Tugor Pressure

Transpiration is also crucial in maintaining water pressure within cells, keeping them rigid so they can support the plant.
The water pressure inside plant cells is called turgor pressure , and it is maintained by a process called osmosis . Technically speaking, osmosis is the movement of water across a differentially permeable membrane from a place where water concentration is higher to one where the concentration is lower.
Fluids like to reach a state of equilibrium. If I pour milk into my coffee, it doesn’t all stay in one place but diffuses throughout the cup. If you put a drop of food coloring in a basin of water, it diffuses until all the water is tinted.
Plant cells maintain a delicate balance of water and various dissolved salts and sugars. If the fluid inside the plant cell is "saltier" than the surrounding fluid, water molecules move in to try to reach equilibrium. If there were no cell membrane, then at the same time the salty water would diffuse out, until the salt concentrations inside and outside the cell were equal.
BUT: the cell membrane is "differentially permeable," meaning that water molecules can enter, but the salt molecules are too large to escape. The result is that water pressure builds inside the cell, causing the cell membrane to exert pressure on the cell wall—in much the same way a balloon inflated inside a box would exert pressure on all sides of the box.

These rigid, stacked "boxes" keep the plant upright. If the "balloons" deflate, then the boxes collapse. Plants must maintain their internal water pressure, or turgor pressure, to keep stems rigid and leaves expanded to the sunlight. This means that water must be available to the plant whenever it needs it. If water isn’t available, cells collapse and the plant wilts. So, next time you have a dry spell, and are commiserating with your gardening friends, you might just say, "Boy, the soil is dry as a bone. All my beautiful plants are losing turgor pressure!"