A double fertilization takes place during the sexual reproduction of the bed covers (flowering plants). Two sperm cells fuse with two cells in the embryo sac , from which on the one hand the zygote and from it the embryo and on the other hand the endosperm arise. This process is a peculiarity of flowering plants to which there is no parallel in other living beings. It was first described in 1898 by Sergei Gavrilowitsch Navashin .
At the end of the maturation process, two haploid cells are usually created in the pollen grain : a larger vegetative cell and a smaller generative cell. The vegetative cell surrounds the generative cell.
As part of the pollination the pollen grain lands on the stigma of a fruit sheet or of the multi-carpels punch in the flowering . The pollen grain germinates when the vegetative cell produces a tubular outgrowth, the pollen tube . This growing through the pen to the ovary down. In contrast to other parts of the plant, the growth of the pollen tube is not associated with cell division and it takes place at the tip (apical), as is also characteristic of the hyphae of fungi . Before or mostly shortly after pollen germination, the generative cell divides into two sperm cells, the male gametes , and these migrate down the pollen tube with the nucleus of the vegetative cell. The small sperm cells consist only of a nucleus, which is surrounded by very little cytoplasm and a cell membrane .
The tip of the pollen tube penetrates the ovary and up to the egg apparatus, which consists of the egg cell and two other cells, the synergids . This usually happens through the micropyle , an opening in the integuments that enclose the ovule . The pollen tube then fuses with one of the two synergids and empties into it. While the vegetative pollen core with the relevant synergid perishes, one of the two sperm cells fuses with the neighboring, also haploid egg cell to form a diploid zygote. The embryo later develops from this . The second sperm cell fuses with the adjacent diploid or binuclear central cell of the embryo sac to form the triploid endosperm cell . From this, the secondary endosperm develops , a nutrient-rich tissue, which normally surrounds the embryo and supplies it with nutrients.
The cell fusions are possible because the egg cell, the synergids and the central cell, unlike normal plant cells, are only partially surrounded by thin cell walls . After fertilization, many free nuclear divisions quickly take place in the endosperm cell without subsequent cell divisions , which creates a rapidly growing coenocytic (multinucleated) endosperm. The zygote does not develop into an embryo until later.
The evolutionarily more original haploid endosperm of the naked samos , in which mostly no double fertilization takes place, is called the primary endosperm . A special case within the Nacktsamer are the genera Ephedra ( sea crow ) and Gnetum , in which two zygotes result from double fertilization, but mostly only one of them develops into an embryo. In contrast, in the are conifers (pine trees) and Welwitschia namely two sperm cells are present, but only one of them fertilizes the egg.
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