The loss of water by evaporation from a plant surface. Over 90% escapes through the open stomata, while about 5% is lost directly from the epidermal cells. It has been shown that although the combined area of stomatal pores is on average only 1-2% of the total leaf area, the amount of transpiration they allow is 90% of the transpiration that occurs from a water surface the same area as the leaf. Transpiration rates are greatest when the leaf cells are fully turgid and when the external * relative humidity is low. Water forms a film around the mesophyll cells and evaporates into the substomatal chamber from where it diffuses into the air. The degree of opening of the stomata (stomatal resistance) is of prime importance in governing the rate of water loss. The width of the * boundary layer at the leaf surface is also important. In dry conditions, transpiration can cause wilting and so the plant may develop features such as waxy cuticles to minimize the problem. See also antitranspirant , cohesion theory. |
