THE MICROBIOLOGY OF THE ATMOSPHERE 



75 to I, and although counts in London were less than half those at 

 Rothamsted, they still averaged 6,500 per cubic metre during the 1954 

 season. With some species, diurnal changes in concentration tended to be 

 less pronounced in London than in the country (Fig 21); this may per- 

 haps have indicated that spores of the species trapped in London came 

 mostly from distant sources. 



Results of extensive surveys on a roof at Manhattan, Kansas, are based 

 on daily sampling at approximately 09-00 local time. A Pady-Rittis slit 

 sampler was used for visual identification and the G. E. Electrostatic 

 sampler for making cultures on Rose-Bengal agar. The maximum numbers 

 of fungus spores recorded at one time was 100,000 per cubic metre in 

 visual traps and 20,000 per cubic metre in culture (Pady, 1957; Rogerson, 

 1958; Kramer et a I. 1959). The main constituents were identified as 

 follows : 



EFFECT OF WEATHER 



Atmospheric spore concentration fluctuates according to meteorological 

 conditions. It also fluctuates for biological reasons such as growth and 

 differentiation of the spore-producing organisms. Studies by Hirst (1953) 

 show that the pollens, and spores of Cladosporium, Erysiphe, Alternaria, 

 smuts, and rusts (which together form the main components of the day- 

 time 'dry-air' spora), are mostly removed by prolonged rain which, 

 however, soon puts into the air a characteristic damp-air spora. 



Fluctuation is a property of the fine-weather air-spora, but some types 

 depend on rain to get into the air, and occur in high concentration only 

 after measurable rainfall. Keitt & Jones (1926) showed that liberation of 

 ascospores of the apple scab fungus {Venturia inaeqiialis) is correlated 

 with rain. Hirst et al. (1955) trapped no ascospores of this fungus during 

 dry weather in orchards, and during the first hour after the onset of rain 

 they found only a few ascospores ; yet high concentrations occurred in the 

 second and third hours. Rain at night led to lower concentrations than 

 an equal amount of rain falling by day. In general, perithecia of most 

 ascomycetes possessing them must be wetted before ascospores are 



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