To study the aerobiological processes directly, it is important to differentiate between fresh and older pollen grains. Different methods were tested for this purpose using birch (Betula) pollen. Simple general stains stained also dead grains. More specific histochemical techniques (nitro blue tetrazolium and isatin) stained a high percentage of the grains even after eight days, although most of the germination ability was lost already in 2#x2013;3 days. Of the methods tried, thein vitrogermination test was thus the most sensitive in differentiating between fresh and older pollen grains. To test the germination ability of airborne pollen it was collected by a suction pump on filters. Sectors of filters were incubated on a thin layer of agar on objective glasses in moist Petri dishes in +30#xB0;C for 24 hours. The germination ability was presumed to vary during the pollen season, but this was not found in this preliminary study. The diurnal variation in germination ability was clear. During or near diurnal peak concentrations the percentage of germination ability significantly higher than during diurnal minima and during intermediate concentrations.
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