Transpiration is the loss of water vapor from a plant.  The surface of a plant, called the epidermis, has microscopic pores called stomata, which allow for gas exchange between the interior of the plant and external atmosphere.  The gas exchange of oxygen and carbon dioxide is essential to the processes of photosynthesis and cellular respiration.  Most water lost by transpiration escapes through the stomata.  Although all aboveground parts of plants commonly have stomata (except for woody stems), leaves usually have the greatest total number and thus account for most of the water lost from a plant by transpiration.  Each stoma is opened and closed by the actions of a pair of guard cells, which border the opening of the pore.  In many plants, other specialized epidermal cells, called subsidiary cells or accessory cells, border the guard cells and also contribute to the operation of the stomata. 

Water moves by osmosis from the soil across cell membranes into cells on the root hairs.  From the roots, water moves up the stem to cells within the leaves through specialized vascular tissue called xylem.  Water evaporates from the surfaces of these cells into the intercellular spaces until those air spaces are saturated.  Unless the air outside the leaves is also saturated, the water potential inside the leaf will be greater than outside the leaf and water vapor will diffuse from the leaf through the stomata to the surrounding air.  This lowers the water potential of the leaf spaces to below that of the leaf cells, causing more water to diffuse from the cells into the spaces, and so on.  This loss of water and lowering of water potential exerts a powerful tension or pull on the water contained within the specialized water-conducting tissues of the stem and helps move water from the roots to the aerial portions of the plant. 

The cohesive properties of water assist in its transport through the plant.  Water molecules form hydrogen bonds with one another, and a chain of water molecules is “pulled” from the roots to the leaves as transpiration occurs.  As a water molecule exits the plant through a stoma, another moves into its position.  This process continues as long as water molecules are available in the soil to diffuse into the roots.