endosperm

Endosperm containing tissues of two different types is called mosaic endosperm. It occurs in plants like corn. In this case, endosperm lack of uniformity in the tissues. The endosperm contains patches of two different colours. It forms a sort of irregular mosaic pattern. The part of endosperm is starchy and part is sugary.

Perisperm

In this case, a part of nucellus may persist in embryo in the form of an apical cap. It acts as a nutritive tissue and called perisperm. It occurs in some dicots such as pepper and water-lily.

Hypothesis about the Nature of the Endosperm

There are different hypothesis about the nature of endosperm. These are: