double fertilization The production of 2 sperm nuclei, both of which contribute to fertilization. The male gametophyte, comprising the pollen grain and pollen tube, contains 2 sperm nuclei and 1 vegetative nucleus. The vegetative nucleus degenerates once the pollen tube has penetrated the embryo sac. The 2 sperm nuclei enter the embryo sac. One unites with the egg nucleus to form the zygote, which develops into the embryo. The other unites with both of the polar nuclei or with the secondary (definitive) nucleus formed by their fusion, to form the primary endosperm nucleus, from which the endosperm develops. Double fertilization occurs only in angiosperms, with the single exception of the gymnosperm genus Ephedra, in which 2 sperm nuclei are produced, 1 of which fuses with the egg nucleus. The other unites with an adjacent cell, but develops no further.

double fertilization A process, unique to flowering plants, in which two male gamete nuclei, which have travelled down the pollen tube, separately fuse with different female nuclei in the embryo sac. The first male nucleus fuses with the egg cell to form the zygote; the second male nucleus fuses with the two polar nuclei to form a triploid nucleus that develops into the endosperm.

double fertilization In flowering plants, the union of one sperm nucleus with the egg nucleus and of a second sperm nucleus with the two polar nuclei to form a triploid endosperm nucleus. The male gametophyte (the pollen grain and pollen tube) contains three sperm nuclei, but one (the vegetative nucleus) degenerates once double fertilization has been accomplished.