THE MICROBIOLOGY OF THE ATMOSPHERE 



Trichia^ expose small, dry spore-masses on stalks at most a few millimetres 

 high. The spores are set free by twisting movements of hygroscopic 

 elaters which take place with changes in air humidity (Ingold, 1939), 

 or, in a few species, spores may be removed by eddies from shallow 

 wind-cups. 



FUNGI 



Adaptations facilitating air dispersal show more diversity in the fungi 

 than in any other group — except, perhaps, adaptations for seed dispersal 

 among the flowering plants. They vary from the passive but quite effective 

 processes in the Fungi Imperfecti, to the spectacular ballistic feats of the 

 ascus gun. The various mechanisms have been summarized by Dobbs 

 (1942, 1942^/) and Ingold (1953, i960), and they formed one of the main 

 topics of the classical work of Buller (1909-50). 



In contrast, spores of many other species of fungi rarely get into the 

 air but are carried by insects, on seeds, or in soil. Mere dispersal by in- 

 sects may be relatively unimportant; but, where the insect actively 

 inoculates the substratum or host, it is a mechanism comparable in 

 efficiency with insect pollination of flowering plants. Passive liberation 

 by the action of external energy depends on 'spore presentation' (Hirst, 



1959)- 



(i) Shedding of spores under gravity. Stepanov (1935) concluded that 



spores of some CiinninghameUa species, and of some Fungi Imperfecti, 



including Botrytis cinerea, Monilia sitophila and Hehninthosporium sativum, 



as well as the macroconidia of Fusarium, could be shed under gravity. 



However, as he also showed that minor air currents released spores of 



some of these fungi, the effect remains uncertain. 



(ii) Shedding in convection currents. Stepanov (1935) placed open 

 Petri-dish cultures at the bottom of glass cylinders 10 to 12 cm. high in 

 which convection currents were induced by differential heating. Sticky 

 slides or a surface of inverted sterile medium at the top of the cylinder 

 trapped spores which might become detached and carried aloft by con- 

 vection. With temperature differences of the order of io°C., conidia of 

 Monilia sitophihi and Botrytis cinerea were freely transferred upwards, but 

 CoUetotrichum Uni was not. Smaller temperature differences, such as 

 resulted from the slight heat produced by a mould culture or an electric 

 lamp shining on the floor, were ineffective. 



(iii) Blowing away ('deflation'). This occurs commonly with dry- 

 spored fungi including moulds, smuts, and rust uredospores. The spores 

 are often 'presented' on an elevated sporophore, any stem or leaf patho- 

 gen usually being adequately raised on its host tissue. Qiiantitative studies 

 so far are insufficient to lead to a theory of 'deflation'. Little is known 

 about the quantitative effect of wind-speed on liberation, but there is 

 good evidence that the higher the wind-speed the more spores are carried 

 away. 



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