The embryos arising from the maternal sporophyte tissues are called adventive embryos. In polyembryonate species the adventive embryo arises by the proliferation of the nucellus cells.
An embryo develops directly from a diploid cell other than egg like that of nucellus and integument, e.g., Citrus, Opuntia. It gives rise to a condition called polyembryony or the phenomenon of having more than one embryo. There may be more than one egg cell in an embryo sac or more than one embryo sac in an ovule.