THE MICROBIOLOGY OF THE ATMOSPHERE 



metres. This zone may be nearly cleared of pollen by sedimentation, 

 while above it a much larger spore and pollen concentration may be 

 retained in the turbulent layer. 



Sedimentation, we may therefore suppose, is only noticeable in the 

 laminar layer which normally extends only a few millimetres above the 

 surface but, under exceptional conditions at night, may reach several 

 metres. Wind-tunnel experiments confirm that the effect of sedimentation 

 is slight at wind-speeds of 2 m. per sec. and upwards. 



IMPACTION 



When a small surface, such as that of a leaf or twig, projects into the 

 wind, spores may be deposited by impaction on its windward side. Wind- 

 tunnel experiments confirm that impaction is inefficient when small 

 spores approach large obstructions at low wind-speeds. Conversely, 

 impaction is more efficient when large spores are blown towards small 

 objects at high wind-speeds. So it seems that large spores, in addition 

 to carrying a bigger food reserve, have the advantage of a favourable size 

 for impaction on surfaces. Dry-spored, airborne leaf-pathogens usually 

 have comparatively large spores (e.g. uredospores, aecidiospores, Phyto- 

 phthora^ Helmifithosporium^ etc.). 



On the other hand, dry-spored soil inhabitants are characterized by 

 small spores, unsuitable for impaction (e.g. Penicillium and Aspergillus). 

 Among vegetation, where the wind-speed normally reaches an upper 

 limit of about 2 metres per sec, spores of Ly coper don per latum 4-5 /x in 

 diameter would not be impacted at all, even on objects as narrow as 

 I mm. ; and it can be calculated that spores of this size would require a 

 wind of about 25 metres per sec. to be impacted with only 10 per cent 

 efficiency on a blade of grass. Evidently we must look to processes other 

 than impaction to deposit minute spores such as those of puffballs, earth- 

 stars, Ustilago^ and the common moulds. The loose smuts of cereals, 

 Ustilago spp., with spores in the 7-9 n range, would not be impacted 

 efficiently on leaves and stems ; but on narrow surfaces such as glumes and 

 stigmas of grasses, they would be expected to reach an efficiency as high 

 as 50-75 per cent. 



Agaricus {Psalliota) campestris has spores of about 7 X 6 |U, which 

 should be near the low^er limit of size for impaction on grass leaves and 

 stems, at the limiting wind-speed in closed vegetation of about 2 metres 

 per sec. Uredospores o^ Puccinia graminis and conidia oi Erysiphe graminis 

 would impact on a leaf of wheat with efficiences near 40-60 per cent. 

 Botrytis polyblastis^ a leaf pathogen of Narcissus, with spores up to 90 fji 

 diameter, comes into the same group of pathogens that are relatively 

 efficiently impacted on leaves and stems. 



The difficult problem of impaction on objects as wide as the trunk of 

 a tree seems to have been solved by some of the lichens. Pertusaria 

 pertusa, as I have myself observed, can shoot single ascospores, measuring 



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