Leaf structural characteristics and gas-exchange measurements were used to determine whether photosynthetic tissue of Typha Iatifolia L. (cattail) utilized CO2 from the aerenchyma gas spaces, part of an internal pathway for gas transport in this wetland species. The partial pressure of CO2 (pCO2) in these aerenchyma gas spaces can be more than 10 times atmospheric pCO2. The photosynthetic tissue occurred in structurally similar adaxial and abaxial palisades, which were distinctly separated from each other by the aerenchyma gas spaces. In each palisade there were three to four layers of tightly packed, nonchlorophyllous cells separating the photosynthetic tissue from the aerenchyma gas space. Different lines of evidence indicated that CO2 conductance in the light was significantly greater across the epidermal surface than across the internal surface of both palisades. However, at an epidermal pCO2 of 350 [mu]bars and an internal pCO2 of 820 [mu]bars, the net rates of CO2 uptake (PN) across the epidermal and internal surfaces were about equal. PN across the internal surface was greater than across the epidermal surface at higher internal pCO2. Gas space pCO2 can be greater than 820 [mu]bars in the field, and therefore, PN across the internal surface could be a significant proportion of epidermal surface PN.