ULTRASTRUCTURAL CHARACTERIZATION OF APOSPORY IN PANICUM-MAXIMUM

The nucellar ultrastructure of apomictic Panicum maximum was analyzed during the meiocytic stage and during aposporous embryo sac formation. At pachytene the megameiocyte shows a random cell organelle distribut ion and sometimes only an incomplete micropylar callose wall. The chal azal nucellar cells are meristematic until the tetrad stage. They can turn into initial cells of aposporous embryo sacs. The aposporous init ials can be recognized by their increased cell size, large nucleus, an d the presence of many vesicles. The cell wall is thin with few plasmo desmata. If only a sexual embryo sac is formed, the nucellar cells ret ain their meristematic character. The aposporous initial cell is somew hat comparable to a vacuolated functional megaspore, It shows large va cuoles around the central nucleus and is surrounded by a thick cell wa ll without plasmodesmata. In the mature aposporous embryo sac the stru cture of the cells of the egg apparatus is similar to each other. In t he chalazal part of the egg apparatus the cell walls are thin and do n ot hamper the transfer of sperm cells. Structural and functional aspec ts of nucellar cell differentiation and aposporous and sexual embryo s ac development are discussed.