Plants also need water for photosynthesis. Photosynthesis is what plants do to create their food, and water is critical to this process. Water enters a plant's stem and travels up to its leaves, which is where photosynthesis actually takes place. Once in the leaves water evaporates, as the plant exchanges water for carbon dioxide. This process is called transpiration, and it happens through tiny openings in the plant's leaves, called stomata. The water from the leaves evaporates through the stomata, and carbon dioxide enters the stomata, taking the water's place. Plants need this carbon dioxide to make food. Transpiration - this exchange of water for carbon dioxide - only occurs during the day when there is sunlight. This is why you might find dew on plants in the morning. The plants contain a lot of water because all night long water has been entering through the stem and being pulled into the leaves where it can't evaporate. Since the water doesn't evaporate at night, the water has no where to go so it remains on the leaves as dew.