THE MICROBIOLOGY OF THE ATMOSPHERE 



giving a highly distorted picture of the air-spora because it preferentially 

 selects the larger particles. Terminal velocity increases as the square of the 

 spore's radius (Chapter II), and terminal velocities are such that under 

 ideal conditions, while for example a slide is receiving the puffball spores 

 in a layer of air 0-5 mm. thick, it also receives rust uredospores from a 

 column 10 mm. thick, and grass pollen from a layer 50 mm. thick. A 

 volume of air containing puffball spores and grass pollen in equal con- 

 centration would be recorded by the gravity slide as having 100 times 

 as many grass pollen grains as puffball spores. To correct this distortion 

 Scheppegrell (1922) tried to calculate the volumetric concentration from 

 gravity-slide deposits using a formula based on particle diameter^ which 

 was later corrected by Cocke (1937) to particle radius. 



Because of edge effects and turbulent deposition at high wind-speeds 

 (Chapter VI), the gravity-slide catch is very difficult to interpret; but in 

 spite of these defects it has been widely used and has contributed much 

 of our still meagre knowledge of the air-spora. 



(ii) The gravity Petri dish. A few workers have exposed Petri dishes 

 horizontally in outside air, to provide pollen deposits for visual examination 

 (Hesselman, 191 9; Ludi & Vareschi, 1936). 



More commonly, Petri dishes of sterile media are exposed in the open 

 air for periods of from i to 10 minutes to investigate the cultivable bac- 

 terial or mould flora of the atmosphere. (Some workers expose empty 

 sterile dishes and pour the medium on returning to the laboratory.) 

 Indoors, the method is subject only to distortion owing to the sedimenta- 

 tion rate. Out-of-doors, however, the method is also subject to aerody- 

 namic effects from the edge of the dish — unless the dish is sunk below 

 a plane surface (p. 73). For sampling out-of-doors the method has been 

 used in tests by: P. F. Frankland & Hart (1887), Saito (1904, 1908, 1922), 

 ZoBell & Mathews (1936), Dye & Vernon (1952), Menna (1955), Richards 

 (1954, 1954^, 1954/'), Werff (1958), and many others. 



Apart from convenience and economy, the method is valued for the 

 precision with which organisms can be identified in resulting cultures. 

 Its defects are: sensitivity to particle size, wind-speed, and aerodynamic 

 effects (p. 73); also the small volume of air sampled intermittently. 

 Continuous sampling is impracticable and diurnal changes of the air- 

 spora are not revealed. Its restriction to viable and cultivable particles 

 may be developed with advantage to a high degree of selectivity when 

 sampling is aimed at a limited group of organisms. With pleomorphic 

 fungi, positive growth in culture may still leave the origin in some doubt; 

 a Phojna or a Fusarium culture might have arisen either from a 

 conidium or an ascospore — a detail which it is sometimes important to 

 resolve. 



(iii) Conical funnels have been exposed for trapping conidia of downy 

 mildew of the vine {Plasmopara viticola) by Savulescu (1941), for fore- 

 casting mildew outbreaks in Rumania. 



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