446 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



papers served, however, to kindle an interest in the subject in the 

 United States. Also noteworthy in the history of aerobiology was 

 a thesis published in 1935 by K. M. Stepanov of Leningrad [9]. From 

 research based on the work of these and others during three genera- 

 tions it is possible — though our information is still meager — to picture 

 the circulation of plant spores and other microbes in the atmosphere, 

 and to assess its bearing on medicine, agriculture, and the biological 

 sciences [4] . 



TECHNIQUES OF AEROBIOLOGY 



Much has been learned about the microbial flora of the atmosphere 

 (here termed the "air-spora" and taken to include the pollen of flower- 

 ing plants) by examining deposits on sticky-surface traps exposed 

 to the wind. But results obtained by this method are difficult to inter- 

 pret quantitatively, because the catches depend on factors that vaiy 

 greatly. For quantitative information about the air-spora it is neces- 

 sary to use apparatus that removes spores efficiently from a measured 

 volume of air. Such apparatus requires a means of drawing a meas- 

 ured volume of air through a filter, or of accelerating the air so that 

 particles carried in it adhere to a sticky surface or are trapped in 

 liquid. 



Suction to draw a measured volume of air through the filter medium 

 is required by sampling devices such as Pasteur's aspirated plug filter 

 and the newer membrane filters. Another series of devices act by 

 forcing the air through a narrow jet and directing it toward a sticky 

 surface. The General Electric electrostatic air sampler applies the 

 dust-collection principle worked out by Oliver Lodge. Each of these 

 sampling devices has its virtues and lunitations, but can give quanti- 

 tative data if properly used. In outdoor work, high accuracy is not 

 usually required at present, as results already obtained show that the 

 spore content of the air differs enormously with place and time. 



The results of sampling by different methods are difficult to com- 

 pare. Some samplers deposit particles directly onto a microscope 

 slide, where totals of the larger spores and pollen grains can be counted 

 visually and classified. Others allow bacterial and yeast colonies, 

 f migus mycelia, or whole moss plants to develop in culture, and identi- 

 fication of the cultivable fraction of the air-spora can then be more 

 precise. This gain m precision of identification over the visual method 

 is, however, balanced by loss of information about the total number 

 of organisms, some of which may not be viable. A few workers have 

 used both kinds of sampler simultaneously. 



THE AIR-SPORA NEAR GROUND LEVEL 



Most abundant in numbers near ground level are bacteria and 

 fungus spores. When some abmidant species of plant is in flower, 



