Transpiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers an roots. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots. Mass flow is caused by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere. Water is absorbed at the roots by osmosis, and any dissolved mineral nutrients travel with it through the xylem.