Plants have holes in their leaves called Stomata on the surface of their leaves. The Stomata are opened to allow CO2 in for Photosynthesis, but all the time they are open, water vapour is lost. This is called Transpiration.
Stomata can be opened and closed by the Guard Cells that surround them. Losing water is the side effect of allowing CO2 in.
As water evaporates from the surface of leaves, water is pulled up through the Xylem to take its place. The constant movement of water molecules from the roots to the leaves is known as the Transpiration Stream. It is driven by the evaporation of water from the leaves, so anything that affects the evaporation affects the Transpiration Stream.
Firstly, conditions that effect the rate of Photosynthesis will increase the rate of Transpiration. This is because the more Photosynthesis that is happening, the more Stomata are open to let CO2 in, and therefore let water escape. So warm, sunny conditions increase the rate of Transpiration.
Also, conditions that affect the rate of evaporation of water when the Stomata are open will also make Transpiration happen more rapidly. Hot, dry, windy conditions increase the rate of Transpiration
Plants have a constant problem, they need to open their Stomata to let in CO2 to Photosynthesise, but at the same time don't want to lose water. So, some plants have special adaptions to reduce the amount of water lost!
Most leaves have a waxy, waterproof layer known as the Cuticle to prevent uncontrolled water loss. Most Stomata are found on the underside of the leaves, meaning they are not as exposed to the light and heat of the sun, reducing the time they are left open for.
If plants begin to lose water faster than it can be replaced by the roots, it will take Drastic measures. The plant can Wilt. This is where all the leaves collapse and hang down so the surface area for water loss by evaporation is greatly reduced.
The Stomata will close, which stops Photosynthesis and risks overheating. However this prevents most water loss and any further wilting. The plant will remain wilted until the Temperature drops, the sun goes or it rains.