LONG-DISTANCE DISPERSAL 



heat radiation, a spore is seldom at the same temperature as the air in 

 which it is suspended; though, because of its small heat-capacity, the 

 difference is not likely to be great. Improvements in the technique of 

 freeze-drying show that damage to organisms is greatly affected by the 

 temperature and speed at which they are desiccated. Repeated wetting 

 and drying often lowers viability. 



As physiological experiments show, many of the regular components 

 of the air-spora are resistant to desiccation. Less hardy organisms are 

 probably better able to survive when they are high above the Earth's 

 surface. Most micro-organisms will survive in a resting condition longer 

 at the temperatures found in the upper air than they will at ground-level; 

 temperatures in the atmosphere are preservative rather than lethal for 

 most of the air-spora {see Meier, 1936^). 



Radiation is a much more serious hazard, and on this there is a rapidly 

 increasing literature. The most quickly lethal radiations in the atmosphere 

 are in the ultra-violet region, and these are largely absorbed by the air 

 before they reach the ground. Ascent to the upper air therefore brings 

 the risk of greatly increased dosage of ultra-violet radiation — except in 

 the shelter of clouds. Pigmented spores, and bacteria carried on larger 

 rafts, may also be effectively screened from radiation. The interactions of 

 humidity, temperature, and radiation are not well known, and it may be 

 that low temperature and desiccation partly protect a spore against 

 radiation damage. In addition, when a spore returns to ground-level, the 

 recently studied phenomenon of photo-reactivation by visible light of 

 organisms which have been exposed to ultra-violet, may perhaps reduce 

 the damaging effects of radiation received at high altitudes. Photo-reacti- 

 vation is defined by Jagger (1958) as: 'the reversal with near-ultraviolet 

 or visible light of ultraviolet radiation damage to a biological structure.' 

 Visible light reverses lethal effects of ultra-violet radiation in many 

 microbes — including bacteria and actinomycetes, fungi and yeasts, and 

 also protozoa — but the survivors of high-altitude passage may be expected 

 to show an increased mutation rate on their return to Earth. 



The suitability of the atmosphere for the survival of 'aerial-plankton' 

 is summed up by Gislen (1948) as follows: 'While the lower cloudy air- 

 strata — let us say under 3,000 to 4,000 metres — form a suitable medium 

 for the transport of micro-organisms, the higher layers are very inhospit- 

 able to them, not so much because of the low temperature, drought and 

 barometric pressure, as because of destructive radiation.' 



N 193 



