452 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



nent a northward migration of wheat-rust spores in early summer is 

 followed by a return migration in autumn. Yet the distribution of 

 the species and races of the rust fungi is not worldwide : oceans and 

 large tracts of mountain and desert seem to present almost uncross- 

 able barriers. 



Apart from death by desiccation or irradiation while airborne, the 

 flight of a microbe ends either by dry deposition on the ground or by 

 washing out of the air by rain, snow, or hail. The phenomenon of 

 washout has never been systematically investigated, and sound tech- 

 niques have still to be worked out. Results from examining hail are 

 particularly unambiguous, because the surfaces of hailstones can be 

 sterilized to eliminate possible contamination from the ground. Fall- 

 ing raindrops sweep up a substantial proportion of the suspended 

 microbes in their path, and all precipitated water brings down from 

 the sky a rich flora of bacteria, algae, spores of fungi and mosses, and 

 pollen. Precipitated water is not sterile, whether collected over the 

 land, the ocean, or the polar regions. Although a spore is most likely 

 to be deposited dry by sedimentation to ground or by impact with a 

 surface within a few hundred meters of takeoff, most spores that 

 escape into the free air probably have their flight ended by rain. 



Conditions in outer space beyond our atmosphere, as far as they 

 are known, would appear to offer a highly uncongenial environment to 

 unprotected micro-organisms. If attempts are made to detect viable 

 spores in interplanetary space, special techniques will be required that 

 owe little to the methods of aerobiology. However, experience gained 

 in sampling our own atmosphere can be applied to some of the prob- 

 lems of sampling in the atmospheres of other planets. Conventional 

 methods of sampling aerosols of single bacterial cells indoors are de- 

 fective when applied to taking samples of large spores from moving 

 air, and we need to develop better sampling methods, especially for 

 continuous sampling in culture. 



The glimpses we now have of the circulation of minute organisms 

 in the atmosphere of our planet with all the implications in agriculture, 

 medicine, and theoretical biology tantalize us by their incompleteness. 

 It is unfortunate that exploration of our atmosphere has scarcely be- 

 gun, and that we are not yet adequately equipped with technical meth- 

 ods for the task, at a time when the opportunity of probing the atmos- 

 pheres of other planets is hastening upon us. 



REFERENCES 



1. Bateman, A. J. Heredity, vol. 4, p. 353, 1950. 



2. Durham, O. C. Journ. Allergy, vol. 18, p. 231, 1947. 



3. Erdtmann, G. An introduction of pollen analysis. Chronica Botanica, 



Waltham, Mass., 1943. 



