THE MICROBIOLOGY OF THE ATMOSPHERE 



minuscuhi (i). Myxomycete : Physarum nutans (i). Moss: Brachythechim 

 rutcibulum (i). 



Another worker who made a re\^ arding study of autotrophic plants in 

 rain-water was Pettersson (1940), working at the Zoological Station at 

 Tviirminne, Finland, in the summer of 1936. Glass funnels (176 sq. cm. 

 in area) were lined with filter-paper, sterilized and taken, covered, to the 

 trapping site. After exposure, the filter-paper was sprayed with a nutrient 

 solution, and the funnel was covered with a glass lid and left to stand in a 

 light place for a few days. Developing organisms were picked off and 

 transferred to new culture vessels to continue their growth. 



The originality of the method lies in the medium being unfavourable 

 for the development of bacteria and fungi because the cellulose of the 

 filter-paper was the only carbon source provided. Pettersson, like Over- 

 eem, was therefore able to explore a novel part of the air-spora. Snow-traps 

 were also used, consisting of shallow glass dishes 15-20 cm. in diameter, 

 with a thick bed of blotting paper and an upper layer of filter-paper. 

 These two methods gave an unexpectedly rich harvest. A sample of snow 

 taken at Pikis (Piikio) from the start of snowfall on i March 1936, gave 

 thirt)'-six lichen thallus fragments and a moss gemma. On the next day, 

 2 hours after the start of another snowfall, a sample corresponding to 

 625 ml. of water yielded nineteen lichens and two Chlorococcum colonies. 

 A third sample of 805 ml. of water, taken 4I hours later, yielded six 

 lichens, and three mosses which were identified after 6 months' growth as 

 Brachythecium velutiniun^ Hypnum cupressiforme, and Pylaisia polyantha 

 {see also Pettersson, 1936). 



Pettersson's rain-trap yielded a wealth of information from the fourteen 

 samples investigated, for details of which the original paper must be 

 consulted. The interest was taxonomic and qualitative rather than 

 quantitative. For the early samples the funnel was placed on a low rock, 

 2-5 metres above sea-level, on open grassy soil. Some of the organisms 

 caught may possibly have come by splash from the ground, but not many 

 can have done so because the largest catch of mosses belonged to a 

 genus hitherto unrecorded in Finland {see Chapter XIV). The precaution 

 of raising the funnel on a wooden base i metre high was adopted in later 

 tests. 



In a total of 1,373 ^- of rain collected, Pettersson obtained 1,200 

 conifer pollen grains, 300 liverwort spores (all of Marchantia polymorpha 

 except for one of Metzgeria), Myxomycete spores {Stemonitis fusca three 

 times and Arcyria denndata), and numerous algae. Blue-green algae were 

 scarce, being represented only by Nostoc commune and Gloeocapsa sp. in 

 separate samples. Green algae were abundant in almost every sample, 

 those identified including: Chlamydomonas nivalis^ Chlorella vulgaris, 

 Chlorococcum humicolum, Cystococcus pseudostichococcus, Prasiola stipitata, 

 Roy a sp., and Tetraedron punctulatum. Some of these, the author suggests, 

 may have originated from lichen soredia. Lichen spores and soredia were 



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