THE MICROBIOLOGY OF THE ATMOSPHERE 



about 5 minutes at the right akitude (Chatterjee, 193 1) ; but this method was 

 discontinued in favour of sticky sHdes attached to kites which could be 

 kept aloft in the wind for 2 or 3 hours. It is now obvious that when carried 

 by a free balloon (contrary to a kite or captive balloon), even if the slide 

 faced the wind, its trapping efficiency would be low as there would be 

 little or no movement of the slide relative to the air. 



Sampling from aeroplanes in the early days was by sticky microscope 

 slide or Petri dish, exposed at right angles to the line of flight. The wooden 

 paddle and bottle devised by Stakman et al. (1923) was also used by 

 Mehta (1952) and others, and, at aircraft speeds, should be a reasonably 

 efficient collector except for the smallest spores. Newman (1948) claimed 

 an increase in efficiency by placing a leading wire in front of the slide, 

 to break the stagnation zone when sampling from aircraft. 



(ii) Vertical cylinder. A removable sticky coating applied to a vertical 

 cylinder was apparently first used by Rempe (1937) for studying airborne 

 pollen at Gottingen. To measure pollen 'drift', cellulose film coated with 

 petroleum jelly was wrapped around the surface of small brass tubes, 

 14 mm. wide by 45 mm. long. These tubes were hung vertically from 

 trees or on stakes at the required height. Rempe compared pollen 'drift' 

 with pollen 'deposition' (which he measured on horizontal slides i cm. 

 above the ground), and found that drift was usually greater than deposition. 

 At 100 metres horizontal distance from a Cory Ins bush, and at a height 

 of 3 metres, the drift was 25 times as great as the ground deposition. Day 

 and night values for a whole month were obtained on the level roof of the 

 Gottingen Botanical Institute. The vertical sticky cylinder (0-5 cm. in 

 diameter) has also been used by Turner (1956) for field studies on cereal 

 powdery mildew {Erysiphe graminis). 



The lower limit of cylinder diameter, and at the same time the maxi- 

 mum efficiency, is probably reached in the glass fibres of Kordyum & 

 Bobchenko (1959), and in the 'flag sampler' of Harrington et al. (1959). 

 This latter consists of a miniature wind-vane of cellulose tape folded over 

 a pin which rotates in a low-friction bearing. 



Sticky-surface traps have usually been scanned visually but they can 

 be used to provide cultures. Thus Martin (1943) showed that spores can 

 be picked off an exposed slide by Hanna's (1928) wet-needle method. 



Sticky gravity slides, vertical slides, and cylinders, are all convenient 

 and cheap. Their defects are: (i) theoretically a zero catch of large particles 

 in still air and of small fungus spores even at ordinary wind-speeds ; and 

 (2) very great changes of efficiency with wind-speed {see Fig. 10). 



Landahl & Herrmann (1949) claimed that the amount of aerosol 

 'deposited on a vertical slide does not change much if it is oriented at 

 various angles within 30° of the wind'. Our results indicate that with 

 Lycopodium spores this is true only at w'ind-speeds of about 5 metres per 

 sec. Rotating the slide from 90° to 60° increases the efficiency by 10 



94 



