SEMINAR ON DOUBLE

FERTILIZATION

Presented by :- mohd abdullah haiderym.Sc – 1 sem (1)

Department of botany Sant gadge baba amravati university

amravati

Introduction Structure of flower Development of male gametophyteDevelopment of female gametophytePollination Fertilization i.e. double fertilizationReferences

CONTENTS:-

Introduction:-

Double fertilization is a complex fertilization mechanism that has evolved in flowering plants (angiosperms). This process involves the joining of a female gametophyte (mega gametophyte), with two male gametes (sperm)

Q:- How the female and male gamete produce and fertilized ?

STRACTURE OF FLOWER :-

Microsporogenesis and male gametophyte:The formation of microspores inside the microsporangia (or pollen sacs) of seed plants. A diploid cell in the microsporangium, called a microsporocyte or a pollen mother cell, undergoes meiosis and gives rise to four haploid microspores. Each microspore then develops into a pollen grain (the micro gametophyte).

Development of male gametophyte Formation of generative and vegetative cells

Megasporogenesis The formation of megaspores inside the ovules of seed plants. A diploid cell in the ovule, called a megasporocyte or a megaspore mother cell, undergoes meiosis and gives rise to four haploid megaspores. In most plants, only one of the megaspores then goes on to develop into a mega gametophyte within the ovule, while the other three disintegrate. In the ovules of angiosperms, megasporogenesis takes place within a structure called a nucellus, and it is the megaspore farthest from the micropyle of the ovary that survives.

Megasporogenesis :-

Female Gametophyte :-

Pollination :-The transfer of pollen from the anthers of a flower to the stigma of the same flower or of another flower.Dihescence of anther is the prerequisite for pollination. Once the anther is dehisced the pollination can occur through the agencies of water ,air, insects,etc. It is of two types:-

Self pollination Cross pollination

Fertilization :-When the pollen grain is shed from the anther it has two cells a generative cell and a tube cell . The generative cell give rise to male two male gametes. The pollen tube with two male gametes grows down through the style and enters into the ovule usually through the micropyle. It releases two male gametes into the embryo sac. One fuses with the egg and forms zygote. This fusion of male and female gamete is known as fertilization. The second male nuclei fuses with the two polar nuclei which form endosperm. So there is two fertilization usually called as double fertilization or triple fusion.

Germination of pollen grain After being deposited on the stigma the pollen grain absorb liquid from the moist surface of the stigma ,expand in size and the intine protrudes out through a germ pore . The small tubular structure also known as pollen tube continue to elongate ,and make its way down the tissue of the stigma and style . Only the distal part of the pollen tube possess living cytoplasm. In most of cases a single pollen tube comes out from each pollen grain . In other cases e.g. Malvaceae ,cucurbitaceae several pollen tube. But usually only one of them is functional. The stigma plays an important role in the germination of pollen grain. The chief function of the stigmatic secretion is to protect the pollen as well as the stigma from desiccation.

Path of pollen tube into ovule :-Depending upon its place of entry, three types of fertilization have been recognised. They are :Porogamy:- Here the entry of the pollen tube into the ovule takes place through the micropyle.Chalazogamy:-In plants like Casuarina, the entry of pollen tube takes place through the chalaza.Mesogamy:- In Cucurbita, pollen tube enters the ovule through the integuments.

Entry of pollen tube in the ovule; A. Porogamy, B. Chalazogamy, C. Mesogamy

Entry of Pollen Tube in the Embryo-sacIrrespective of the place of entry into ovule the

pollen

tube always enter the embryo sac through the

micropylar region . It may enter the embryo sac i.e. one of the following routes

between the egg cell and the synergids without destroying the latter

between the wall of the embryo sac or both the synergids

between the two synergids without destroying either of them or

directly penetrate one of the synergids .

Movement of Sperms toward Egg and Polar Nuclei

The content of pollen tube are released and the egg cell and synergids are in close contact sperm do not have to transfer long.

The sperm show amoeboid movement one of the male gamete moves towards the egg and the other to the polar nuclei .

The factor or forces which propel one of the sperm into the egg and the other in the central cell with remarkable regularity and precision are not properly known .one of the two sperm fuses with the egg the receptiveness of the egg is lost once a sperm has fuse with it .

Gametic fusion After the discharge of the content of the pollen tube into the embryo sac one male gamete fuses with the egg (syngamy) and the other with the two polar nuclei (triple fusion).male gamete may be spherical (Erigeron),ellipsoidal(Livisticum) rod shaped(Urtica) vermiform (Lilium) in structure.In normal case one male gamete unite with the egg to form the Zygote and the second sperm with the secondary nucleus. This process is called double fertilization and also termed as triple fusion . With the result of the syngamy a zygote (2n) is formed , whereas the triple fusion results in the formation of endosperm nucleus(3n).

B.P.Pandey(1992) “Text book of Angiosperm & Embryology” S. chand publishing house, New Delhi-110002.P.Maheshwari,(1990) “ An introduction to the embryology of angiosperms” East west press , New DelhiBhojwani and Bhatnagar S.P (2000), “ the embryology of angiosperms” Vikas publishing house, New Delhi http://en.wikipedia.org

Reference :-

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