I've noticed that not all plant cells are the same. Some plant cells, such as onion cells, have that rigid shape that you see in text books; but some other plant cells such as duckweed cells have a more rounded shape.
Here are a couple of views of onion cells:
For contrast, here are cells from a duckweed leaf:
The duckweed cell walls are thick, but they aren't rectangles, like the cell walls of the onion cells.