Abstract: The constituent ceils of the female germ unit in the embryo sac of Pelargonium homman Bailey just before fertilization were studied using transmission electron microscope. It was revealed that a large area between the egg and synergicls and between the synergids were sepemted by their plasma membranes at the chalazal portion of the egg apparatus. Only a small portion at the micmpylar end of the cells had normal wall. Common wall was also lacking between the egg and the central cell at the chalazal end. Furthermore,the spacebetween the two plasma membranes was alternatively narrow and wide. There were a large number of single or double membrane-bound vesicles in the wider space. These vesicles appeared to be originated from the cortical ER in the egg and central cell, and probably contained substances essential for the formation of the zygote wail. The egg had numerous giant mitoehondria, starch-contained plastids and RER. Based on the ultrastructural observations, it is proposed that the egg cell is provided high potential of physiological synthetic activity but still being quiescentas compared to the synergid. In the central cell, the location of the polar nuclei (or secondary nucleus) toward the micropylar end is essential for the simultaneous fusion of the sperm and egg and sperm and central cell during double fertilization. At the micropylar end of the central cell, lateral wall ingrowths developed abutted against the nucellus which was characteristic as transfer cell and indicated important site for the megagametophyte to obtain nutrients from the sporophyte. A great number of mitochondria were located toward the filiform apparatus in the synergids, which was quite similar as occurred in many other plants with the exception of the following features. Lipid bodies were formed at the end of RER. The nucleus of the synergids contained micronucleolus. After pollination but before the entrance of the pollen tube in the embryo sac, one of the synergid cells began to degenerate.

Key words: Egg, Synergid, Central cell, Ultrastructure, Pelargonium hortonan