THE MICROBIOLOGY OF THE ATMOSPHERE 



successively in winds of increasing speeds. Tests were made at angles of 

 0°, 45°, and 90°. The percentage of the original deposit that was retained 

 after i minute at each wind-speed was estimated by counting. At the 

 highest wind-speed, 9-5 metres per sec, slight traces of grease on the 

 slide greatly increased retention when the slide was horizontal, and, 

 unless the surface was carefully cleaned before use, erratic results were 

 obtained under these conditions. 



With the slide vertical (90°), blow-off was nearly linear with wind- 

 speed, 98 per cent being retained at i-i metres per sec, and 60 per cent 

 at 9-5 metres per sec. Blow-off was least at 45°, and at this angle retention 

 was 100 per cent at wind-speeds up to 5-5 metres per sec, and 95 per cent 

 was retained even at 9-5 metres per sec. By contrast, blow-off was greatest 

 with the surface horizontal (0°), when 77 per cent was retained at 1-7 

 metres per sec, and only 26 per cent at 9-5 metres per sec. These results 

 illustrate the way the laminar boundary layer acts as a dust trap (p. 25), 

 but the actual values probably have little application to plant surfaces. 



Deposition and Retention on Potato and Bean Leaves 



The tests described in the preceding sections were all on artificial 

 surfaces, approaching ideal conditions, and gave information on principles 

 of particle deposition from air. This is useful in devising apparatus for 

 sampling airborne particles. We now need to ask how relevant this work 

 is to spore deposition on plant surfaces, especially on leaves which, though 

 rough, are not particularly sticky, and which flap in the wind. 



To imitate natural conditions, shoots of potato {Solanimi tuberosum) 

 with rough leaves, and broad bean {Vicia faha) with smooth leaves, were 

 placed in the turbulent wind-tunnel and exposed to clouds of Lycopodium 

 spores in the usual manner. The petiole of the leaf was clamped and the 

 leaflets allowed to flap freely, trailing the leaf-tip downwind. After ex- 

 posure, the leaf surfaces were examined under the microscope and the 

 deposit w^as counted on zones across the tip, middle, and base of the 

 lamina (upper and lower surfaces), the deposition efficiency being calcu- 

 lated (Table X). 



Considerable differences from deposition on a rigid, sticky horizontal 

 slide are apparent. Turbulent deposition either fliiled to develop at the 

 higher wind-speeds, or the spores were shaken off again in the wind. 

 Deposit on the undersides of the leaves was small at all wind-speeds, but 

 on the upper side it was up to 5 per cent of area dose at winds of 0-5 

 to i-o metres per sec — both on potato leaves and on the still smoother 

 broad-bean leaves. At low wind-speeds the deposit w^as similar to that 

 expected from sedimentation under the influence of gravity; but potato 

 leaves had more spores at the base, and broad-bean leaflets had more at 

 the tip. 



74 



