NATURAL DEPOSITION 



at a height of i-o to i-6 metres above ground-level, from 93 to 99 per cent 

 of the spores carried a positive charge. He concluded that this interesting 

 phenomenon might aid adhesion of spores to healthy pine-needles on the 

 tree. 



In the normal, fine-weather electrical field of i volt per cm. over a 

 flat surface with the Earth negatively charged, a Ganoderma spore would 

 gain a negligible addition of 0-05 per cent to its terminal velocity. Ranz 

 & Johnstone (1952) have shown that aerosol particles 0-5 /x in diameter 

 could easily carry charges by which their deposition or suspension would 

 be controlled by the Earth's field rather than by gravity. Whether such 

 comparatively large particles as fungus spores or pollen grains are repelled 

 from the ground or, alternatively, attracted to projecting surfaces such as 

 leaves and tree tops, is as little known as are the causes of changes in the 

 Earth's field itself. Clearly this phenomenon is worth further investigation. 



MINOR DEPOSITION MECHANISMS 



It is also known that small particles tend to move down a temperature 

 gradient (Cawood, 1936; Watson, 1936), and that forces exist which cause 

 particles, at least up to 2 ju. in diameter, to be repelled by a hot surface 

 and attracted by a cold one. Smoke particles and Lycopodium spores are 

 also known to move aw^ay from light in the phenomenon of 'photophor- 

 esis' (Whytlaw-Gray & Patterson, 1932). 



RAIN-WASHING ('Scrubbing', 'rain-out', 'wash-out') 



Natural raindrops vary in size up to a maximum diameter of about 

 5 mm., above which they become unstable and break up during fall into 

 two smaller drops. They have terminal velocities of fall varying from 2 

 to 9 metres per sec. (Best, 1950; see also Gunn & Kinzer, 1949). 



The pick-up of small spheres in the path of falling raindrops was 

 studied theoretically by Langmuir (1948) in connection with artificial 

 rain-making in Hawaii. To judge from Langmuir's figures, the minute 

 spores of Lycoperdon and of the soil-inhabiting Penicillia w ould fail to be 

 collected at all by drops much below i mm. in diameter, and efficiency 

 of collection would rise to a maximum of about 15 per cent with droplets 

 about 2 mm. in diameter, decreasing again with still larger drops. 

 Basidiospores of Agaricus {PsaUiota) campestris should begin to be col- 

 lected by raindrops over 0-2 mm. in diameter, reach a maximum of 30 

 per cent efficiency with raindrops of 2-0 mm. in diameter, and decrease 

 slightly with drops of larger diameters. Spores of TiUetia caries^ Pucchiia 

 uredospores, and conidia of Erysiphe graminis, could be collected by any 

 possible raindrop, and collection would reach a maximum of about 80 

 per cent efficiency with drops 2-8 mm. in diameter. 



The optimum size of spore for deposition in rain varies with the size 

 of the prevalent raindrops. As we have seen, spores o( Lycoperdon perlatimi 



85 



