DEVELOPMENT OF 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.
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
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.
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.
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:
- Gametophytic nature: Endosperm is formed in the embryo sac by free nuclear division. Therefore, some botanists take it vegetative tissue of the female gametophyte. But this hypothesis is not accepted because it develops as a new structure after triple fusion.
- Sporophytic nature: The endosperm nucleus produced as a result of the fusion of second male gamete with the secondary nucleus. Therefore, some botanists consider it a sporophyte tissue homologous to embryo. But the product of this fusion is not a new plant. Therefore, this fusion cannot be regarded as fertilization. This fusion forms a simple triploid (3n) nutritive tissue, not an embryo.
- Special undifferentiated nature: According to this view, it is neither sporophytic tissue nor gametophytic tissue. But it is special undifferentiated triploid tissue. It provides nourishment to developing embryo in angiosperms. It is most accepted hypothesis.
DEVELOPMENT OF EMBRYO
Development of Dicot Embryo
The dewiopment of Capsella bursa-pastoris (Shepherd’s purse) embryo is taken as model organism for the study of development of embryo of dicots. Following developmental changes take place in the embryo Capsella hurca pctstoris.
- First division of Oospore: Its oospore increases in size. It divides transversely in two cells. The cell toward the microphyll end is called suspensor cell. The cells towards other side is called embrymial cell. Embryonal cell forms the major portion of embryo.
- Formation of suspensor and radicle: The suspensor cell undergoes few transverse divisions. It produces short filament of cells called suspensor. The first cell of suspensor enlarges very much. It becomes basal cell. It pushes the embryo down into the developing endosperm. Suspensor also acts as conductive tissues for the nutrients. The last cell of suspensor adjacent to embryonal cell is called hypophysis. Hypophysis divides further to form radicle.
- Formation of octant: They embryonal cell increases in size. It divides by three divisions. Two divisions are vertical and one division is transverse. These divisions form eight groups of cells called octant or pro-embryo. The four octants towards the chalazal end are the epibasal or anterior octant. The other four octants which are adjacent to suspensor are hypobasal or posterior octant.
11. Formation of cotyledons and plumule: The epibasal cells further divides to fora two cotyledons and plumule. Further divisions occur in the cotyledonary cells and bibbed mass of cells is formed. These lobes are primary cotyledons. The plumule and epicotyl is produced in the notch between two depressions. Therefore, plumule in dicot is terminal in origin.
12. Formation of bypocotyl: The hypobasal octants divide to form mass of cells called hypocotyl. Hypocotyl is elongated. It carries radicle at its tip.
13. Folding of embryo: The developing embryo increase in size. Therefore, it become curved or folded in different ways. The way of folding of embryo in seed is characteristic feature of each plant.
14. Formation of basic layers of meristem: Two successive divisions occur in octants. It produces three layers. The outer layer is called dermatogen, middle is called periblem and central one is called plerome. Dermatogen gives rise to epidermis. Periblem gives rise to cortical portion. Plerome forms the stele in the centre.
Development of Monocot Embryo The development of Sagittaria sagittifolia embryo is taken as model organism for the study ofembryology of monocots. It undergoes following changes:
- Its zygote divides by a transverse wall into a terminal and a basal cells
2. The basal does not divide further. It enlarges to form a vesicular cell. The terminal cell divides transversely to form proembryo.
3. The proembryo upper, middle and basalupper, middle and basalThe lowermost cell of the proembryo divides by a longitudinal wall. It then divides by transverse and longitudinal walls. Thus
eight cells are formed. These are arranged in two tiers. Each containing four cells.
- Each of the eight cells undergoes periclinal division and form dermatogen. Thus the entire region grows. It differentiates into a single terminal cotyledon.
- The middle cell of the proembryo undergoes a transverse division and two cells are formed. The lower of these two cells give rise to lateral shoot apex. The upper cell forms the hypocotyl, the tip of the root and a short suspensor. The suspensor is composed of 3-6 cells.
or Abnormal Embryonal Development
Apomixis includes all those cases of embryonal development in which the normal process of fertilization is not involved. Certain species of the following genera show different cases of apomixis Iris. Pea, Lilium, Malus, Crepis, Hypericum and Ulmas. Apomixes includes apogamy, apospory and parthenogenesis:
- Apogamy: The development of embro from any cell of the gametophyte without the normal process of fertilization is called apogamy.
- Apospory: The development of an embryo-sac from the
sporophytic cell, generally the nucellar cells, without undergoing the usual meiosis or reduction division is known as apospory. In apogamous cases the normal oosphere or one of the synergids, or one of the antipodal cells may develop into an embryo without the inyolvement of normal fertilization. If the cells involve involved are haploid then the embryo would also be haploid. The resulting plants are generally sterile. If such are diploid then the embryo and the resulting plant would also be diploid. It will be fertile pant.
- Parthenogenesis: The development of a gametophytic cell or oosphere without undergoing fertilization is also known as parthenogenesis. It occurs in banana.
Production of more than one embryo in an ovule is known as polyembryony. It is very rare in the Angiosperms. Citrus is a very good example showing different cases of polyembryony. There are different forms of polyembryony. These are:
1. Cleavage polyembryony. In this case, more than one embryo
may be produced from a single oospore. In such cases, all the embryos may not survive till the maturation of the seed due to the mutual competition.
- Adventitious polyembryony: More than one embryo may be produced in a single ovule due to the development of certain nucellar cells. These cells changes into embryos in addition to the normal embryo which develops from the oospore. Such cases are known as Adventitious polyembryony. In the case of Citrus upto ten embryos have been recorded in the mature seed.
- Sometimes, an ovule contains more than one functional megaspores. They develop into embryo sacs and oosphere. These oosphere are fertilized and produce more than one embryos.
- Sometimes, embryos may develop from synergids or antipodal. Embryo from oospore is also there. Thus polyembryo are • formed.
Development of Seed and Fruit
The stimulus of fertilization leads to the development of embryo and endosperm in the. It also stimulates enormous changes in the ovule. These changes leading to the development of seed, and in the ovary wall resulting in the formation of fruit.
Development of seed
The ovule increases in size during development of embryo. Its integument becomes thin, dry and hard and forms testa. In certain seeds it may be differentiable into two layers. The inner one is generally thin and membranous. It is known as the tegmen. Tegemn represents the inner integument. The developing embryo may or may not utilize the whole of the endosperm. Thus endospermic or non endosperinic seeds may formed. In certain seeds a small amount of the nucellus persists as a nutritive tissue known as the perisperm. In the non endospermic seeds the cotyledons become massive. They contain the stored food material. This food is utilized by the embryo during the germination of the seed. In case of endospermic seeds the persisting endosperm is utilized by the embryo during the germination of the seed. In certain seeds outgrowths of variable sizes are produced. These outgrowths form aril or earuncle. A scar left on the seed. It represents the point of attachment of the ovule. It is known as the hilum.
Development of Fruit
The stimulus of fertilization also causes changes in the ovary wall. It becomes the fruit wall or pericarp. The ovary wall may become dry and hard giving rise to dry fruit. Or it may become soft and fleshy giving rise to the fleshy fruits. The development of the fruit from the ovary wall is one of the chief characteristics of Angiosperms. The development of the fruit ensures the protection and maturation of the seed. It also provides an efficient means of seed dispersal. In certain cases otter parts of the flower such as calyx or thalamus may also take part in the formation of the fruit. It some extreme cases, the whole inflorescence may be involved. Such fruits are called pseudocarps. Examples of these types are pear, apple, pineapple, strawberry, fig, mulberry etc. In certain plants the fruits may be produced even without the process of fertilization. Such fruits are generally seedless and are known as parthenocarpic fruits.