450 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



atmosphere, and in practice a zone of increased concentration often 

 occurs at a height of perhaps 2,000 or 3,000 meters. This fact has led 

 to speculation about a so-called "biotic zone" in the upper air, but the 

 explanation probably lies partly in the different histories of air masses 

 at different heights, and partly in the washing of the lower layers of 

 air by rain. Microbial concentration is sometimes high in the bases of 

 clouds, and spores may perhaps become concentrated there by being 

 collected in droplets poised on ascending convection currents in cumu- 

 lus clouds. The effect of these processes would be particularly notice- 

 able over the ocean, where the air-spora is not constantly being re- 

 newed from the surface. 



Systematic measurements of spore concentrations at different 

 heights over the oceans have still to be made, but observations made 

 by different methods on ships and from aircraft suggest that the 

 gradient may be the reverse of that over land. Far out to sea, the 

 surface air appears to contain exceptionally few microbes, whereas 

 several thousand meters up, the concentrations of bacteria, fungus 

 spores, and pollens may be considerably greater. Studies by S. M. 

 Pady and coworkers [7, 8], for example, indicate fungus-spore and 

 pollen concentrations of tens to hundreds per cubic meter at 3,000 

 meters above the North Atlantic, whereas G. Erdtmann [3], sampling 

 on board ship, found values only a tenth or a hundredth of these. We 

 thus have a picture of air masses carrying over the ocean the spore 

 load they acquired during passage over land, and of the lower layers 

 of air being gradually cleared in passage over the sea both by deposi- 

 tion and by scrubbing by rain showers. 



It is remarkable that the microbial content of the atmosphere above 

 the troposphere still remains almost uninvestigated. Samples were 

 taken in the stratosphere by the balloon Explorer II in 1935, but there 

 seem to have been no later attempts to sample the stratosphere. 



CHARACTERISTICS OF THE AERIAL DISTRIBUTION PROCESS 



The atmospheric concentrations reported in earlier paragraphs 

 result from many spore sources. We must now turn to consider the 

 problem of spatial distribution of spores liberated into the air from 

 a single source. Common experience leads us to expect a decrease in 

 contamination of air or of the ground as the horizontal distance from 

 the source increases. This expectation is abundantly borne out in 

 practice [10] and is a phenomenon exploited widely in isolating 

 healthy from diseased crops, hay-fever patients from pollen sources, 

 and seed crops from foreign windborne pollen which could cause 

 genetic contamination. Plotted on a linear scale, a graph of the de- 

 crease of contamination downwmd from a point source of spores at 

 ground level typically gives an exponential-type curve. The media- 



