Fruticose lichens obtain water by (1) absorption of vapor from the air, (2) absorption of liquid from the soil through the basal part of the thallus and (3) absorption of dew or rain through the upper surface, particularly the tips of the podetia. An important part of the mechanism of uptake seems to be imbibition of water by thick, gelatinized hyphal walls. The rate of water uptake from vapor is much lower than from liquid and the amount which can be gained is less. The water obtained by either route is stored in capillary spaces between hyphae and as imbibition water in the hyphal walls. Water loss occurs through evaporation. My study was undertaken to describe water exchange with the atmosphere by selected species of Cladonia and to evaluate the effect of relative humidity upon water content of the thallus.