NATURAL DEPOSITION 



suggests that, when non-wettable spores reach the ground, they remain 

 in the upper layers of the soil. For the leaf- and stem-parasites, removal 

 from the air by rain is possibly unfavourable, while for small-spored soil 

 inhabitants as well as for smuts and other fungi that infect seedlings, it 

 may be the normal method whereby they come to ground. 



lOO 500 ipOO 



Diameter of drops p 



5000 



Fig. 1 6. — Collection efficiencies of spores of diameters 4 to 40 /x by raindrops 

 of diameters 0-03 to 5-0 mm. (Smooth curves drawn through values computed by 

 Langmuir, 1948.) 



Figs. 15 and i6, compiled from the data of Best (1950) and others, 

 show the proportion of raindrops of various sizes occurring in natural 

 rainfall, and the collection efficiencies of such drops (E per cent) as given 

 by Langmuir (1948). 



Most relevant to our problem is the theoretical study by Chamberlain 

 (1956), who used Best's (1950) results to calculate that, if other causes of 

 diminution of the spore-cloud concentration are neglected, rainfall at the 



87 



