AEROBIOLOGY 



has one other function besides those of increasing the range and colonizing 

 new substrata within the range — that of dispersing genes and transmitting 

 novelties, arising from mutation and recombination, between one estab- 

 lished m5'celium and another. The shift of sexual reproduction from the 

 sedentary spores of the Phycomycetes, which lack hyphal fusions, to the 

 dispersal-spore form in the Ascomycetes and Basidiomycetes, may be a 

 phase in the evolution of this habit. On this hypothesis, a perennial my- 

 celium would be the locus of activity and multiplication of individual 

 nuclei — some perhaps descended from the original spore-colonizer, others 

 descended from outside immigrant sources. The mycelium would remain, 

 whereas nuclei would come and go. The established mycelium would 

 resemble a city rather than an individual. Thus, besides their function as 

 colonizers, spores may perhaps act as a sort of unreliable air-mail service, 

 transmitting genes between established mycelia. 



Dispersal spores of some fungi such as the Gasteromycetes are difficult 

 to germinate. If we reject, as first choice, the hypothesis that they are 

 functioniess, we must suppose that there exist special conditions under 

 which germination occurs naturally. The experiments of Ferguson (1902), 

 who discovered that mushroom spores would germinate readily when in 

 contact with living hyphae of the same species, are suggestive in this con- 

 nection. Other examples are kno^\Tl of spores that are stimulated to germi- 

 nate by hyphae of the same species, an instance being the spermatia (pycnio- 

 spores) of Puccinia helianthi (Craigie, 1933). Experiment may show that 

 this phenomenon of gene interchange plays a bigger part than we have 

 hitherto considered possible. If this speculation is justified, the produc- 

 tion and dissemination of novelty must be a major activity of the 

 fungi. 



Ascospores and basidiospores are the spore forms most likely to con- 

 tain genetic novelty, and they are most commonly dispersed by wind to 

 potentially new environments. The conidia, which are ordinarily dis- 

 persed by wind, splash, or insects, were well named 'repeating spores' in 

 the older literature, functioning as they did for exploitation of the same 

 environment as the parent. However, G. Pontecorvo's discovery of para- 

 sexual re-combination shows that genetic novelty can also arise from 

 conidial forms in several ways. 



BEYOND THE ATMOSPHERE 



Aerobiological technique has much to contribute to research beyond 

 the Earth's atmosphere. 



To counter the idea that living organisms evolved from non-living 

 matter on this planet, Arrhenius (1908) put forward the hypothesis that 

 space is permeated by spores. On this hypothesis, spores might be carried 

 to great heights — for example in the Earth's stratosphere by volcanic 

 activity — and then driven off into space by electrical repulsion. If this 



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