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 ?
The results indicated that the ovules were not fertilized at all and the embryo sacs underwent degradation, and thus embryo abortion was not the reason for the failure in production of selfing seeds.
Male-Derived Haploid Embryogenesis in the Embryo Sac
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.
To find out, her team is determining the makeup, or DNA sequence, of genes that are switched on or off in the male cells and in the embryo sac cells.
The free nuclei were peripheral to the central area of the embryo sac by dense cytoplasm connections (Fig.
Paleobotanists surmise that before the advent of endosperm, all growing plant embryos fed on cells from their embryo sacs -- the maternal tissue that gives rise to unfertilized eggs.
Besides, the inner integument remains uniformly two layered which include numerous starch grains at the stage of mature embryo sac. The asymmetrical integument growth creates the anatropous curvature.
Forty-nine plants (64.4%) formed some aposporous embryo sacs in addition to the normal meiotic sac, and the remaining 27 plants were free of apospory.
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. The archespore forms in the subdermal layer (of the trizonate type), immediately subtended by the central zone.
Embryo sac formation and maturation follows as the Polygonum type.