Modulated chlorophyll fluorescence of the chlorolichen Parmelia sulcata (A), the cyanolichen Peltigera neckeri (B), and the moss Rhytidium rugosum (C). Very strong light pulses (1 s) were given every 500 s.. After rapid desiccation of the organisms, basal fluorescence was at a low F_o^′ level. Light pulses caused small fluorescence responses. Hydration increased fluorescence to the levels F_o and F_m. Slow desiccation suppressed both levels towards F_o^″. For explanation, see text.