THE MICROBIOLOGY OF THE ATMOSPHERE 



show that part of this skewness was due to the presence of clumps of 

 spores which fell faster than single units. It is also clear that, with both 

 uredospores and aecidiospores of rust fungi, a large number of single 

 spores fall very slowly. Measurements are needed to test whether, within 

 one species, the single spores arriving first at the bottom are larger than 

 those arriving at the end of the experiment. Another possibility is that 

 small eddies may have hastened the fall of some spores and retarded that 

 of others. A more serious defect of the method is that a vertical circulation 

 of air by convection in the cylinder might bias the results by introducing a 

 systematic acceleration or retardation of fall. This drawback could be 

 overcome by establishing a small temperature difference between the top 

 and bottom of the column, so that the stratified air would be stabilized 

 as in a 'temperature inversion'. A thermostat may produce artefacts from 

 convection currents set up by rhythmic temperature changes. BuUer 

 (1909) emphasized the difficulty of reducing air to anything like stillness, 

 even in closed beakers. 



In air, spores gain or lose water rapidly and the effect of spore hydra- 

 tion on terminal velocity, noted earlier by Duller, is evidently complex. 

 Weinhold (1955) showed that with uredospores of Puccinia graminis 

 tritici, changes in volume and weight occurred within 3 minutes of transfer 

 to air of different temperature and humidity. Weinhold reported that, 

 contrary to expectation, spores stored at 5 per cent relative humidity fell at 

 1-25 cm. per sec, in spite of being smaller and less dense than spores stored at 

 80 per cent relative humidity, which fell at i-i cm. per sec. Increasing the 

 humidity of air through which the spores fell increased the terminal 

 velocity, which was: 1-03, 1-22, 1-23, and 1-54 cm. per sec. at relative 

 humidities of 24, 45, 52, and 80 per cent, respectively. With increasing 

 temperature, terminal velocity decreased from i-o6 cm. per sec. at 23'4°C. 

 to 0-94 cm. per sec. at 39-9°C. 



We still lack observations on the rate of fall of highly elongated fungus 

 spores found in such genera as Ophiobolus, Epkhloe, Geoglossum^ and 

 Cordyceps, whose unusual shape makes Stokes's law inapplicable. Falck 

 (1927) calculated terminal velocities for a number of species with approxi- 

 mately elliptical spores on the assumption that the expected velocity 

 Ve = vj i^(a/b), where v^ is the fall velocity of a spherical particle of the 

 same volume, and a and b are axes of the ellipse. McCubbin (1944) 

 stressed our lack of observations on asymmetrical spores, and provisionally 

 suggested a method of calculating terminal velocity on the assumption 

 that surface drag accounts for most of the retardation. He showed that 

 observed terminal velocities of most spherical and oval spores fitted the 



r 1 length X width , . . . . 



approximate formula Vs = , where velocity is in mm. per 



40 



sec. and spore dimensions are in microns. Fusiform spores were treated 



18 



