Cytology of Endosperm
Normally the young endosperm is triploid as it is formed by the fusion of three haploid nuclei (male gamete + upper polar nucleus + lower polar nucleus). However, in some plants it shows ploidy of different levels due to variation in the number of polar nuclei which may be 1, 2, 4 or 8, depending upon the type of embryo sac. The number of polar nuclei contributing to the formation of endosperm is one only in Oenothera leading to the formation of diploid endosperm while it is 8 in Peperomia so the endosperm is 9n.
During further development, the cells of endosperm may undergo further polyploidization due to endomitosis and nuclear fusion. The highest level of ploidy is reported in Arum in which the nucleus of endosperm becomes 24576 n. The size of nuclei and the number of nucleoli also exhibit enormous variation.