THE MICROBIOLOGY OF THE ATMOSPHERE 



laminar-air layers, at the interface between the ground or other surface 

 and the atmosphere, in order to enter the freely moving air of the tur- 

 bulent boundary layer, where it stands a chance of being carried into 

 higher layers of the troposphere. 



Many species that are distributed as spores have not solved this 

 problem, but instead have become adapted for dispersal by some other 

 agency such as water, insects, or other animals. There are more insect- 

 pollinated (entomophilous) species of flowering plants than wind-pollin- 

 ated (anemophilous) species, though in the temperate regions at least 

 there are more wind-pollinated individuals because of the preponderance 

 of grasses and anemophilous trees. We may wonder how important in 

 practice is the occasional dispersal of a spore by some agency other than 

 that to which it is adapted. However, it is a fundamental principle that the 

 better a species is adapted to dispersal by one agency, the poorer are its 

 chances of dispersal by another agency — unless, like many fungi, it 

 produces spores of several distinct types that are specialized for different 

 dispersal mechanisms. 



If we wish to control the dispersal process, a precise knowledge of the 

 mechanisms involved is preferable to the vague idea that the spores will 

 get there somehow anyway! Success in colonization or fertilization de- 

 pends on logistics — on getting enough material to the right place at the 

 right time. 



Energy is required to detach spores from their source. It may be an 

 active process through which, by some explosive or hygroscopic mechan- 

 ism, spores are discharged by energy operating through the parent struc- 

 ture. Or it may be passive, by the energy of an external agent — usually 

 wind or the kinetic energy of falling raindrops. Seasonal development of 

 the parent structure and maturation of the spores commonly determine 

 what organisms are in the air at a particular time, but other fiictors modify 

 this pattern. The working of the various discharge mechanisms is more or 

 less affected by external conditions, and the result is that the output of 

 spores of a particular species varies greatly from time to time. Conversely, 

 all the individuals of one species in an area may behave in unison, so that 

 the composition of the air-spora differs vastly on different occasions. 



Take-off Mechanisms in Cryptogams etc. 



Spore- and pollen-liberation mechanisms have formed the subject of 

 classical researches in biology for over a century. The wealth of informa- 

 tion in the scattered literature on land plants is reviewed by Ingold 

 (1939), and knowledge about bacteria by Wells (1955), but for protozoa 

 and algae I know of no comprehensive treatment. In the present connec- 

 tion we are concerned with those aspects of the mechanism which deter- 

 mine when, and under what conditions, spores get into the air. 



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