Transpiration

Transpiration is the evaporation of water from the inside of a plant into the air, mostly through stomata on the leaves. Water from the soil enters the roots, travels up the xylem and moves into the leaf, where it evaporates from the moist surfaces of mesophyll cells into air spaces and diffuses out through the open pores. Only a small fraction of the water a plant absorbs is kept for growth; the great majority is transpired.

Transpiration is what pulls water up a plant. Evaporation at the leaf creates tension that draws the column of water up from the roots, held together by the cohesion of water molecules, so the process is passive and needs no ATP. It also carries dissolved minerals upward and cools the leaf. The cost is water loss, because the stomata that must open to let carbon dioxide in for photosynthesis also let water out. Rates rise with warmth, wind, sunlight and dry air, and fall when humidity is high or when soil water runs short and plants close their stomata.

Together with evaporation from soil and open water, transpiration makes up evapotranspiration, a major route by which water returns to the atmosphere in the water cycle.