Why Does Wilted Celery Become Crisp Again in Water?

When celery bends after storage, its fibres have not necessarily vanished. More often, its cells have lost water and the pressure inside them has fallen. A plant cell has a comparatively rigid wall. When its central vacuole is well filled, the contents press outwards against that wall, creating turgor pressure. Many firm cells together support the tissue, helping celery stand upright and feel crisp when bitten.

Place the cut end of celery that is still fresh but mildly wilted in plain water, and water can move into the tissue and across selectively permeable cell membranes by osmosis. The vacuoles refill, turgor rises and the slack tissue may become firm again. It is therefore incomplete to say only that the fibres swell. The change in texture comes from the mechanical state formed by cells, their walls and water.

Cold water is often used because cooling also slows further water loss and deterioration; available water is what restores turgor. A strongly salty or sugary solution has a higher external solute concentration and can make water entry less favourable. Nor can soaking repair celery that is decayed, badly damaged or injured by freezing. Becoming crisp again is rehydration of tissue whose cells remain sufficiently intact, not a reversal of time.

https://openstax.org/books/biology-2e/pages/5-2-passive-transport