OUTDOOR AEROBIOLOGY — GREGORY 449 



the wetting of the substrate by ram or dew ; they occur in the air in 

 large numbers only after rain. 



THE ORIGIN OF THE AIR-SPORA 



Despite claims to the contrary, there is little doubt that most of the 

 air-spora comes from ground sources on the surface, such as plants 

 and vegetable debris, rather than from the soil itself. Only the 

 sources of the protozoa, bacteria, and yeasts (other than the "mirror 

 yeasts") remain in doubt. The air-spora is not rich in typical soil 

 inhabitants but represents mainly organisms growing above the sur- 

 face. Soil and surface dust raised by wind may possibly be the source 

 of most atmospheric bacteria and yeasts, and the seasonal maximum 

 numbers of bacteria in the air of temperate regions seems to be 

 associated with the tilling of bare ground in spring or with strong 

 winds. Splash droplets from marine and fresh water, and from wet 

 soil, evidently help to make surface organisms airborne. 



THE AIR-SPORA OVER THE OCEAN 



Samples taken on ships show that, with an offshore wind, the in- 

 fluence of the land-spora often extends to several hundred miles from 

 shore, but that in midocean the air is nearly free from microbial con- 

 tamination. The proportion of airborne bacteria requiring sodium 

 chloride for growth is stated to increase in proximity to the ocean. 

 Pollen can sometimes be found in quantity for some miles out to sea, 

 but its concentration usually decreases faster as the land recedes than 

 does the concentration of molds or bacteria. However, even in mid- 

 ocean, on the coasts of Greenland, and on remote oceanic islands, tree 

 pollen falls regularly in small but measurable quantities after being 

 transported for hundreds or thousands of miles by the wind. 



THE UPPER TROPOSPHERE 



The presence of pollen and microbes in air layers above ground 

 has been confirmed by catches on kites, balloons, and airplanes. Theo- 

 retical considerations suggest that spore concentration should de- 

 crease logarithmically with height, on the assumption that spores 

 coming into suspension from the ground reach an equilibrium resulting 

 from the rival actions of stirring up by atmospheric turbulence and 

 sedimentation under gravity. In practice, concentration does at first 

 usually decrease with height above ground level. On some occasions, 

 and more often when several occasions are averaged, the decrease fol- 

 lows approximately the logarithmic law up to a height of several 

 thousands of meters. However, a decrease in concentration according 

 to the logaritlmnic law is an ideal condition seldom attained in the 



