If you've ever wondered how trees transport water all the way from roots to crown, you might be surprised to learn that transpiration -- water loss in the leaves -- is involved. To draw up water, plants must lose water through transpiration.
Each leaf has microscopic pores in its underside called stomata ringed by sausage-like cells called guard cells. The leaf cells need carbon dioxide for photosynthesis, so gas exchange takes place through these pores. The air comes in contact with plant cell surfaces at air spaces inside the leaf. Since the cells are full of water, these air spaces quickly become saturated with water vapor.
Most of the time, the air outside the leaf is drier and poorer in water vapor than the air spaces inside the leaf, so water vapor diffuses outwards in a process called transpiration. Plants can limit the amount of water vapor they lose by closing their stomata. Many plants do so at night, when they are no longer performing photosynthesis.
Transpiration helps generate the force needed to pull water up the plant's stem. In leaf cells, the surface of the water in the pores of their cell walls curves inwards as the cell loses water to transpiration. Water has high surface tension, so this curvature pulls more water out of the cell, generating a negative pressure that draws water up the stem like sucking it through a narrow straw.