THE MICROBIOLOGY OF THE ATMOSPHERE 



forests across the Gulf of Bothnia. Pollen was trapped in open Petri 

 dishes on two lightships, 'Vastra Banken' and 'Finngrundct', situated 

 respectively 30 and 55 km. from land, from 16 May to 25 June 1918 

 (Table XXIX). 



TABLE XXIX 



POLLEN TRAPPED ON LIGHTSHIPS IN GULF OF BOTHNIA, 1 6 MAY TO 25 JUNE 



191 8 (Hesselman, 1919) 



No. of grains per sq. cm. 



Pollen can be carried over much greater distances than this. In the 

 month of June, Dyakowska (1948) found pine and spruce pollen falling 

 on the coasts of Greenland, 600 to 1,000 km. from the nearest pine and 

 spruce trees. Still further north, in Spitsbergen, Polunin (1955) noted a 

 deposition of pine and spruce pollen which must have been equivalent to 

 about 200 grains per square metre per day in July and August (Chapter IX). 

 The long-distance record is probably that noted by Hafsten (195 1), 

 who reported Nothofagus pollen in peat on the island of Tristan da Cunha, 

 4,500 km. from the nearest source in South America. 



An instance of long-distance transport of moss spores is reported by 

 Pettersson (1940), whose investigations of plant spores in rain-water were 

 described in Chapter XI. At Tviirminne, Finland, during 22-23 July 

 1936 (when there were persistent rains and light, mainly easterly winds), 

 104 cc. of rain-water were collected during 15 hours. This sample proved 

 particularly rich in the spores of bryophytes, yielding 300 plants of 

 Marchantia polymorpha and Metzgeria sp. Most remarkable, however, was 

 the occurrence of spores which yielded 278 plants of Aloina brevirostris 

 and 2 which were identified as A. rigida. These are small, annual or bi- 

 ennial mosses of dry calcareous soils, and belong to a genus hitherto un- 

 recorded in Finland. There are a few records oi A. brevirostris in Eastern 

 Europe, but their presence in such large numbers in rain over Finland 

 suggested that the spores must have come from a rich and extensive source 

 area. This, Pettersson suggested, must lie in Siberia, in the region of the 

 River Yenisei — a distance of at least 2,000 kilometres east of Tvarminne. 

 It is true that this conclusion has been questioned by Persson (1944), who 

 thinks that they must have come from a nearer source such as European 

 Russia or possibly Sweden; and by Bergeron (1944), who examined air- 

 mass trajectories for the day in question and reached a similar conclusion 



186 



