Embryo Sac


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Related to Embryo Sac: pollen grain, double fertilization, synergid

embryo sac

[′em·brē·ō ‚sak]
(botany)
The female gametophyte of a seed plant, containing the egg, synergids, and polar and antipodal nuclei; fusion of the antipodals and a pollen generative nucleus forms the endosperm.

Embryo Sac

 

or female gametophyte, the sexual generation of angiospermous plants.

The embryo sac develops in the central portion of the ovule (nucellus), where the maternal macrosporocyte, as a result of meiotic division, forms four haploid cells (a tetrad of macrospores), of which one develops (the rest atrophy). During the development of the embryo sac there are three successive synchronous mitotic divisions of its nuclei, so that their number increases in the progression 1:2:4:8, and they are distributed evenly along the ends of the growing embryo sac. After the third mitotic division, three cells of the egg apparatus are formed at one (micropylar) end of the embryo sac; three antipodal cells are formed at the opposite (chalazal) end. Between these groups of cells a central cell containing two polar nuclei is formed. The cells of the egg apparatus differentiate into an egg cell and two synergids; the polar nuclei in many cases merge, forming a secondary nucleus. The subsequent evolution of this so-called normal type of embryo sac consisted in the emergence of embryo sacs formed by two or four macrospores, reduction of the number of mitotic divisions to two or one, and a change in the distribution of nuclei. Various combinations of these changes caused the emergence of several types of embryo sac, which differ in the number of nuclei (4, 8, 16) and of cell groups and polar nuclei (1, 2, 4, 7–14), as well as in the number of cells in the groups (for example, the egg apparatus may consist of 1, 2, 3, 5, or 7 cells) and in other characteristics. Double fertilization occurs in the mature embryo sac of any type, after which the embryo and the endosperm develop. Formerly, the female gametophyte of gymnosperms was also called an embryo sac; however, it differs in principle from the embryo sac in the development of a massive multicellular gametophyte body and in the formation of archegonia.

REFERENCES

Maheshwari, P. Embriologiia pokrytosemennykh. Moscow, 1957. (Translated from English.)
Poddubnaia-Arnol’di, V. A. Obshchaia embriologiia pokrytosemennykh rastenii. Moscow, 1964.
Romanov, I. D. “Zhenskii gametofit pokrytosemennykh rastenii.” In Materialy Vsesoiuznogo simpoziuma po embriologii rastenii. Kiev, 1968.

I. D. ROMANOV

References in periodicals archive ?
Embryo sac and microsporangium development in Pandanus (Pandanaceae).
Development of the embryo sac and endosperm of Albuca transvalensis Moss-Verdoorn.
Nutrition of the embryo sac and embryo--a morphological approach.
The ovule and embryo sac in Xanthorrhoeaceae sensu lato.
The outermost (dermal) layer undergoes mainly anticlinal divisions at first, although it may later proliferate at the micropylar end to form a nucellar cap, and sometimes may also proliferate around and below the embryo sac.
This mutation is located on chromosome arm 3L and has as a direct effect an increased number of nuclear divisions before cellularization of the embryo sac, which generates in the embryo sac an indeterminate, extra number of micropylar and synergids cells, egg cells, central cells, and polar nuclei within central cells (Evans, 2007; Guo et al.
It is likely that being in the best environment possible (the embryo sac of its same species) with a perfect, in vivo control of all of the nutrients, growth factors and regulatory cues needed, can allow for a sperm cell to proceed through embryogenesis with a high probability of Success.
1, Route 0): (1) in vivo haploid embryogenesis in the embryo sac (Fig.
In this case, it is thought that the wild type ig1 gene is a repressor of proliferation and embryogenesis in the embryo sac (Evans, 2007).
In this review I have summarized the main aspects of the three androgenic routes known so far in plants, in vivo haploid embryogenesis in the embryo sac, in vitro microspore/pollen embryogenesis, and in vitro meiocyte-derived callogenesis.
They found that auxin concentrations determined the fate of the nuclei and, depending on whether auxin levels were high or low, they could predict the appearance or disappearance of egg cells at different positions within the embryo sac.
Finally, the group employed a long series of bio-manipulative techniques to determine that the auxin gradient they had discovered within the embryo sac was due to on-site synthesis rather than transport from a source outside the sac.