Here's why:

* Turgor Pressure: Vacuoles are large, fluid-filled sacs within plant cells. They store water and other dissolved substances. When the vacuole is full of water, it creates pressure against the cell wall, known as turgor pressure. This pressure gives the plant its rigidity and helps it stand upright.

* Cell Wall Support: The cell wall, which is a rigid structure surrounding the plant cell, also contributes to rigidity. However, the turgor pressure from the vacuole is essential for maintaining the cell wall's shape and providing overall support.

So, while the cell wall provides structural support, the vacuole is the key player in making a plant rigid.