AN ELECTRON MICROSCOPIC STUDY OF THE MATURE MEGAGAMETOPHYTE IN ZEA MAYS

Abstract

With light microscopy maize megagametophytes stained with Alcian blue‐periodic acid‐Schiff (AB‐PAS) reveal acid or neutral polysaccharides in various cell walls. Comparative fine structural studies were made of permanganate‐ or OsO4‐fixed material. Organelle distribution is random in the vacuolate and multinucleate antipodal cells; organelles are abundant; starch is scarce. Antipodal cell walls have large openings forming several syncytia. Some walls are papillate. In the central cell (primary endosperm cell) a thin peripheral layer of cytoplasm surrounds the large vacuole; organelle number is moderate; starch is abundant. The central cell wall is also papillate adjacent to the antipodals and around the egg apparatus. In the synergids organelle distribution is non‐random; nuclei and numerous organelles occupy the micropylar cytoplasm of each synergid; vacuoles dominate the chalazal cytoplasm of these cells. The filiform apparatus stains with AB‐PAS and is composed of both lightly and darkly stained amorphous material. In the egg, organelle distribution is perinuclear with vacuoles proximal to the micropyle; mitochondria are large, abundant and polymorphic; starch is abundant. Nucleolar diameter is five times greater in the central cell and egg than in the antipodal cells and ten times greater than in the synergids. Plasmodesmata occur in all cell walls within the gametophyte, but none appear in the gametophyte wall itself. It is suggested that the antipodals and synergids might be secretory, the latter probably being involved in pollen tube attraction, and that stored metabolites in the central cell and egg cytoplasm support rapid increase in metabolism following fertilization.