Structure of an Inverted Ovule PDF

STRUCTURE OF AN INVERTED OVULE  Placenta is a ridge like parenchymatous tissue in the ventral margin of the ovary to which ovule[s] remain attached.  Each ovule is attached to the placenta by a slender stalk known as the FUNICLE.  The point of attachment of the body of the ovule to the funicle...

STRUCTURE OF AN INVERTED OVULE  Placenta is a ridge like parenchymatous tissue in the ventral margin of the ovary to which ovule[s] remain attached.  Each ovule is attached to the placenta by a slender stalk known as the FUNICLE.  The point of attachment of the body of the ovule to the funicle is known as HILUM.  In the inverted ovule the funicle continues beyond the hilum alongside the body of the ovule, forming a sort of ridge called RAPHE.  The upper end of the raphe is the junction of the integuments and the nucellus called CHALAZA.  The main body of the ovule is called NUCELLUS and it is surrounded by 2 coats called INTEGUMENTS.  A small opening is left at the apex of the integuments, this is called MICROPYLE.  There is a large oval cell lying embedded in the nucellus towards the micropyle and this is the EMBRYO SAC.  The embryo sac contains a) 3 cells called the ANTIPODAL CELLS towards the chalazal end. b) Towards the micropylar end it has the EGG APPARATUS. The egg apparatus consists of 1) the EGG CELL/ ovum. 2) Two SYNERGIDS or help cells c) Initially in the centre the embryo sac has 2 nuclei called POLAR NUCLEI. They later fuse to give rise to the DEFINITIVE NUCLEUS.  The egg cell after fertilization gives rise to the EMBRYO.  The synergids help the process of fertilization by guiding the pollen tube into the embryo sac.  The antipodals have no known function.  The definitive nucleus on fertilization gives rises to the ENDOSPERM NUCLEUS which forms the ENDOSPERM of the seed. STRUCTURE OF THE POLLEN GRAIN  Pollen grains are minute in size. Each pollen grain consists of a single microscopic cell which possess 2 coats , the outer EXINE and the inner INTINE.  The exine is tough layer often provided with spinous out growths. Sometimes it is smooth.  The intine is a thin delicate cellulose layer lying internal to the exine.  The exine has certain thin regions called the GERM PORES through which the intine grows out into a tube called POLLEN TUBE, when the pollen grains germinates on the stigma.  At first each pollen grain has only one nucleus. This divides to form 2 nuclei—one of which is larger and is known as the VEGETATIVE NUCLEUS OR TUBE NUCLEUS; and the samller one is called GENERATIVE NUCLEUS.  The generative nucleus later divides and develops into 2 male reproductive units called MALE GAMETES. FERTILIZATION  Fertilization is the fusion of the male gamete nucleus with the female gamete nucleus resulting in the formation of the ZYGOTE.  After pollination, the pollen grains reach the stigma, the intine grows out into a thin tube called the pollen tube , through the weak spots/ germ pores in the exine.  The growth of the pollen tube is stimulated by the sugary substances secreted by the stigma.  The pollen tube penetrates the stigma and pushes its way through the style and through the wall of the ovary carrying with it the tube nucleus and the generative nucleus..  The tube nucleus guides the pollen tube through the style and the ovary.  The generative nucleus meanwhile divides forming 2 male gametes.  The pollen tube runs down the wall of the ovary and finally turns towards the micropyle of the ovule.  It passes through the micropyle and reaches the embryo sac.  Of the 2 male gametes one fuses with the egg cell/ ovum. This fusion is called SYNGAMY. The fertilized egg cell gives rise to the embryo.  The other male gamete fuses with the two polar nuclei or their fusion product, the definitive nucleus. This fusion is termed TRIPLE FUSION as it involves the fusion of three nuclei. The fertilized definitive nucleus now is called the endosperm nucleus and this gives rise to the endosperm of the seed.  Triple fusion and syngamy together constitute double fertilization as the fertilization occurs twice [male gamete+ definitive nucleus and male gamete + egg cell]. This is a characteristic feature seen in angiosperms.  The synergids are supposed to direct the male gametes towards egg cell and definitive nucleus. FATE OF VARIOUS PARTS AFTER FERTILIZATION FUNICLE- becomes Integuments- Raphe, chalaza, Nucellus- forms the the stalk of the become the seed micropyle- form the nutritive perisperm seed. coats. corresponding parts of the seed Synergids – Fertilized egg cell- Endosperm nucleus - Antipodal cells- disorganized embryo endosperm of the disorganized. seed Calyx – may persist[ Corolla- falls off Stigma and style- fall as in capscicum, off brinjal etc] or may fall off