How a boundary layer works. Artists: Neil Dold, Abraham Khan, Julie Kohn, and Chrissy Kaufmann
"Our leaf temperature data support the microclimatic buffering hypothesis; leaf temperatures were usually cooler than ambient, especially during the hottest part of the day. Interestingly, although both size classes protect eggs, larger leaves were consistently warmer during the day than their matched small leaves. Indeed, only small leaves show limited homeothermy as defined by Mahan and Upchurch (Mahan and Upchurch, 1988). This pattern likely reflects differences in the physiology and morphology of smaller D. [Datura] wrightii leaves, which have higher transpiration rates during the day than large leaves (G. Barron-Gafford, unpublished) and higher boundary layer conductance (Jones, 1992)." (Potter et al. 2009:3448-9, 3452)
"Boundary layer – The boundary layer is a thin layer of still air hugging the surface of the leaf. This layer of air is not moving. For transpiration to occur, water vapor leaving the stomata must diffuse through this motionless layer to reach the atmosphere where the water vapor will be removed by moving air. The larger the boundary layer, the slower the rates of transpiration.
"Plants can alter the size of their boundary layers around leaves through a variety of structural features. Leaves that possess many hairs or pubescence will have larger boundary layers; the hairs serve as mini-wind breaks by increasing the layer of still air around the leaf surface and slowing transpiration rates. Some plants possess stomata that are sunken into the leaf surface, dramatically increasing the boundary layer and slowing transpiration. Boundary layers increase as leaf size increases, reducing rates of transpiration as well. For example, plants from desert climates often have small leaves so that their small boundary layers will help cool the leaf with higher rates of transpiration." (Library of Crop Technology Lessons website)