THE UPPER-AIR SPORA 



samples were rare. Concentrations were not uniform but increased strik- 

 ingly at the level of the cloud base. Species identified included : Alicro- 

 coccus radicatus, M. albus, M. Jiubilus, M. aerogenes, Bacillus ubiquitus, 

 B. aurescens, B. aureo-Jlavus, B. terrestris, B. aerophilus, B. submesenter- 

 oides, B. mycoides, and PeniciUium crustaceum — all spore-formers. Above 

 2,500 altitude he found Bacillus terrestris, B. aerophilus, and Sarcina lutea, 

 and, at 4,000 metres. Micrococcus citreus, M. luteus, and PeniciUium 

 crustaceum. Flemming commented on the frequency of pigment-formers 

 among the bacteria and yeasts of the upper air. 



From catches made during balloon flights over southern Germany, 

 Hahn (1909) concluded that, on the average, bacterial and dust counts run 

 parallel with each other and decrease with height because of sedimentation. 

 He claimed that the air above a certain height was germ-free, and that this 

 zone was lower in winter than in summer. 



THE STRATOSPHERE 



We would expect the stratosphere to be almost devoid of organic 

 particles, because of the apparent inadequacy of mechanisms able to carry 

 them above the top of the troposphere. At great heights the intensity of 

 radiation would be unfavourable to survival. However, well-documented 

 evidence is worth more than theory and, for the present, we must admit 

 that we know nothing of the possibilities of life in the stratosphere. 



The only attempt to sample the stratosphere known to me was made 

 with a balloon. Rogers & Meier (1936, 1936^) devised a sampler to be 

 opened and closed by an aneroid between 21,000 and 11,000 metres 

 during the descent of the Balloon 'Explorer H'. They obtained five 

 bacterial cultures, all of which were different species of Bacillus., and five 

 fungi {Rhizopus sp., Aspergillus fiiger, A.fumigatus, PeniciUium cychpium, 

 and Macrosporiutn tenuis) ; in all, these were equivalent to approximately 

 0-14 viable organisms per cubic metre. 



SAMPLING FROM AEROPLANES 



Exploration for microbes in the upper air from aeroplanes was started 

 in 1921 when Stakman et al. (igi^) exposed Vaseline-coated slides over the 

 Mississippi Valley to trap cereal rust spores. Flights from Texas as far 

 north as Minnesota and at altitudes up to 3,300 metres yielded numerous 

 pollen grains and fungus spores, among which Alternaria (often in chains) 

 were most numerous, followed by Puccinia, Helminthosporium, Clado- 

 sporium^ Cephalothecium., Ustilago^ Tilletia^ and Scolecotrichum. Among 

 rust spores the uredo forms predominated, but some teleutospores and 

 aecidiospores were also caught. Spores became relatively scarce at alti- 

 tudes above 3,000 metres. At 5,400 metres (the highest altitude tested), 

 two uredospores of Puccinia triticina were caught. Alternaria from alti- 

 tudes of 1,000 to 3,000 metres germinated readily, as also did uredospores 

 from 2,300 metres. 



135 



