Botanique.org

When the pollen tube penetrates the embryo sac at the micropylar end, it goes through one synergid, burst and release the 2 sperm cells. One sperm cell and the egg fuse into a main zygote, diploid, that will give the seed embryo.

The other sperm cell fuses with 2 well individualized or already fused polar nuclei. It gives a secondary zygote or endosperm that is triploid. It is called albumen(N. M.) TISSU DE RESERVE, GENERALEMENT TRIPLOIDE, RESULTANT DE LA CROISSANCE DE L’EMBRYON ACCESSOIRE OBTENU LORS DE LA FECONDATION DES ANGIOSPERMES, ENTRE UN GAMETE MALE ET LES DEUX NOYAUX CENTRAUX DU SAC EMBRYONNAIRE. in French to make the distinction with the Gymnosperms endosperm that is diploid tissue. Endosperm is usually triploid but its ploidy number can be different given that the number of polar nuclei can vary.

Endosperm is a nutritive tissue which ends its development before that of the main zygote. When this last one will have ended its own development, endosperm has disappeared (seeds of Asteraceae or Fabaceae) or remains to go on its nutritive functions for the mature embryo during seed germination (seeds of Ricinus, seeds of Coco with a liquid endosperm).