Female Gametophyte And Embryo SAC

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.
Ages of development of capsella bursa pastorts

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

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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.
Apomixis

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.
Polyembryony

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.