THE MICROBIOLOGY OF THE ATMOSPHERE 



readily as spores already there when rain starts to fall, so that concen- 

 trations of airborne spores measured during rain represent, not the total 

 released, but the excess of those released over those removed. Rain- 

 scrubbing seems an ideal method of deposition for air-dispersed soil 

 fungi. For foliage pathogens its biological significance is far from clear. 

 Many spores may be lost in "run-off" unless they can attach themselves 

 to the leaf surface or penetrate into crevices they would be unlikely to 

 reach when deposited from dry air.' 



TABLE XXV 



SPORES BROUGHT DOWN BY THUNDER RAIN TERMINATING 7-DAY DRY SPELL, 



ROTHAMSTED, 22 JULY 1 95 1 (Gregory, Hirst & Last, unpublished). 



Number of spores per ml. of rain 

 in I St 0-95 mm. in succeeding 375 mm. 



In contrast Asai (i960), who introduced the useful method of filtering 

 rain through membrane filters under reduced pressure, failed to obtain 

 uredospores of Puccmia grammis, although they were known to be in 

 suspension in the air at the time the rain samples were collected. 



Spores in raindrops appear to play a part in some processes of plant 

 infection. Dry wind-blown spores of barley loose-smut {Ustilago nuda) 

 rarely infect the ears of susceptible barley varieties but, when drops 

 containing spores in suspension fall on flowers, the spores are brought 

 into direct contact with the ovary, and infection follows (Malik & Batts, 

 i960). 



Snow 



Janowsky (1888), Pettersson (1940), and others have found a few 

 organisms in falling snow. Only Gazert (191 2) gave a negative report from 

 the Antarctic on the microbial content of fresh-fallen snow in Kaiser 

 Wilhelm II Land. A. L. McLean (191 8), on the other hand, reported 

 numerous organisms in snow and ice in Adelie Land ; but it is not certain 

 whether they were brought down with the snow or deposited dry in 

 fine weather from the atmospheric dust which settles over the Antarctic. 

 However, on three occasions McLean caught falling snow in a sterile 



152 



