Female Gametophyte

FEMALE GAMETOPHYTE

      

Gynoecium is the female reproductive organ of the plant. It consists of carpels or Megasporophylls.  Each carpel consists of three parts- stigma, style and ovary. The stigma is the terminal end of the style, which receives pollen grains. The style is a slender projection of the ovary. The basal swollen portion is ovary- which bears small oval or roundish ovules.

The ovule

      

The ovule or megasporangium consists of the basic parts- nucellus and integument. It is attached to the placenta of the ovary by means of a short stalk called –funiculus. In a mature ovule the integument encloses the nucellus completely except a small opening – micropyle. The basal portion is called the chalaza. The female gametophyte i.e. the embryo sac remains embedded in the parenchtmatous nucellus. Integuments are generally two, the inner and the outer.

Development of ovule

      

The ovule primordium appears as a small protuberance on the surface of the placenta. It grows rapidly and develops into a prominent conical structure with rounded tip. The tip grows into a projecting mass of tissues by growth and division of its cells. One or two layers of the tissue develop from the base of the nucellus forming the integuments. The inner integument develops first and thereafter the outer one develops. The growth of the integuments is much faster than the nucellus and thus, they completely enclose the nucellus except for the micropyle.

     

Forms of ovule-

      

 On the basis of the relative position of the micropyle, chalaza and funiculus, the mature ovules may be of Ortrotropous, Anatropous, Amphitropous, Hemianatropous, Campylotropous and Circinotropous types.

  

Megasporogenesis

           

At early stage in the development of the ovule, when initiation of integumentary primordial is taking place, a single hypodermal cell- the primary archesporial cell, becomes large, with deep staining nucleus and dense cytoplasm. In tenuinucellate ovules it directly functions as Megaspore Mother Cell (MMC). In crassinucellate ovules, the archesporial cell divides periclinally and cuts off a primary parietal cell above and inner primary sporogeneous cell below, which acts as MMC.

         

 Normally, only one cell of nucellus functions as archesporial cell. But, sometimes more than one or a group of cells may act as archesporial tissue.

           

The MMC divides meiotically to form a tetrad of four megaspores. This process is known as megasporogenesis.

            

Within the tetrad of megaspore only one is functional and other three get crushed and eventually degenerate.

Female gametophyte

          

The functional megaspore forms the embryo sac (female gametophyte). Based on the number of megaspore nuclei that take part in the construction of the embryo sac, the embryo sacs are categorized into three main types- monosporic, bisporic and tetrasporic. In the first one, only one spore gives rise to embryo sac, in the second two spores and in the third all the four spores participate in the embryo sac development. Only one type is being described in detail, others are expressed in the fig.

         

Monosporic types- They are of two types-

            

1) Polygonum or Normal type-

                       

 It is found in about 82% of angiosperms. Her, the MMC undergoes meiosis to form a linear tetrad of megaspores, of which the three micropylar cells degenerate and the lowermost becomes the functional. It enlarges and its nucleus divides to form two nuclei, which move towards two poles- 2 nucleate stage. The two nuclei again divide to form 4 nucleate stage, and again divide to form 8 nucleate stage with 4 nuclei at each pole.

                        

These nuclei organize themselves. Of these four nuclei of micropylar pole, one migrates to the center. Similarly, one nucleus from the chalazal quadrate also moves to the centre of the embryo sac. These two nuclei fuse together to form a diploid secondary nucleus, the three nuclei at the chalazal end form the antipodal cells and the three at micropylar end form the egg apparatus with two synergids and one egg cell.

           

 2) Oenothera type-

Here the mature embryo sac bears only 4 nuclei. Its development and movement of nuclei is expressed in the given figure.

            

Bisporic types- 

They are of two types-

             1) Allium type-                   8 nucleate embryo sac

                2) Endymion type-              8 nucleate embryo sac

           

Tetrasporic types- They are of eight types

             1) Adoxa type-                   8 nucleate embryo sac

             2) Drusa type-                    16 nucleate embryo sac

             3) Pepromia type-                   16 nucleate embryo sac

             4) Penaea type-                        16 nucleate embryo sac

             5) Plumbago type-                   8 nucleate embryo sac

             6) Chrysanthemum

                  cinerariafolium                   6,9 or 10 nucleate embryo sac

             7) Frittilaria type-                    8 nucleate embryo sac

             8) Plumbegella type-                   8 nucleate embryo sac

Organization of the embryo sac and structure of cells.

               

In majority of angiosperms, the organization of the embryo sac shows a uniform pattern. It consists of-

              

The egg apparatus-

                  

Typically the egg apparatus consists of an egg and two synergids. The synergids are elongated cells; they are notched and possess a prominent hook. They lack a complete cell wall. In lower side, the cell wall is absent. The nucleus lies in or just below the hook and the lower part of the cell contains a large vacuole. In some cases, the upper part of the cell is occupied by a filiform apparatus, which is a mass of finger like projections of the wall into the cytoplasm. The synergids are generally ephemeral, degenerating soon after fertilization. But, sometimes they may persist and become haustorial.

                 

Synergids –

help in directing the pollen tube growth by secreting a chemotropic substance. The filiform apparatus in the synergids may help in the absorption and transportation of nutrition from the nucellus into the embryo sac.

                 

The egg is median and has a common wall with the synergids. The micropylar end of the cell has a large vacuole and the nucleus is situated at the lower end. The cytoplasm of the egg cell is rich in organelles during development. It contains mitochondria with less cristae, plastids, large number of ribosomes and a little starch.

             

Central cell-

                   

This diploid cell is formed by the fusion of two polar nuclei. It occupies a large area in the embryo sac. Sometimes, the two polar nuclei may not fuse for a long time and remain as free nuclei. The cytoplasm of the central cell has a large amount of reserve food material showing a high rate of metabolism. All the cell organelles are present in their maximum concentration. Plastids are rich in starch and proteins. The central cell is connected to the remaining cells of the embryo sac through plasmodesmata. It gives rise to the endosperm.

            

Antipodal cells-

                  

These are three uninucleate cells, but sometimes, more with multinucleate or polyploidy nuclei. The antipodals are usually short-lived and generally degenerate soon after after fertilization. But sometimes, they may persist and become haustorial. These cells have abundant mitochondria, dictyosoms and a large number of plastids. The cytoplasm is rich in small vesicles. The cell wall of antipodal cells, near nucellus, show a number of pappilate ingrowths, sometimes resembling the filiform apparatus. The cytoplasm is rich in inclusions such as Ascorbic acid, Sulphydril compounds; starch, lipids, proteins; RNA and polysaccharides are very less in concentration.