Double Fertilization

The pollen grains that get on the stigma one way or another begin to germinate. The grain swells, a pollen tube is formed from the vegetative cell. Thus, germination begins. The shell breaks through in the thinnest place with a pollen tube. Special substances are emitted from the tip of the tube. They soften the tissues of the post and stigma.

The nucleus of the generative and vegetative cells passes into the pollen tube. The generative cell divides and forms two sperm. Through the micropyle (hole in the membrane) of the ovule, the pollen tube penetrates into the embryo sac. Inside the bag, the tube is torn, and all of its contents are poured out.

Then, one of the spermatozoa merges with the egg. Thus, a zygote is formed, which subsequently gives rise to a seed embryo. Together with this, the second sperm merges with the central nucleus. This leads to the formation of a triploid nucleus, which later becomes a triploid endosperm, which is secondary.

The entire process described above is called "double fertilization in flowering plants".

Antipodes and synergids (polar nuclei in the embryo sac) after dissolution of the process dissolve, and integuments (integuments) form the seed skin.

Double fertilization in plants was discovered by the Russian scientist SG Navashin.

In this process, the nuclei in the embryo sac are generally 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 may vary. For example, in some Compositae they may be elongated, sometimes crimped, without nucleoli. As a rule, spermatozoa are represented in the form of round 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 female and male nuclei.

Thus, during the pre-dentist process, the sperm nucleus immerses into the female nucleus, where despiralization (unwinding) of the chromosomes occurs. In this case, the combination of sets of chromosomes of both nuclei is performed in the interphase.

Postmototic double fertilization is the entry of the female and male nucleus, retaining the envelope, into the prophase, which ends with their unification. The formation of interphase nuclei that contain chromosomal sets occurs after the first mitotic division in the zygote passes.

When double fertilization occurs in the egg, two haploid (containing half sets of chromosomes) nuclei merge, so 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 promotes the manifestation of the predominant symptoms in the endosperm of seeds (hybrid) endosperm from the paternal plant. When several pollen tubes penetrate the embryonic sac, sperm are involved in the process from the first tube, in the remaining tubes of the sperm they degenerate. Double fertilization with two spermatozoa is extremely rare.

In several embryological studies using various representatives of angiosperms (Compositae, lutonic, lilac), Navashin proved the belonging of endosperm, like the embryo, to the sexual process in flowering plants. This phenomenon is characteristic only of angiosperms in the plant kingdom.

It should be noted that Navashin's work made it possible to clarify the origin of the endosperm. Later the scientist's conclusions were further developed.