THE MICROBIOLOGY OF THE ATMOSPHERE 



colonize apparently favourable environments. Not all of these necessarily 

 represent the failure of transport : genetic differences in host populations, 

 or the pre-establishment of competitors, may explain many puzzling 

 failures. 



Diverse views have also been held about Bryophytes and Pterido- 

 phytes; however, most authors accept the possibility of their dispersal 

 over great distances, although the minimizing view has been held by some 

 {see Pettersson, 1940, p. 22). 



Probably these differences in viewpoint will be resolved by quanti- 

 tative studies. The maximizing view, if held too strongly, may lead to 

 fatalism and to the neglect of local hygiene and of reasonable precautions 

 when transporting plants from place to place. However, evidence presented 

 in earlier chapters stresses the overriding importance of local sources. 

 The minimizing view, if applied to certain diseases, may lead to over- 

 reliance on local hygiene and neglect of protection by chemical and genetic 

 measures. It may also increase the danger of introduction of a vigorous 

 organism by neglect of quarantine precautions. 



Long-distance dispersal will be discussed under two headings: (i) 

 diffusion theories extrapolating from short-range experiments; and (2) 

 observations on distant dispersal of inorganic and radioactive particles, of 

 rust fungi, and of other micro-organisms. The problem is complicated by 

 the curvature of air-mass trajectories, by the possibility that spores are 

 re-concentrated within cumulus and other clouds, by unpredictable 

 removal in precipitation, and by loss of viability. 



Theoretical Discussion 



Presumably the tropopause limits the vertical expansion of the spore- 

 cloud, and an extensive temperature inversion at a lower level may have 

 the same effect. After the spore-cloud has travelled perhaps 20 km. or 

 less, diffusion will become two-dimensional, and concentration may be 

 expected to decrease more slowly with increasing distance than it would 

 when the spore-cloud was nearer the source, where its diffusion was three- 

 dimensional. In the limiting case of a land-mass acting as a long strip or 

 area source, the decrease of concentration at 20 km. or more out to sea 

 can be expected to depend only on depletion of Q^^ (the fraction of the 

 cloud remaining in suspension). 



Sutton's theory appears satisfactorily to describe dispersal of microbes 

 in air up to the limit of distance studied experimentally, but there is 

 some doubt about its use for distances greater than i km. or heights 

 above 30 metres. This is emphasized by Pasquill (1956), who dispersed 

 fluorescent dusts and sampled, both on the ground and by aeroplanes, 

 at distances of up to 64 km. and at heights of up to 1220 metres. In these 

 tests the height of the cloud was much less than the width : the width 

 did not increase uniformly with distance and the angle subtended at the 



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