Embryo Sac

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

embryo sac

n. The female gametophyte of a seed plant, within which the embryo develops.

Fig. 147 Embryo sac . Vertical section.

embryo sac

the female GAMETOPHYTE of flowering plants, consisting of a sac-like structure inside the OVULE in which are found six HAPLOID cells (without cell walls) and two haploid nuclei. See Fig. 147 . All cells have originated from one haploid nucleus and so are all genetically identical. The name ‘embryo sac’ comes from the fact that the plant embryo develops from within the sac. Two MALE GAMETE NUCLEI enter the embryo sac from the POLLEN TUBE and carry out a ‘double’ fertilization. One male nucleus fuses with the egg cell to give a DIPLOID zygote (the future plant), the other male nucleus fuses with two polar nuclei giving a triploid fusion nucleus (3n) which develops into the ENDOSPERM (the future food supply in some species).

单词	embryo sac
释义	embryo sac embryo sac n. The female gametophyte of a seed plant, within which the embryo develops. embryo sac n (Botany) the structure within a plant ovule that contains the egg cell: develops from the megaspore and contains the embryo plant and endosperm after fertilization em′bryo sac` n. (in flowering plants) a large cell of the rudimentary seed, within which the embryo develops. Also called megaspore. embryo sac An oval structure within an ovule of a flowering plant that contains the egg cell. Together with the fertilized egg cell, it develops into a seed. Translations Embryo Sac 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 embryo sac embryo sac n. The female gametophyte of a seed plant, within which the embryo develops. Fig. 147 Embryo sac . Vertical section. embryo sac the female GAMETOPHYTE of flowering plants, consisting of a sac-like structure inside the OVULE in which are found six HAPLOID cells (without cell walls) and two haploid nuclei. See Fig. 147 . All cells have originated from one haploid nucleus and so are all genetically identical. The name ‘embryo sac’ comes from the fact that the plant embryo develops from within the sac. Two MALE GAMETE NUCLEI enter the embryo sac from the POLLEN TUBE and carry out a ‘double’ fertilization. One male nucleus fuses with the egg cell to give a DIPLOID zygote (the future plant), the other male nucleus fuses with two polar nuclei giving a triploid fusion nucleus (3n) which develops into the ENDOSPERM (the future food supply in some species). ThesaurusSeeovule
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embryo sac

embryo sac

embryo sac

em′bryo sac`

embryo sac

Embryo Sac

embryo sac

Embryo Sac

REFERENCES

embryo sac

embryo sac

embryo sac