OVULE DEVELOPMENT IN ORNITHOGALUM CAUDATUM (LILIACEAE) WITH A REVIEW OF SELECTED PAPERS ON ANGIOSPERM REPRODUCTION III. NUCELLUS AND MEGAGAMETOPHYTE.

Ornithogalum ovules are crassinucellate, and the nucellar epidermis forms a nucellar cap which is secretory. The nucellus accommodates archespore and megaspore mother cell expansion by a concurrent increase in its size. It accommodates functional megaspore and megagametophyte expansion by simultaneously undergoing growth and partial degeneration. Nucellar degeneration is centrifugal and appears to be an autolytic process rather than a result of mechanical pressure from the growing megagametophyte. Megagametophyte development is of the Polygonum type. Mitotic divisions of the functional megaspore and its daughter cells are characterized by a perinuclear distribution of organelles. Some starch is stored in the micropylar end of the nucellus, but the majority is stored in the chalazal end around the hypostase. A strand of cytoplasm extends between the egg apparatus and antipodals. The micropylar polar nucleus moves across the central cell's vacuole in this strand to reach its mate. Antipodals initially are vacuolate but later fill with dense cytoplasm and numerous starch grains. Preliminary evidence from light microscopic investigation indicates a thin layer of wall material surrounds the egg apparatus. However, around the chalazal end of the egg this layer may be disjointed islands of wall material rather than a continuous wall. Synergid cytoplasm contains an abundance of organelles and a large filiform apparatus (FA) projects into it. In addition to the FA, synergid walls have a knobbed in growth around their micropylar end. We propose that the antipodals and central cell function initially as storage cells for nutrient reserves and later as nurse cells for embryo and endosperm whereas the synergids function as secretory cells which produce an agent(s) which attracts pollen tubes and, via the filiform apparatus, as cells which receive the pollen tube. [ABSTRACT FROM AUTHOR]