Why are onion cells rectangular?

Plant cell structures are very similar across the range of plants that exist on Earth. All plant cells contain specific components, one of which is a cell wall. Cell walls, which do not exist in animal cells, are rigid in structure. Cell walls provide shape to form tissues and play a large role in determining what individual parts of a plant look like.

Because all plant cells have cell walls, plant cells have fixed rectangular shapes. If you look at an onion peel (or any piece of an onion) under a microscope, you will see clear evidence of a rectangular cell supported by the rigid structure of the cell wall. In contrast, animal cells, which do not have cell walls, will appear to be round.