DEPOSITION IN RAIN, SNOW, AND HAIL 



not the main source of lichens in the traps, for the hchcns mostly originated 

 as thallus fragments and were evidently of fairly local origin. 



The 2,000 moss plants cultured from the spores caught in Pettersson's 

 traps included specimens of: Aloina brevirostris, A. rigida^ Amblystegium 

 serpens^ Brachythecium velutiniwi, Bryum spp., B, argentemn, B. pa liens, 

 Ceratodon piirpureus, Funaria hygrometrica, Leptobryum pyriforme, Mnio- 

 bryum carnemn, Pohlia cnida, P. nutans, and Pylaisia polyantha. 



Observations on micro-organisms in rain were made at Rothamsted 

 Experimental Station in 195 1 by Gregory, Hirst, and Last {see Hirst, 1959) 

 while they were comparing various spore-trapping techniques. Two 

 conical glass funnels 20 cm. in diameter were exposed on a wooden 

 structure at a height of 2 metres above ground-level. One funnel was open 

 to rain (rain-trap), while the other (dry-trap) was protected by a flat 

 asbestos-cement disk held 25 cm. above the mouth of the funnel — to keep 

 off rain but still allow^ dry deposition. Washings from both funnels were 

 collected daily and the fungus spores separated by sedimentation onto a 

 glass cover-slip. On dry days the fully exposed rain-trap consistently 

 caught fewer microbes than the dry-trap ; but, as might be expected, this 

 was reversed during rain — especially in the first rain after dry weather 

 (Table XXIV). 



TABLE XXIV 



GEOMETRIC MEANS OF RATIOS OF CATCHES BY R.AIN-TRAP TO DRY-TRAP, 2 

 METRES ABOVE GROUND, ROTHAMSTED, JUNT-SEPTEMBER 1951 (HirSt, 1959). 



Rain falling during one thunderstorm was studied in detail (Gregory, 

 1952; Hirst, 1959), and a detailed account of changes in the air-spora 

 during this period, observed with the aid of the Hirst automatic volumetric 

 spore-trap, has already been published (Hirst, 1953, pp. 382-5). A y-day 

 spell of warm, dry weather ended in a thunderstorm at 13-25 hours 

 on 22 July 1 95 1. The rain-trap was cleaned immediately before the rain 

 started, and the first i mm. of rain which fell in the first half-hour of the 

 storm was collected separately from the succeeding 375 mm., which 

 contained manv fewer spores (Table XXV). 



As Hirst (1959) remarks in discussing this series of observations: 

 'Spores released during rain are presumably removed from the air as 



151 



