Perisperm differs from endosperm in

Enclosed within the integuments is a mass of cells called the nucellus. Cells of the nucellus are diploid and have abundant reserve food materials. Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent.  This residual, persistent nucellus is the perisperm. Therefore, it is diploid. 

One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy.  This results in the formation of a diploid cell, the zygote. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN). The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm. The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo.