ULTRASTRUCTURAL STUDIES ON THE EMBRYO SAC OF VISCUM-MINIMUM .1. MEGASPOROGENESIS

Changes taking place during megasporogenesis of a mistletoe (Viscum mi nimum) were examined at both light and electron microscopy levels. No distinct ovules, integuments, or ovarian cavity are present at any sta ge of development. The multicellular archesporium originates in the ce nter of a solid ovary. Several functional megasporocytes are developed from the archesporial cells, either adjacent to each other or separat ed by unspecialized cells. The megasporocyte is much larger than surro unding cells, is invested by a thick wall, and possesses a large nucle us and amyloplasts. Although plasmodesmata are absent even between the adjacent megasporocytes, cells enter meiosis simultaneously. Followin g meiosis a linear tetrad is formed. Double and treble linear tetrads are frequently observed. The development of the embryo sac conforms to the monosporic or Polygonum type of megasporogenesis. However, the bi sporic or Allium type of development is occasionally observed in prepa rations. Factors determining the pattern of development are discussed. As in other plant species which follow the monosporic type of develop ment, only one functional megaspore cell undergoes further development while others degenerate. Unlike the healthy functional megaspore cell , the degenerating cells have large starch grains and electron-dense c ytoplasm. At a later stage of development, the degraded cells are abso rbed by the surrounding tissue.