The ultrastructure of the egg and central cell of Petunia

The egg and central cell of Petunia hybrida undergo a number of changes and become mature during anthesis. The egg greatly enlarges and becomes highly vacuolated. The nucleus and the major part of the cytoplasm of the mature egg are located at the chalazal pole of the cell. The number of organelles decreases slightly during maturation. The ribosomes of the mature egg are clustered in polysomes. The chalazal part of the mature egg seems to be surrounded by the plasma membrane only, whereas the remainder of the cell has a normal cell wall. The number of mitochondria and plastids and the amount of endoplasmic reticulum (ER) in the central cell are almost constant during maturation. The number of ribosomes, however, increases greatly. Almost all ribosomes are free and the impression is that they are monosommes. A large amount of starch is formed in the plastids. T wo types of dictyosomes are present in the mature central cell. The outer membranes of the polar nuclei are sometimes connected by ER membranes.