In leaves of Sdndapsus aurea L. after a mild heat treatment (5 min 46-49 “C), the chlorophyll fluorescence yield, measured with a pulse amplitudě modulation (PAM) fluorometer and excited with short pulses of actínic radiation (50-500 ms), increased during excitation up to the maximum F,„. The fluorescence yield then decreased to the minimum (where Fn^„ may be lower than the initial fluorescence yield Fq) and rose again to the second maximum F„^, which occurred some seconds (2-20) after that pulse of the actínic radiation.
Most of the work on exact models representing gravitational waves in general relativity has been concerned with spacetimes which are,at least in some directions, asymptotically flat. Here we treat gravitational waves in one of the simplest but everywhere curved cosmological model. We give the physical interpretation of exact "type - N solutions" of Einstein´s equations with cosmological constant presented recently by García Días and Plebanski. We show that some of these solutions may be interpreted as gravitational waves propagating in the de Sitter universe. (According to present consensus, this unvierse represents the inflationary stage of our Universe.) The analysis also reveals that tha solutions can serve as the illustration of the cosmological no-hair theorem: they approach the de Sitter space-time asymptotically in time.