SPORE LIBERATION 

 VIRUSES 



The viruses are little adapted to independent air dispersal. Some 

 viruses infecting the animal respiratory tract are forced into the air on 

 droplets during coughing and sneezing; but most bacterial and plant 

 viruses, if they occur in the air at all, only get there on 'rafts' of debris or 

 water droplets. Some of the so-called polyhedral viruses infecting insects 

 are exceptional. Reports of outbreaks among pests of forest trees in 

 eastern Europe speak of copious yellow deposits of the polyhedral bodies, 

 shed by parasitized insects, which coat the surfaces of vehicles travelling 

 through the forests. Study of air dispersal of viruses might explain 

 some of the anomalies in the behaviour of insect viruses. To prevent 

 contamination by an airborne infective particle of the dimensions of a 

 virus may well require quite unusual experimental precautions. 



BACTERIA 



Moving air does not normally detach bacterial cells from the surface 

 of a colony, at least when this is slimy, and in the absence of an active 

 discharge mechanism natural processes capable of producing an aerosol 

 of single bacterial cells are unknown. Mechanical disturbance of dust, 

 clothing, surgical dressings, etc., however, carries into the air contamin- 

 ated particles of substratum acting as 'rafts' and bearing clumps of bac- 

 teria (Bourdillon & Colebrook, 1946). Rafts of soil or dust particles are 

 raised by wind, by 'dust-devils' when the ground is heated by solar 

 radiation, and by animal and human activity such as cultivation of bare 

 ground. Rain splash, breakers, and sea spray continuously throw minute, 

 potentially bacteria-laden, droplets into the atmosphere. Droplets 

 expelled by coughing and sneezing are important indoors {and see p. 158), 

 yet processes which put bacteria into the air are still not satisfactorily 

 known. This is also true of the yeasts whose frequent abundance in the 

 air remains unexplained, except for the Sporobolomycetaceae which show 

 the ballistospore discharge mechanism (pp. 37-38). 



ACTINOMYCETES 



The mycelial organization of this group allows the Streptomycetaceae 

 to develop aerial hyphae bearing &r^\ powder}'' spores — the first example 

 of the sporophore elevation device, common in more elaborate organisms, 

 for raising the spore-producing organ above the substratum and towards 

 the moving layers of the atmosphere. Take-off conditions in the Actino- 

 mycetes seem not to have been investigated. 



MYXOMYCETES (Mycetozoa, Myxogastrales) 



The slime-moulds are a group thoroughly adapted to wind dispersal. 

 Some, such as Reticidaria^ merely expose a dr}'', powder)'' spore-mass on a 

 cushion raised above the substratum. Others, such as Stemonitis and 



c 33 



