Answer:
Vacuole
Step-by-step explanation:
Apart from cell wall, vacuole plays an important role in maintaining rigidity of plant cells. It is a membrane bound fluid filled sac. Vacuole can expand in size greatly.
When the outside environment is hypotonic, plant cell takes in excess of water. This water is taken up by vacuole and it can stretch to fill out around 90% of cell without losing its integrity. The cell is said to be turgid and all other organelles press against the cell wall without facing any damage. The development of this hydrostatic pressure helps to maintain the rigidity of the cell along with cell wall.