Variations in the morphology and physiology were noted when parts of the Peltigera aphthosa Willd. thallus differing in age were examined. The many small cephalodia on the growing apex of the lichen showed a lower heterocyst frequency (14%) than those on the rest of the thallus (21–22%), which was reflected in the nitrogenase activity. In contrast, highest levels of photosynthesis (14CO2 uptake and O2 evolution) were noted at the growing apex. while respiration rates were fairly stable over the thallus. The water-holding capacity was greatest in the midparts.
Cephalodial biomass represented an average of 2.6% of total thallus biomass. while the number and size of these structures varied considerably. A minor part of the total carbon fixation (14CO2) and net oxygen evolution (O2 electrode) was performed by the blue-green phycobiont (Nostoc) in light. A rapid excretion of ammonia from isolated cephalodia was noticed, which together with a comparatively constant C:N ratio throughout the thallus indicated a rapid transport of metabolites facilitated by close physical contact (electron microscopy).