Carbon dioxide exchange in lichens: Estimation of internal thallus CO2 transport resistances

The gaseous exchange pathways of Sticla latifrons Rich. and Pseudocyphellaria amphisticta Kremp. were examined using both light and scanning electron microscopes. The size and frequency of the pores in the gas exchange structures (cyphellae and pseudocyphellae) and in the medulla were measured and from these CO₂ diffusion resistances were calculated. Pseudocyphellae were found to be smaller and more widely spaced than cyphellae, consequently the resistance of the pseudocyphellae, was much greater than that of the cyphellae. Medulla resistances were low in both lichens and are probably unimportant, even at high water contents. No evidence of hyphal swelling was found. Gas exchange structure resistances were more than five fold greater than medulla resistances. It is suggested that this arrangement of resistances may simultaneously encourage refixation of respired CO₂ and maintain a non desiccating environment for the lichen algae. The internal transport resistances calculated in this work approximate experimentally obtained values.