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Double fertilization is a complex fertilization mechanism of 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 (triple fusion). 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. The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm, arise by mitosis from the same single meiotic product that gave rise to the egg. The maternal contribution to the genetic constitution of the triploid endosperm is double that of the embryo. In a recent study done of the plant ''Arabidopsis thaliana'', the migration of male nuclei inside the female gamete, in fusion with the female nuclei, has been documented for the first time using in vivo imaging. Some of the genes involved in the migration and fusion process have also been determined.〔 〕 Evidence of double fertilization in Gnetales, which are non-flowering seed plants, has been reported.〔 〕 ==Brief history== Double fertilization was discovered more than a century ago by Sergei Nawaschin in Kiev, Russian Empire, and Léon Guignard in France. Each made the discovery independently of the other.〔 〕 ''Lilium martagon'' and ''Fritillaria tenella'' were used in the first observations of double fertilization, which were made using the classical light microscope. Due to the limitations of the light microscope, there were many unanswered questions regarding the process of double fertilization. However, with the development of the electron microscope, many of the questions were answered. Most notably, the observations made by the group of W. Jensen showed that the male gametes did not have any cell walls and that the plasma membrane of the gametes is close to the plasma membrane of the cell that surrounds them inside the pollen grain.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Double fertilization」の詳細全文を読む スポンサード リンク
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