Pollen grains that have fallen on the stigma in one way or another begin to germinate. The grain swells, a pollen tube forms from the vegetative cell. Thus, germination begins. The shell erupts at the thinnest point with a pollen tube. Special substances are released from the tip of the tube. They soften the tissue of the column and stigma.
The nucleus of the generative and vegetative cells pass into the pollen tube. The generative cell divides and forms two sperm. Through the micropile (hole in the membrane) the ovule penetrates the pollen tube into the embryo sac. Inside the bag, the tube breaks, and all its contents are poured.
Then there is a merger with the egg of one of the sperm. Thus, a zygote is formed, which subsequently gives rise to a seed germ. Along with this, the second sperm merges with the central nucleus. This leads to the formation of a triploid nucleus, which subsequently passes into the triploid endosperm, which is secondary.
The entire process described above is called "double fertilization in flowering plants."
Antipodes and synergides (polar nuclei in the embryo sac) dissolve after the process is completed, and integments (integuments) form a seed coat.
Double fertilization in plants was discovered by the Russian scientist S. G. Navashin.
During this process, the nuclei in the embryo sac are usually much larger than the sperm nuclei, which is due to the presence of the first in the interphase (intermediate phase). In this case, the state and shape of the nuclei of the embryo sac can vary. So, in some Asteraceae, they can be elongated, sometimes crimped, without nucleoli. As a rule, sperm are presented in the form of rounded interphase nuclei with nucleoli, in some cases not differing from the structure of female nuclei.
Double fertilization can be of two types, depending on the nature of the union of the female and male nuclei.
So, in the premitotic process, the sperm nucleus is immersed in the female nucleus, where despiralization (unwinding) of the chromosomes takes place. Moreover, the combination of sets of chromosomes of both nuclei is carried out in interphase.
Postmitotic double fertilization is the entry of the female and male nucleus, preserving the membrane, into prophase, which ends with their union. The formation of interphase nuclei that contain chromosome sets occurs after the first mitotic fission in the zygote passes.
With double fertilization in the egg, two haploid (containing half sets of chromosomes) nuclei merge, therefore the zygotic nucleus is diploid (with a full set of chromosomes).
Depending on the number of polar nuclei contained in the central cell, the number of chromosomes of the endosperm nuclei is formed.
Double fertilization contributes to the manifestation of the prevailing traits in the endosperm of the seeds of the (hybrid) endosperm from the paternal plant. When several pollen tubes penetrate into the embryo sac, sperm from the first tube are involved in the process, and sperm degenerate in the remaining tubes. Double fertilization with two sperm is extremely rare.
In several embryological studies using various representatives of angiosperms (Asteraceae, Ranunculaceae, Liliaceae), Navashin proved that the endosperm, like the embryo, belongs to the sexual process in flowering plants. This phenomenon is peculiar only to angiosperms of the plant world.
It should be noted that the work of Navashin made it possible to clarify the question of the origin of the endosperm. Subsequently, the findings of the scientist were further developed.