LONG-DISTANCE DISPERSAL 



The source of supply at the place of origin may be too weak or, because 

 of dilution and deposition, the spore-cloud may arrive at a concentration 

 below the threshold for detection. Or again, when the spore-cloud arrives, 

 few of the spores may be viable, or perhaps all may be dead. 



The dispersion equations given in Chapter XIII ignored the death- 

 rate of spores in transit, and treated merely their diffusion as particles — 

 regardless of whether they were living or dead. The justification for this 

 is that the physical processes controlling dispersal likewise fail to dis- 

 criminate between living and dead spores. Further, although a plant 

 pathologist or plant breeder is concerned only with living cells, to an 

 allergist a spore or pollen grain is equally significant whether alive or dead. 

 None of the gradients measured so far contains any hint that loss of 

 viability is affecting the dispersal process: it may very well do so, but the 

 distances covered in any quantitative study are still too short, and our 

 methods too coarse, to allow us to detect a decrease in viability along a 

 gradient. Yet, over longer distances, loss of viability may well dominate 

 the gradient. Some organisms may lose viability while travelling quite 

 short distances in air; but this effect has apparently not yet been disen- 

 tangled from effects of changes in concentration caused by diffusion and 

 deposition. 



Uredospores of some cereal rusts as, for example, Piiccinia graminis 

 and P. rubigo-vera, can evidently remain viable after travelling many 

 hundreds of kilometres and reaching the top of the troposphere, though 

 we suspect that others, such as those of P. glumarum and P. polysora, are 

 less robust. Aecidiospores of rusts in general are believed to travel shorter 

 distances in the viable state, and the basidiospores (sporidia) of the white- 

 pine blister-rust {Cronartium ribicola) may well have their viable range 

 restricted to a few hundred metres. Intensive study of this phenomenon 

 in the open air is needed before we can know the quantitative significance 

 of death for the dispersal gradient. 



PHYSIOLOGICAL STUDIES OF VIABILITY 



The immense literature on microbial survival and death-rates throws 

 much light on factors influencing percentage viability in a population, 

 under almost all imaginable conditions — except during suspension in air. 

 Of changes in the viability of organisms during air-dispersal we have only 

 circumstantial evidence, except for bacteria which are small enough to 

 allow their longevity to be studied while suspended as aerosols in the ro- 

 tating steel drum devised by Goldberg et al. (1958). Using this method, 

 Webb (1959, 1959^) concluded that bacterial cells suspended in air die 

 as a direct result of loss of bonded water from their protein. Loss of via- 

 bility takes place in two stages : a rapid killing in the first second of time, 

 followed by a slow death-rate which might be delayed by some bacterio- 

 static substances whose presence might actually increase survival. 



191 



