Nuclear Type - Endosperm
The primary endosperm nucleus divides. The cell wall is not laid. These nuclei and their division products form a large number of free nuclei. The first few divisions are synchronous as a result nuclei are seen in multiples of two i.e. 4, 8, 16, 32 and so on. Later the nuclear divisions are non-synchronous i.e. the nuclei may be seen in different stages of divisions and the number of endosperm nuclei are not in multiples of two. The free nuclei thus formed remain suspended in the cytoplasm of the embryo sac. After some time the nuclei become gradually pushed towards the periphery by an expanding central vacuole.
A large number of nuclei accumulate towards the micropylar and chalazal ends. The nuclei may increase in size either by the fusion of two or more or by their independent growth. The process of cell formation starts with the centripetally growing walls from the periphery proceeding towards the centre of embryo sac or from the apex progressing towards the base. To begin with a single layer of uninucleate cells is formed. Subsequent anticlinal and periclinal divisions of these cells lead to complete cellularization of the endosperm. In some plants only one or two peripheral layer of endosperm cells may develop and the entire embryo sac may remain in the he nuclear state or cell formation may be restricted only to the micropylar end of the embryo sac. In a few plants wall formation may not take place at all and the endosperm has free nuclei. Normally the endosperm cells are only uninucleate; sometimes more than one nucleus may be enclosed within a cell. The number may further increase by nuclear divisions.
The development of endosperm in coconut is interesting. When the fruit is young the embryo sac is filed with a clear fluid containing numerous free endosperm nuclei. Later, the periphery becomes jelly-like, containing several cells. As the fruit matures, and the cellular endosperm along the periphery becomes very massive, the central part contains a sweet liquid with a large number of nuclei. The cellular endosperm constitutes the edible copra rich in stored fat. In the betel nut and fruits of several other palms, the cellular endosperm becomes very hard and woody.