The traditional view of the role of morphology in the water relations of lichens has been that the lichen thallus is purely passive with respect to water uptake and loss (Blum 1973). While water uptake and loss are certainly physical processes (Harris 1976), an overreliance on this view ignores the important significance of thallus mor phology and anatomy in controlling water relations. In the long term response, it is certainly true that lichen thalli of any morphology will come into equilibrium with the vapor pressure of the surrounding air. However, it is the short-term response of differential rates of water uptake and water loss where a great variety of aspects of thallus morphological form and anatomical structure have a very strong influence on thallus water relations.