THE MICROBIOLOGY OF THE ATMOSPHERE 



to a distance of a fraction of a millimetre or more. In species which form 

 the basidia on exposed surfaces, as in many lower Basidiomycetes, the 

 spore after discharge has a chance of being picked up by an air current. 



The higher Basidiomycetes often show great elaboration of a stalked 

 fruit-body with the basidia lining the vertical surfaces of folds, gills, 

 pores or spines. Here, in cavities protected from wind and adverse 

 conditions, the basldiospores are discharged into still air and fall under the 

 influence of gravity into the moving air-current below the cap-shaped or 

 bracket-shaped fruit-body. Spore discharge in the higher Basidiomycetes 

 often goes on continuously throughout almost the entire life of the fruit- 

 body — to all appearances little affected by wind, temperature, or humidity, 

 though it must be emphasized that accurate quantitative studies on the 

 effects of these factors are lacking. Just how a basidiospore is shot off" 

 the sterigma remains a major puzzle of mycology; several explanations 

 have been advanced, but none seems entirely satisfactory. Nevertheless 

 the process is highly efficient and basidiospores are a conspicuous com- 

 ponent of the air-spora. 



The same mechanism occurs in the mirror-yeasts (Sporobolomy- 

 cetaceae), which may possibly have evolved from lower Basidiomycetes 

 (unlike the Saccharomycetaceae, which are clearly Ascomycetes). To 

 avoid prejudging the issue by calling the spores of the mirror-yeasts 

 'basidiospores', the term 'ballistospores' has been coined to include all 

 spores showing the drop-excretion discharge mechanism. A moist sub- 

 stratum is necessary for spore discharge in the Sporobolomycetaceae. 



LICHENS 



The fungus component of lichens discharges ascospores from typical 

 apothecia or pcrithecia, or basidiospores from basidia. Fragments of the 

 thallus including both fungal and algal components are blown about freely. 

 Rounded groups of algal cells surrounded by fungal hyphae, separating 

 off" from the lichen thallus as soredia, are also blown away; but we 

 know little as yet about the relative importance of these various modes of 

 reproduction. 



ALGAE 



Adaptations facilitating take-off" into the air are unknown in the algae, 

 though some of the simpler types of algal cells get into the air regularly. 

 Pettersson (1940) suggested that Chlamydomonas nivalis is carried away 

 from its habitat on snow-fields and glaciers in melt-water and becomes 

 airborne by splash in mountain torrents. Lichen soredia may possibly 

 aid the dispersal of algae when they become grounded in a habitat moist 

 enough for the algal component to dominate the fungus, and where the 

 resulting colony will be an alga instead of a lichen. Again, some terrestrial 

 and epiphytic algae may crumble and blow away. 



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