Is correct, lichen is a symbiotic association between an alga and a fungi, which live together for their mutual benefit. If both are separated from each other then they will die. The fungus holds water, provides protection and ideal housing to the alga. The alga in turn supplies carbohydrate food for the fungus. If the alga is capable of fixing nitrogen (e.g., Nostoc), it supplies fixed nitrogen to the fungus. The kind of mutual interdependence helps lichens to grow on dry, barren rocks, where the other plants fail to exist. Morever, the algae or the fungi alone cannot grow in such places. Thus, both the partners cannot survive without each other.

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 fertilisation 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 fertilisation. The zygote then develops into embryo and PEN develops into endosperm which provides nourishment to the developing embryo.