What is double fertilization and triple fusion?
Double Fertilization is a complex fertilization mechanism that has evolved in flowering plants (angiosperms). This process involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm). It begins when a pollen grain adheres to the stigma of the carpel, the female reproductive structure of a flower. The pollen grain then takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle opening in the ovule. The pollen tube proceeds to release the two sperm in the megagametophyte.
One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the megagametophyte. The haploid sperm and haploid egg combine to form a diploid zygote, while the other sperm and the two haploid polar nuclei of the large central cell of the megagametophyte form a triploid nucleus (some plants may form polyploid nuclei). The large cell of the gametophyte will then develop into the endosperm, a nutrient-rich tissue which provides nourishment to the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which protects the seeds and may function to disperse them.
In normal case one male gamete unites with the egg to form the zygote and the second travels a little farther and unites with the secondary nucleus. This process is known as double fertilization. (Navashin, 1898; Guignard, 1899). This is also termed triple fusion. The process of double fertilization is characteristic of angiosperms. Actual course of fusion of the gametic nuclei, has been studied in Crepis caillaris by Gerassimova (1933). In this case, at the time of the approach to the egg nucleus, the male nucleus becomes like a continuous thread rolled into a ball and it soon begins to unwind and spread out with its entire surface adjacent to the nuclear membrane of the egg.