DOUBLE FERTILIZATION

Structure of the flower

• Flowers vary enormously in their structures,yet all flowers follow the same basic plan .
• A generic flower consists of 4 whorls or circles.
• The two outermost- the Sepals and the Petals , are vegetative structures.
• The two innermost- the Stamens and the Pistil ,are the male and female reproductive structures.
• At the base of pistil or female structure is the ovary ,which contains one or more ovules.

DOUBLE FERTILIZATION
STAGE 1
FORMATION OF FEMALE GAMETE/EGG

• Within each ovule ,a single large diploid cell called the Megaspore mother cell is present.
• Which undergoes the mitosis to produce four Megaspore cells.
• Three out of these cells degenerate and only one survives . This cell undergoes three  meiotic division to produce an Embryo sac with eight haploid nuclei.
• Subsequent cell divisions produces a mature embryo sac in which eight haploid nuclei are segregated in seven cells.
• One of these cell is the Egg (n), another is the large Central cell consists of two polar nuclei (2n), the rest cells are the haploid synergids(n) and antipodal cells(n).

                            

FORMATION OF MALE GAMETE /POLLEN GRAIN

• The male structure or stamens ,are surrounded by the pistil and consists of Anther perched on a stalk called filament.
• The anther contains a large number of microspore mother cells, each of which undergoes meiotic divisions to form uninucleated, single celled Microspore.
• The microspore are then  srrounded in heavy resistant, outer walls and the nucleus divides mitotically, forming two cells-a Tube cell and a Generative cell-within the original spore wall. This is called as mature Pollen grain.

A mature Pollen grain showing tube cell and generative cell. The diploid generative nucleus divides meiotically to produce two haploid sperm nuclei.

STAGE 2

• Mature Pollen grain are shed from the anthers and carried to stigmatic surface of the pistil by insects, wind or some other vector.
• Once the pollen grain lands on the stigmatic surface by an event called Pollination.
• The pollen grain lands takes up the water and sends out Pollen tube that grows down the style of pistil towards ovule.
• The tube nucleus start moving down the Pollen tube and appears to direct its growth.
• The cell wall of the generative cell breaksdown and the generative nucleus divides into two sperm nuclei that follow the tube nucleus down the tube as it elongates.
• Pollen tube growth towards the embryo sac requires a calcium gradient, but the precise signaling mechanism remains uncharacterized .

            A germinating pollen grain

STAGE 3

• In the final stage ,the elongating pollen tube enters the ovule by growing through micropyle (the space between two integuments) and releases two sperm nuclei into the embryo sac.
• Ultimately, one of the sperm nuclei enters the egg cell and fertilize  the egg cell nucleus to form a diploid cell called ZYGOTE (2n). This process is called as SYNGAMY.
• The second sperm nuclei enters the large central cell and fuses with the two polar nuclei to form a triploid PRIMARY ENDOSPERM NUCLEUS (3n).This process is called as TRIPLE FUSION.
• This process of fusion of two sperm nuclei with egg cell and large central cell to give zygote and primary endosperm nucleus  is called as DOUBLE FERTILIZATION.
• This is a characteristic feature of flowering plants or angiosperms.