About 95% of water absorbed by a tree is transpired through which part?

• A. Leaves
• B. Roots
• C. Bark
• D. Trunk

Answer: (A)

Transpiration primarily occurs through the leaves because that is where the vast majority of stomata are located. Stomata are tiny openings on the leaf surface that open to allow carbon dioxide in for photosynthesis and, in the process, water vapor escapes to the atmosphere. Leaves present the largest surface area and have a high stomatal density, so when the stomata are open, evaporation from the leaf interior drives most of the water loss. The regulation of these openings means leaves are the main control point for transpiration, and the water that moves up through the xylem from the roots is supplied to replace what is lost. Bark, trunk, and even roots don’t provide the same extensive pathway for vapor to escape, so they account for only a small fraction of transpiration.