Step 1: Look at what each genus is known for: Fucus and Ectocarpus are brown algae, Polysiphonia is a red alga, and none of these show cap cell formation.
Step 2: Oedogonium, a filamentous green alga, divides by laying down internal wall rings just below the apex of the cell. As the cell elongates and divides, remnants of these rings remain as visible cap-like markings.
Step 3: This cap cell formation is a textbook identifying feature of Oedogonium and does not occur in the other three genera listed.
Correct genus: \[\boxed{\textit{Oedogonium}}\]