The phenomenon that two sperm formed by the male gametophyte of angiosperms, one fused with the egg to form a diploid zygote, and the other fused with the polar nucleus (usually two) of the central cell to form a primary endosperm nucleus. After double fertilization, the zygote develops into embryo and the primary endosperm nucleus develops into endosperm.

The polar nucleus in the embryo sac of angiosperms is fertilized like an egg, which usually produces triploid endosperm with paternal and maternal inheritance. Feeding the embryo with this endosperm may make the offspring more stable and more energetic, so some people think that double fertilization is an important reason for the prosperity of angiosperms.

The stages before gamete mating can be seen as follows: ① Pollen germination and pollen tube growth in the style. After pollen is transmitted to the stigma of pistil through various media, it interacts with pistil immediately. In the case of affinity, pollen germinates to grow pollen tubes and drill into the stigma. Pollen tubes continue to grow in the style after entering the stigma. Pollen tube depends on its terminal growth. When it grows to a certain length, all the substances originally contained in pollen are concentrated in the front end of pollen tube. During the growth process, pollen tubes may not only use their own storage materials, but also absorb nutrients from styles. ② The pollen tube enters the embryo sac and releases the contents. After the pollen tube extends into the ovary, it continues to grow along the inner wall or placenta of the ovary, reaching the ovule, entering the ovule through the micropyle and finally reaching the embryo sac. Pollen tubes grow along a certain path, which is generally considered to be attracted by chemicals secreted by recipient cells.

Usually, the fusion of sperm and egg is called gamete coordination, while the fusion of sperm and polar nucleus is called trinucleation. These two kinds of fusion happened almost simultaneously. 1973, American plant embryologist W.A. Zhan Sen put forward a model diagram of sperm transfer to egg cells and central cells during double fertilization in angiosperms. Sperm is first released into degenerated helper cells (plasma membrane has disappeared). Due to the releasing power of pollen tube contents, two sperm were transferred to the plasma membrane of the egg cell and the plasma membrane of the central cell respectively and contacted with them, and the plasma membrane at the contact point was dissolved, and the two sperm nuclei were transferred to the cytoplasm of the egg cell and the central cell respectively. According to Zhan Sen's sperm entry model, only sperm nuclei are transferred to female cells. In fact, whether sperm cytoplasm is involved in the fertilization of angiosperms is still inconclusive. The basic procedure of the fusion of androgens and androgens in angiosperms is as follows: ① Contact between androgens and androgens. ② Nuclear membrane fusion between male and female nuclei. ③ Sperm chromatin is scattered in the egg nucleus, and male nucleoli appears. ④ The chromatin of male nucleus is mixed with that of female nucleus. ⑤ The male and female nucleoli fuse into one big nucleoli. The fusion process of sperm nucleus and polar nucleus (or secondary nucleus) is basically the same as that of sperm nucleus and egg nucleus.

The time from pollen falling to stigma to complete fusion of male and female nuclei varies from plant to plant. The interval is as short as ten minutes and as long as one year. Most plants are between a few hours and 48 hours. Sperm-egg fusion and sperm polar fusion usually occur at the same time, or sperm polar fusion may occur earlier, and generally complete fusion and enter division faster. In this way, endosperm develops earlier than embryo, which can provide necessary nutrition for embryo development. The interval between pollination and fertilization is also affected by environmental factors, especially temperature. Temperature has a direct effect on pollen germination and the growth rate of pollen tubes.

Normally, an embryo sac only accepts two sperm, but sometimes there will be more than one pair of sperm in the embryo sac because more than one pollen tube enters or more than one pair of sperm is formed in one pollen tube. The existence of excess sperm in embryo sac may cause the following two abnormal fertilization effects: ① Multiple sperm enter the egg. There are two different situations: one is that the extra sperm entering the egg degenerates in the egg, and the other is that the extra sperm also fuses with the egg nucleus. This abnormal fertilization form has only cytological evidence in very few plants. ② Excess sperm is fertilized with other cells in embryo sac. This phenomenon has been reported in a kind of godmother. The synergid morphology of this plant is similar to that of an egg. Two pollen tubes enter the embryo sac and release two extra sperm. Sometimes three proembryos exist in the fertilized embryo sac, and two pollen tubes are in the upper part of the embryo sac. Therefore, it is speculated that the helper cells are fertilized with the incoming extra sperm and produce multiple embryos.