H 



^OODS 

 HOLE, 

 MASS.^ 



VII 



NATURAL DEPOSITION 



Having become airborne and been transported by the wind, the spore 

 must quit the turbulent layers of the atmosphere and re-cross the boundary 

 layer before coming to rest in the still layer of air on a solid or liquid 

 surface, which may or may not prove favourable to growth. Some char- 

 acters of spores seem to have been evolved in response to problems of 

 take-off, while others may well have been evolved as adaptations for 

 deposition. 



Little was known about deposition processes until the development of 

 wind-tunnel techniques — originally for research in aerodynamics — 

 made it possible to experiment on the behaviour of spores in controlled 

 winds in the laboratory. The principal methods of spore deposition 

 in nature can now be tentatively suggested: impaction, sedimentation, 

 boundary-layer exchange, turbulent deposition, rain-washing, and electro- 

 static deposition. 



Measurement of Deposition 



The relation between x, the cloud concentration, and 'd', the surface 

 deposition, has been little studied, although it is a relation of considerable 

 biological importance — for instance in pollination, and in epidemics of 

 plant diseases and their control by protectant dusts and sprays. With wind 

 flowing across a smooth surface, it would be possible to calculate deposition 

 directly from a knowledge of concentration, wind-speed, and terminal 

 velocity. However, it is not clear what factors dominate the situation under 

 the complex conditions obtaining in nature, and so the problem must be 

 approached experimentally. With some insight, obtained from Chapter 

 VI, into deposition under relatively simple conditions in a wind-tunnel, 

 we can better understand the complex factors of deposition in nature. 

 In wind-tunnel studies, and in calibrating spore-traps, it is convenient to 

 use percentage efficiency of deposition, E = (Trap dose/Area dose) 



X 100. 



For field conditions, two expressions have been used for deposition 

 on the ground. The 'deposition coefficient', p = d/n (where 'd' = num- 

 ber of spores deposited per sq. cm. of surface, and 'n' = number sus- 

 pended per cc. of air), measures the thickness of the slice of cloud cleared 

 while travelling over unit length of ground surface (Gregory, 1945). 



76 



