THE UPPER-AIR SPORA 



depends mainly on the history of the air mass. This was clearly shown in a 

 series often flights over northern Canada (Kelly & Pady, 1953; Pady & 

 Kapica, 1953) between September 1948 and August 1949. The history 

 of air masses encountered during sampling and the positions of fronts 

 were correlated with the results of sampling. The electrostatic sampler, 

 loaded alternately with Petri dishes and siliconed slides, gave the most 

 consistent results. (However, as sampling was non-isokinetic, pollen and 

 large spores may have been underestimated.) 



On these flights bacteria varied in concentration less than fungi. At 

 the end of December many samples were blank and the air was almost 

 sterile. Fungi were much more plentiful in June, July, and August, than 

 in the rest of the year, but bacteria were most numerous in spring and 

 autumn. Kelly and Pady suggest, reasonably enough, that the bacteria come 

 mainly from soil which is exposed and cultivated in spring and autumn, 

 giving the opportunity for wind erosion; but their suggestion that the 

 fungi also come from soil seems to be contradicted by the predominance 

 of fungi in summer, and all evidence points to the fungi coming mainly 

 from vegetation and debris above ground-level. All the bacteria isolated 

 and examined in detail were regarded as typical soil forms; they were 

 classified as : aerobic spore-formers (37-9 per cent of the total recorded 

 catch), gram-positive pleomorphic rods (23-8 per cent). Micrococcus 

 (i8-8 per cent), gram-negative rods {Flavobacterium^ Achromobacter^ or 

 Pseudomonas, 4-8 per cent), and Sarcina (4-6 per cent). 



Fungi obtained in culture were : Cladosporium (73 per cent), Alternaria 

 (7 per cent), PenicilUum (2-9 per cent), Streptomyces (2-9 per cent), 

 Stemphylium (1-5 per cent), Aspergillus (0-7 per cent), yeasts (0-7 per cent), 

 and other fungi (11 per cent). Many more fungi could be counted visually 

 on silicone-coated slides than could be grown in culture — an effect 

 which was exaggerated by the numerous smut spores that were obtained 

 in one flight over the prairies in October 1948, which yielded: smut 

 spores 52-4 per cent, Cladosporium 32-4 per cent, Alternaria 3-3 per cent, 

 Helminthosporium 0-3 per cent, and rusts o-i per cent. 



AIR MASSES OVER MONTREAL 



In a further survey of microbes associated with different air masses, the 

 McGill workers used the electrostatic sampler and the slit sampler between 

 10-00 and 13-00 hours on 1 13 days between September 1950 and December 

 1951, at the top of a building high in Montreal (Kelly & Pady, 1954; 

 Pady & Kapica, 1956). Ten t^'pes of air mass were recognized, classified 

 on the basis of exposure to agricultural land ; but as most examples of any 

 one type occurred at one time of the year, the effects of differences in 

 origin of the air mass may be confounded with seasonal effects. Further, 

 at the altitude of 130 metres, sampling at midday is likely to be done in the 

 frictional turbulence layer and to be dominated by local ground sources 



143 



