Water Makes a Leaf Stand Upright

Leaves wilt, most commonly, when water volume in its cells drop.


Turgidity

A Deflated Balloon
Source - simplystatedbusiness.com

A wilted cell is like a balloon that has lost its air. The pressure of the air inside the balloon, against the balloon wall, provides the rigidity required to maintain the shape of a balloon. 

Healthy Parsley Leaves
Source - faithforeverliving.org

The water inside a cell is like the air in balloons.
As the amount of water drops in a cell, the cell looses its ability to maintain its shape - the cell wilts like a deflated balloon because the pressure (known as Turgor Pressure) from the water in the cell against the cell wall drops.


Many Many cells

Cellular Structure of a Leaf
Source - exploratorium.edu

A leaf is made up of many, many cells. As individual cells loose the water contained in each cell, each cell wilts. Eventually this wilting is clearly visible as a wilted leaf.