THE MICROBIOLOGY OF THE ATMOSPHERE 



and deciduous forests, and some semi-desert lands, are dominated by 

 species which reach up to shed pollen into the turbulent boundary layer. 

 Tropical rain forest, on the other hand, is dominated by insect-pollinated 

 plants. Probably the number of anemophilous individuals is many times 

 greater than the number of entomophilous individuals in the world, yet 

 only about one-tenth of the species of flowering plants are anemophilous. 

 By contrast, tropical rain forest is noted for its extraordinary number and 

 diversity of species of flowering plants. Perhaps it is because of meteorolog- 

 ical factors such as rain or the difficulty of access for turbulent wind that 

 these plants are mostly insect-pollinated, and clearly the two phenomena 

 are inextricably linked. The wider possibilities of gene dissemination in 

 wind-pollinated plants has tended towards relative uniformity over wide 

 areas, while the statistically more 'normal' and localized character of 

 insect polhnation has favoured specialization and speciation. 



The role of air-dispersal is evidently two-fold. Its role in leading to 

 colonization is clear enough with seed plants and many micro-organ- 

 isms, and the estabhshment of infection is an example of this process in 

 pathogens. 



A second important role, obvious enough in wind-pollinated flowering 

 plants but curiously overlooked in microbes, is that of gene transmission. 

 In the fungi this process is probably a major function of spore dispersal 

 (Gregory, 1952). So far as well-established fungi are concerned, the im- 

 mense output of spores probably does little to promote the extension of 

 the range. When some accidental or cyclic effect offers a suitable environ- 

 ment within the range, the area involved rapidly becomes colonized and 

 the average number of mycelia of the species is at least maintained. From 

 this we need not necessarily deduce, as has been usual hitherto, that all 

 the rest of the spores are functionless. The higher fungi possess one charac- 

 teristic that is unmatched in other organisms — their ability to form 

 vegetative hyphal fusions which lead to a mixing of cytoplasm and nuclei 

 from spores of different origin — and it is tempting to speculate, as Tran- 

 seau (1949) did with Coprinus variegatus, that far more spores germinate 

 and fuse with an already-established mycelium than ever themselves 

 succeed in establishing a new mycelium. 



The work of H. M. Hansen & Smith (1932), on heterocaryosis, shows 

 that wild mycelia may contain genetically different nuclei. Some of these 

 may be due to mutations in mycelia derived from a single nucleus, but ex- 

 perimental evidence shows that hcterocaryons can also be produced by 

 artificial mixing of suitable, genetically different myceHa, and it seems 

 highly probable that this mixing process also occurs in nature. The value 

 to a species of storing an adequate supply of mutant genes to be drawn 

 upon in future, and so giving plasticity under varying conditions, has been 

 stressed by workers on fungus genetics, including Craigie (1942) and 

 Whitehouse (1949). 



Our speculation would suggest that spore dispersal of the higher fungi 



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