Difference between PEN and PEC?
The nucleus formed in the embryo sac of a seed plant by fusion of a sperm nucleus with two polar nuclei or with a nucleus formed by the prior fusion of the polar nuclei is known as PEN (primary endosperm nucleus). PEN cells are filled with reserve food materials and hence it provides nourishment to the developing embryo.
PEN cells are often called PEC (primary endosperm cells), so there is no difference.