Plants make their own food through the action of sunlight on chlorophyll, and is called photosynthesis. Water is released as vapor through small openings in the leaves called stomates which regulate water loss to some extent by opening or closing. As water is lost, more water is drawn into the plant from the soil as long as there is available soil moisture. Some plants move water faster than others. Some plants go dormant when drought occurs and shed their leaves so that water loss is reduced.
Some plants go dormant in winter.
Many plants also have a daily cycle whereby they transpire during the morning and evening but close up during the day. This can also be measured using the same technique. Through the process of photosynthesis, oxygen is also released from the leaves with the water vapor. Carbon dioxide, which is the building block of food production, is taken in. Therefore, if a plant is manufacturing food it is releasing oxygen and water and taking in carbon dioxide.