When plants wilt, their soft stems and leaves begin to droop. What is going on inside the plants' cells that causes plants to droop like this?

I'm not entirely sure and this response is late but I'm pretty sure it's when the central vacuoles lose their water and can no longer support the cell. When the vacuole of a cell is full it pushes outward against the cell wall. When it has nothing in it, you can assume that because the vacuole is so big and is such a big component of this that this would be the answer.