question_answer 40)

  Explain why sexual reproduction in angiosperms is said to take place through double fertilization and triple fusion. Also draw a labeled diagram of embryo sac to explain the phenomena.

Answer:

                  An angiospermic plants sexually reproduces by the formation of male and female gametes. The male gamete is a pollen which contain two male nuclei and female gamete is egg cell produced in ovule (female gametophyte). The pollen grains germinate on the stigma of a flower and the resulting pollen tube grow through the tissues of stigma and style and reach near the egg apparatus. The two male gametes are discharged within the embryo sac. One of the male gamete fuses with the egg cell to form a diploid zygote. This fusion is known as fertilization or syngamy. The second male gamete fuses with the diploid secondary nucleus and forms the triploid Primary Endosperm Nucleus (PEN). This fusion is known as triple fusion. Because of the involvement of two fusion, this event in angiosperms is termed as double fertilization. The zygote then develops into embryo and PEN develops into endosperm which provides nourishment to the developing embryo. The process of fertilization and double fertilization