NATURAL DEPOSITION 



some 1 50 X 50 /x, to a record horizontal distance of 40 mm. Extrapolation 

 from wind-tunnel data suggests that, once launched into a wind of only 

 2 metres per sec, these giant spores (nearly the largest in the fungi) 

 would impact on a tree-trunk of 8 inches diameter with about 50 per cent 

 efficiency. 



Although a high impaction efficiency may be necessary for fungi 

 which attack leaves and stems, it may be positively disadvantageous for 

 spores produced among dense vegetation. Johnstone et al. (1949) point 

 out that the ability of an airborne particle to penetrate close vegetation is 

 the inverse of its impaction efficiency. In close vegetation a high impaction 

 efficiency would reduce the chances of a spore getting very far from its 

 point of liberation. It is possible that the 10 /x-diameter spore represents a 

 working compromise between the conflicting requirements of dispersal 

 and deposition, evolved under the normal range of winds encountered 

 among vegetation. The large-spored leaf- and stem-pathogens appear to 

 be specialized impactors^ while the minute-spored puffballs and moulds 

 appear to be specialized penetrators — perhaps normally deposited by 

 processes other than Impaction. 



TURBULENT DEPOSITION 



Turbulent deposition has been observed in the wind-tunnel and on 

 artificial spore-traps in the open air. Spore-laden air flowing over hori- 

 zontal surfaces will deposit spores at rates much greater than those 

 calculated for sedimentation under the influence of gravity. In the wind- 

 tunnel turbulent deposition increases with increasing wind-speed and, at 

 5 to 10 metres per sec, deposition may be as great on the underside of a 

 horizontal surface as it is on the upper-side — an effect which clearly 

 cannot be caused either by impaction or by sedimentation under gravity. 

 The effect has been noted on pollen traps by Durham (1944, p. 233), who 

 found the catch on the lower surface of a horizontal slide to be as high as 

 50 per cent of the upper surface. Rishbeth (1959) found that spores of 

 Fomes annosiis and Peniophora gigantea can be deposited on the under- 

 surface of pine stem sections exposed a few^ metres above ground-level. 



Turbulent deposition in the open air is also illustrated by experiments 

 with Lycopodiwn spores liberated just above short grass at Rothamsted 

 Experimental Station (Table XV). Deposition was recorded on the upper 

 and lower surfaces of horizontal traps held clear of supports on long pins 

 projecting from a wire frame. Catches shown in Table XV are for traps 

 at the same level as the spores were liberated (h + 0), and at 25 cm. above 

 this level (h + 25), in a downwind direction. The dimensions of the traps 

 were 18 X 18 X 4 mm. in the x, y, and z directions, respectively. 



The greater deposit on the undersurfaces of traps at 25 cm. above the 

 level of emission, together with the unexpectedly large deposition on 

 ground near the source as shown in Table XII, is evidence of factors near 

 the source of a diffusing cloud which still remain to be elucidated. 



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