Polar nuclei

The polar nuclei are cells located within the ovary, embryo sac or female gametophyte, which are involved in the fertilization process of plants.

Each embryo sac has different types of cells: the fertile ones are the egg and the infertile ones are the polar nuclei. The so-called synergids and antipodals also collaborate in the process.^{[citation required]}

Double fertilization of angiosperms

In gymnosperms there is no double fertilization, since only one of the male gametes is functional and the other degenerates.

Pollen is made up of 3 haploid cells. When the pollen grain germinates, one of these cells forms the pollen tube that passes through the style and through this tube the other two cells reach the embryo sac. In it, one of these cells fertilizes the egg cell and forms a diploid zygote that, through mitosis, will give rise to the embryo.

The other sperm nucleus fuses with the two polar nuclei, each of them is haploid and forms the endosperm, which is a nutritional tissue from which the embryo will be formed in its first stages of development.

Arrangement and number of polar nuclei

The polar nuclei are normally located in the center of the female gametophyte, their number varies depending on the species. Many angiosperms have an embryo sac of the type found in the genus Polygonum with two polar nuclei and therefore produce a triploid endosperm. In few species there are more than two polar nuclei and in these cases the haploidy will be even greater. On the other hand, there are botanical families with a single polar nucleus and therefore give rise to a diploid endosperm.