THE MICROBIOLOGY OF THE ATMOSPHERE 



rust was severe near barberry bushes which are now known to be the 

 akernate hosts for the fungus: 'the effects are striking and desolating in 

 the distance of ten to twelve paces, I have also perceived them visibly 

 at 50, 100, 150 paces and a final attack at above 1,000 paces.' Similarly, 

 dispersion of the pollen cloud made it possible for Blackley (1873) to 

 advise his hay-fever patients to keep away from grass fields during the 

 flowering season of the grasses. Attempts to formulate the process of 

 spore dispersion through the atmosphere have been based on geometrical, 

 empirical, or meteorological considerations. 



The geometrical approach is suggested by analogy with the laws of 

 radiation. Niigeli (1877) stated that the amount of dust which comes on an 

 air current from one place falls off with the inverse square of the distance, 

 whereas E. Fischer & Gaumann (1929) stated that, with linear increase 

 of the distance, the chance of infection by rust spores decreases in cubic 

 progression. Kursanov (1933) stated that, in the absence of wind, the 

 number of fungus spores would fall off inversely as the cube of the dis- 

 tance from the source. The ideas that underlie the geometrical approach 

 are simple. Spores travel away from the point of liberation: at greater 

 distances the volume of air which they can occupy increases as the cube 

 of the distance or, alternatively, the surface of the ground on which they 

 could fall increases as the square of the distance. A third possibility would 

 be a simple inverse relationship with distance, as the areas of successive 

 annuli around a point increase in arithmetical progression. 



The geometrical method is unsatisfactory because, although in a 

 general way it illustrates the features of dispersion, it is not clear why 

 spores should travel and spread out in the manner predicted. The particles 

 interesting to us here are passively borne and do not behave like radiations, 

 because the air which carries them is not in process of being continuously 

 generated at some point in the atmosphere; consequently some totally 

 different concept is needed. 



The approach by empirical curve-fitting has been based on field 

 records of dispersal gradients, such as the scatter of seeds or seedlings 

 on the ground, contamination of seed crops by foreign pollen, or the 

 incidence of plant diseases. Using such data, a curve is fitted to the ob- 

 served points, either graphically or by the statistical method of least 

 squares, and an attempt is made to find an empirical formula to fit the 

 curve. These methods will be referred to in detail in Chapter XIII, after 

 the subject of spore deposition has been discussed. In general the empirical 

 method has the advantage that an equation can usually be obtained, 

 containing at most three parameters, which gives a good fit to any one set 

 of field data. On the other hand, it is difficult to compare results obtained 

 by different workers. The parameters are calculated from the data and 

 correspond to no obvious natural phenomena ; consequently it is difficult 

 to use empirical formulae to predict a dispersal pattern under conditions 

 differing from the original one. 



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