Embryo Sac of Gnetum
Embryo Sac of Gnetum:-
> Lotsy described the embryo sac of G. gnemon as showing an interesting deviation from those in other species of Gnetum.
> Instead of containing only free nuclei at the fertilization stage, the embryo sac of G. gnemon was described as containing a compact antipodal tissue, sharply distinct from the micropylar chamber with its free nuclei.
> As a consequence, the embryo sac of G. gnemon has been used ever since as illustrating a female gametophyte intermediate in structure between the tissue-filled sacs of Ephedra and Tumboa on the one hand, and the sacs of other species of Gnetum, which contain only free nuclei.
> Later the same investigator in reporting parthenogenesis in G. ula described the embryo sac of that species as being of the G. gnemon type.
> At an early stage of the embryo sac, eight nuclei are observed grouped near the center, the sac being invested by the loose tissue of the nucellus.
> At a somewhat later stage the nucellar cells at the chalazal end of the sac are strikingly differentiated, becoming more and more compactly arranged, gradually obliterating the intercellular spaces, and taking on the appearance of glandular cells.
> As vacuolation proceeds in the sac and the free nuclei become parietally placed, this "pavement tissue" becomes more compact and extends deeper into the chalaza.
> Still later it spreads laterally below, until it becomes fan-shaped in section, but it is always very distinct in contour and sharply marked off from the surrounding nucellar tissue.
> At the fertilization stage the sac contains only free nuclei, which become somewhat grouped at the antipodal end, but there is no walled tissue.
> Spreading below the sac, however, the mass of nucellar pavement tissue shows a definite contour, which might be merged in imagination with that of the sac and thus mistaken for a compact tissue within the antipodal end of the sac.
> It will be noted that after the fertilization stage is reached (fig. g) the pavement tissue begins to lose its glandular character; and later it is destroyed entirely by the growing endosperm.