Answer: (1) Nucellus
Citrus’s adventive embryo is due to Nucellus. Nucellus is the tissue within the integuments of an ovule surrounding the embryo known as megasporocyte. Nucellus normally grow into perisperm.
But in certain plants, these cells may create embryos which are genetically identical to the parent plant, in addition to the actual embryo. This phase is referred to as the adventive embryony.