Triple fusion is the characteristic feature of all angiospermic plants. It is a part of double fetilisation. During triple fusion, second male gamete fuses with two polar nuclei. This results in the formation of a triploid primary endosperm nucleus which later on develops into a triploid, multi-nucleated, multi-celled structure, called endosperm. Triple fusion does not occur in gymnosperms, pteridophytes, etc.