The boundary layer is a thin layer of relatively still air that surrounds the surface of a leaf. Because air movement immediately adjacent to the leaf is slower than the surrounding atmosphere, this layer acts as a buffer between the plant and its environment.
The boundary layer helps regulate the exchange of water vapour, heat, carbon dioxide, and oxygen. A thicker boundary layer slows transpiration and heat loss, while a thinner boundary layer increases the rate at which water and gases move between the leaf and the surrounding air. Wind, fans, leaf size, leaf shape, and surface characteristics such as hairs all influence boundary layer thickness.
The boundary layer works together with the cuticle and stomata to control water loss. When the boundary layer is disrupted by strong airflow, leaves may cool more effectively and receive carbon dioxide more readily, but they can also lose water more rapidly if root water uptake cannot keep pace.
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