Final answer:
Elodea and onion cells have similar shapes because they are plant cells with a rigid cell wall, a central vacuole, and other shared structural features.
Step-by-step explanation:
Elodea cells and onion cells have the same overall shape because they are both plant cells, which typically have a more rigid and defined structure compared to animal cells. This structure is largely due to the presence of a cell wall made of cellulose, which provides support and determines the shape of plant cells. Additionally, both Elodea and onion cells have other common plant cell features such as a central vacuole that maintains cell rigidity, chloroplasts (more prominent in Elodea due to its function in photosynthesis), and a defined cell nucleus.