THE MICROBIOLOGY OF THE ATMOSPHERE 



and lower Basidiomycetes are more likely to discharge spores when the 

 substratum is wet. Spore shedding in higher Basidiomycetes is less 

 affected by air humidity and wind speed. Spores of some Fungi Imperfecti 

 may depend on wind for removal, or on changes in humidity for hygro- 

 scopic movements, or on rain for splash dispersal. Soil- and dust-borne 

 bacteria and protozoa are probably borne aloft in high winds from heated 

 or mechanically disturbed ground. The nature of the 'take-off' mech- 

 anism profoundly affects the occurrence of different kinds of spores 

 or pollens in the atmosphere — with consequent significance for hay- 

 fever patients, seed-crops, plant diseases, evolution, and geographical 

 distribution. 



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