Ovule,Megasporogenesis and Female Gametophyte

                Ovule,Megasprogenesis and Female Gametophyte

Ovule :-  Ovule is the Megasprangium of angiosprem in which the female gametophyte or embryo sac develops.

Types of Ovule in Angiosperms :- 

      Orthotropus :-     

                         It is the basic type of ovule found in angiosperms ,in which micropyle,chalaza and funicle lie in same axis .e.g. Polygonum.

      Anatropus :  

                  In this type ,the nucelleus remains straight .So micropyle,chalaza from a straight line .

                   The ovule completely bends along the funicle and gets fused with it to look onverted.e.g.:-Solanacea.

       Amphitropus :-

                         In this type ,the body of ovule is bent only halfway so that it comes to lie at right angle to the funicle .

                            The fusion of integument and funicle is very little,and so hilum,chalaza and  micropyle lie apart.

E.g- Ranaulculacea.

Campylotropus :-

                               Here the body of the ovule is bernt upon itself like a horse –shoe with the result that micropyle comes lie near the funicle .e.g.- Caryophyllaceae.

      Circinotropus:-

                                      In oputia,the funicle makes a complete turn around the ovule and looks as an extra cover.it is called Circinotropus and the nucellus remain straight as in orthotrpous.

Types of Ovules

Typical Angiospermic Ovule :-

                                                                                   Ovule occur in the ovary attached to the placenta.Each ovule has a stalk called funicle.

                       The point where the ovule is attached to its funicle is called Hilum.

                       In some cases ,the funicle is fused with the body of ovule halfway on on e side,and so the ovule   appears inverted or anatropus.

                       The fusion forms a ridges called Raphe .Ovule consist of a parenchymatous mass diploid cells called Nucellus.

                       It is covered by one or two Integument.Onthe basis of their Integuments these are Ategmic if Integumant is absent ,Bitegmic if to integument is present , tritegmic if three integument are present.

Typical Anatropus Angiospermic Ovule

                      The integuments covers the ovule excepts one side and this side is called Micropyle .Just opposite to micropyle end another end is present is called chalaza end .

                     Deep inside the Nucellus Female gemtophyte Embyo sac present.

                      According  to  thickness of Nucellus  These are of two types Tenuinucellate(Thin) and crassinucleate  (Thick).

Development of ovule :-

                                      The ovule primodium appears as a small primodium on the surface of placenta.

                    It  grows rapidly and forms a rounded tip,which is the starting  point of nucellus .

                    First ,the inner integument appears 

.

                     It is followed by the outer intrgument.Nucellus grows gradually into a projecting  mass of tissue.

Megasporogenesis :- 

                                         All the cells of the nucellus are alike in the beginning .

                                                 A single hypodermal cell soon differentiated at the  micropyle end just below the epidermis and it is called primary archesporial cell,Two or three archesporial occurs in hydrilla,Primary archesporial cells differs from the neighbouring cells in having a large and deeply staining nucellus and granular cytoplasm .

                                                  The arechesporial cells divides priclinal wall to form an outer primary parietal cell and inner primary sporogenous cells .

                                                    The primary parietal cell may divides one or twice and it keeps the nucellus thin.Such a nucellus is called  tenunicellate.

                    In Tenunicellate nucellus,the onnermost layer of inner

integument may develop Into integumentary tapetum or endothecium,which acts as a

nutritive tissue.sometimes the nucellus may becomes thick and is made up of many layers

of cells,and it is called Crassinucellate.

                                                     The primary sporogenous cell directly acts as megaspore

mother cell .Development of megaspore mother cell varies in different genera.Generallly

linera tetrad of four megaspires is formed.In linera tetrad upper three degenerate and the

lower most one is functional.

  Development of Femlae Gametophyte :-

                                        Polygonum type of Development is seen most commonlyand it is called Monosporic eight nucleate type .

                                      A single Megaspore farthest from the micropyle is functional 

and the rest three degenerate.The surviving megaspre enlarge.

                                      Its nucellus Mitotically and forms two haploid cells .One nucellus migrates to each pole .

                                      Each nucellus divides two more time and forms Eight nucellus.and tose eight nucelli

arranged like three is at micropylar side three at chalaza end and two nucelli present

slightly towards the micropylar end and two nucelli forms one cell is called central cell

and the Nucelli is called Polar nucelli.

Typical embryo sac :-   

                                       A typical polygonum type of embryo sac consist of an egg appartus at the micropylar end,three antipodal at the chalaza and two polar nucelli in the middle .

                                      The egg apparatus has three cells –a middle egg and two lateral synergids.

(a) Egg cells :-  

                    Egg occupies the middle poistion in the egg apparatus .in the egg the cytoplasm occurs at lower end upper and has a large vacoule.

                   The cytoplasm contains plastids,ribosomes,mitochondria and few dictysomes.Egg nucellus as larger than that of synergids.

(b) Synergids  :-  

                      Mitochondria ,ER and dictosomes are abudent in these cells.

                        The synergids show finger like projections called filliform apparatus .In may help in the distribution in the embryo sac and secreation of chemicals to attract the pollen tube.

                      it also provides mechanical strength to synergid .Synergid are also called helper cells .

(1)    They acts as a haustoria and play an important role in the nutrition on embryo sac.

(2)The substances secreated by filliform apparatus attracts pollren tube  at the time of fertilization .

(3) Synergids after their degeneration ,provide sufficient space foe pollen tube dischrge.

Typical Embryo Sac

(d)   Antipodal cells :-

                            Antipodal have a cell wall of their own .The cytoplasm of these cells contains plentry of mitochondria,plastids and dicytosomes .

                 Enzyme such as peroxidase,cytochrome oxidase and so on are present .Antipodal Perhaps takes part in the nutrition of embryo sac .