SEDIMENTATION IN STILL AIR 



as consisting of an intercalated cylinder (length = l/it) between two axial 

 cones (each of axial length = x/x), Vs being again in mm. per sec. 



During fall in still air, an asymmetrical particle will assume a charac- 

 teristic orientation. Hydrodynamical theory requires that the orientation 

 assumed will be that in which the resistance of the air to the motion of the 

 particle is greatest. This phenomenon can be observed with the naked eye 

 if minute airborne particles of fibre are watched in a beam of light in a 

 still, darkened room. 



We know very little as yet about spore orientation. Duller (1909) 

 observed that some slightly elongated spores tend to fall with their long 

 axis horizontal, as is to be expected for dynamical reasons. Sometimes 

 factors other than shape seem to influence the orientation of an asymmetric 

 spore. When Yarwood & Hazen (1942) watched the smooth conidia of 

 Erysiphe graininis, measuring 32 X 20 /x, during fall in vertical glass tubes 

 7 mm. in diameter, they saw that half of the spores fell with the long axis 

 horizontal and the other half with it vertical. This might indicate an 

 uneven distribution of materials of different density in the cell contents; 

 but, more likely, the vertical position was due to drag at the wall boundary, 

 because if the tube is made even narrower, all the spores fall vertically. 

 The present author has seen the filamentous ascospores of Cordyceps 

 gracilis similarly oriented whilst being carried up by convection currents 

 beside a vertical glass surface. Further, while watching the tailed spores 

 of the puffball, Bovista phwihea, falling in a small chamber on the stage of 

 a horizontal microscope, the tail was seen to trail behind the spherical 

 spore. In Chapter VI it will be indicated that spores tend to be deposited 

 with characteristic orientation on a surface. 



Stokes's law holds for smooth spheres. Few pollens or spores are 

 spheres, but a large proportion of them are microscopically smooth. 

 Others, when highly magnified, are seen to possess warts, spines or other 

 projections, or even to be pitted. These roughnesses would be expected to 

 increase friction during movement through air and to retard fall, but we 

 have as yet no experimental evidence of this. 



Viewed over the whole range of spore and pollen size of, say, 4 to 

 100 /x diameter, and of terminal velocities of from 0-05 to 10 cm. per sec, 

 it is clear that Stokes's law gives a good idea of terminal velocity in still 

 air, but that asymmetry and surface roughness may play a part as yet 

 unmeasured. 



Effects of Sedimentation 



The effects of spore fall in still air can be observed indoors, particu- 

 larly if a room is left closed and unoccupied — a fact noted early in the 

 study of air hygiene by workers using Hesse's horizontal tube method of 

 air sampling, or some modification of it {see Chapter I, p. 10). Although 

 all these investigations suffer from the defect of being based on highly 



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