Final answer:
The Z ring's function in binary fission is to assist in cell division by forming a contractile ring that separates the daughter cells.
Step-by-step explanation:
The function of the Z ring in binary fission is cell division. During binary fission, the Z ring, composed of the protein FtsZ, forms a contractile ring at the site where the cell will divide. This Z ring is crucial for pinching the cell envelope and successfully separating the cytoplasm of the two new daughter cells. It is intricately involved in the recruitment of other proteins and new membrane materials to form a septum, enabling the physical division of one cell into two genetically identical daughter cells.