448 ANNUAL REPORT SIVnTHSONIAN INSTITUTION, 1961 



mirror yeasts (sporobolomycetes) that flourish on the surfaces of 

 living and aging leaves, mushrooms, and toadstools, averaging 4,400 

 per cubic meter and predominating at night. Recognition of ballisto- 

 spores as numerically important components of the air-spora was long 

 delayed by two causes. First, these very small spores were inefficiently 

 collected by the sticky-surface traps used in much early aerobiological 

 work, and, second, most microbiologists were not familiar with the 

 spores of the higher fungi. Spores of various plant-pathogenic fungi 

 such as the rusts, smuts, and mildews are often present in the air in 

 large numbers, but their occurrence, like that of the pollen of flower- 

 ing plants, is highly seasonal. 



The figures given above are for average frequencies over a period of 

 many weeks of continuous recording. Hourly means are often much 

 higher or lower; for example, Cladosporium may reach 100,000 and 

 Sporoholomyces about 1 million per cubic meter. There is evidence 

 that shorter-term fluctuation may be still greater: ragweed pollen in 

 spot tests lasting a couple of minutes has given concentrations of over 

 10 million per cubic meter [2] . 



Protozoan "eggs" in the air were estimated by Miquel at 0.1 per 

 cubic meter, but later work by Puschkarew, based on fewer tests, 

 suggests 10 times that figure. Blue and blue-green algae may average 

 1 to 10 per cubic meter, but spores of myxomycetes are probably less 

 abundant. Spores of ferns and mosses are sometimes plentiful for 

 short periods. 



Concentrations of the few organisms that have been studied in 

 detail fluctuate with a characteristic diurnal periodicity, as also doas 

 grass pollen. Miquel found two maxima and two minima in the daily 

 cycle of bacterial numbers when sampling hourly at Montsouris for 

 over a year. Nothing similar has been attempted with bacteria since 

 1884, however, and the work needs extending and repeating. 



Spores of fungi show various diurnal periodicities, but normally 

 any one type has only a single daily maximum and minimum. For 

 example, in England spores of Phytophthora infesfans, the fungus 

 causing potato blight, are most abundant shortly before noon, whereas 

 the numbers of spores of Cladosporium and of some rust fungi reach a 

 maximum in the afternoon. Spores of Sporoholomyces, and basidio- 

 spores of mushrooms, toadstools, and bracket fungi are all most 

 abundant during the night. Little is yet loiown about difl'erences in 

 these cycles in various parts of the world. These diurnal cycles are 

 clearly determined largely by the effect of meteorological factors on 

 spore liberation and dispersal in ways understood for only a few 

 species of fmigi. Some, such as two important crop pathogens, 

 Ophioholus graminis and Venturia inaequaJis (causing take-all of 

 wheat and apple scab respectively), depend for spore liberation on 



