One thing that land plants do to prevent dehydration is to close their leaf openings to the outside air, called stomata. One problem with that is the plant can't bring in CO2 for photosynthesis. Another thing that plants have is a way to maintain continuous flow of water from the roots with a circulatory system made of xylem. The main force which brings the water into the system is osmosis and the pressure of osmosis forces the water up the tubes in the xylem.
Plants do lose water constantly through transpiration through the stomata and directly at the surface of the leaves depending on how adaptive the structures are for preventing water lose. A supply of water to the roots is essential for most plants and in arid areas many plants have adapted structures which retain water. Their leaves produce water proof oily substances which coat the leaves. Plants also as mentioned, close their stomata during the day and at night open them. There are mechanisms in some plants which chemically store the CO2 collected at night so they can use it during the day when the light will allow them do photosynthesis.