Green algal photosynthetic units in the genus Stereocaulon (phyllocladia or areoles) are typically white to pale gray, but certain species develop a darker central portion. Structurally, these dark-centered areoles of crustose Stereocaulon are identical to the phyllocladia of the S. vesuvianum group. The dark-centered photosynthetic units may have an adaptive role, separating tissue for gas exchange from tissue for water uptake and light transmission. I hypothesize that the white portions of cortex on the photosynthetic units have reflective extracellular deposits that optically occlude the cortex but allow gas exchange through hydrophobic air channels. In contrast, the dark centers apparently provide transparent and water-transmissive windows into the algal layer through an otherwise relatively opaque cortex. Cortical windows are apparently unusual in lichens and may be an adaptation to provide gas exchange in cold, wet environments. For crustose species, the areoles often protrude from a thin film of water over rock and the dual cortex appears to provide both light transmission and gas exchange. Similarly, for the S. vesuvianum group, the dual cortex may provide light transmission and gas exchange for water-saturated thalli in oceanic environments.