Formation of endosperm is initiated by mitotic division of primary endosperm nucleus, which is triploid and is formed by triple fusion of two polar nuclei or secondary nucleus (2n) with male gamete (n). Endosperm is formed before the division of zygote.

In angiosperms, endosperm development is of the three types : 

(i) Nuclear type : It is the most common type of endosperm formation. In this type, the primary endosperm nucleus divides repeatedly and gives rise to a number of free nuclei which remain as peripheral layer and a large central vacuole appears in the embryo sac. Cell wall formation takes place later leading to the formation of cellular endosperm e.g., Coconut, Areca. In coconut, central part remains free nuclear.

(ii) Cellular type : In this type, first and subsequent divisions of primary endosperm nucleus are accompanied by wall formation, hence endosperm is cellular from the very beginning e.g., Petunia, Datura.

(iii) Helobial type : It is an intermediate type between the nuclear and cellular types. The first division of endosperm nucleus is followed by wall formation but the subsequent divisions are free nuclear. The chamber towards the micropylar end of embryo sac is much larger. A large number of nuclei are formed in it by free nuclear divisions while the nucleus of chalazal chamber divides to form a fewer, free nuclei or may not divide at all e.g., Drimya.