174 RESEAKCHES ON FUNGI 



has calculated that the maximum effect of slip is to increase the 

 terminal velocity by 50 per cent. Therefore, if we assume that 

 the slip actually takes place, the discrepancy between observation 

 and theory would be fully accounted for. It seems, however, that 

 the spores are not small enough to permit of our assuming slip 

 to the extent required. 



The most serious objection to my method for testing Stokes' 

 Law seems then to be that it has so far been found impossible 

 to get the spores to fall with a quite constant speed in an 

 apparently saturated chamber. Now the spore diameters were 

 measured when the spores were in water, i.e. when fully turgid, 

 just as spores are upon the sterigmata immediately before their 

 fall. In order, therefore, to observe the fall of the spores when 

 they were in as turgid a condition as possible, the two precautions 

 (1) of placing the field of the microscope immediately under the 

 gills, and (2) of saturating the chamber so far as possible with 

 water-vapour, Avere taken. If the velocity of the spores could 

 have been measured immediately they left the sterigmata instead 

 of when they came into view beneath the gills, probably it would 

 have proved even greater than that recorded. 



All the measurements for density, size, and velocity of the 

 spores could be made with great exactness. It seems to me 

 most unlikely that the large discrepancy between theory and 

 observation can be due to errors in these measurements. Unless 

 loss of water from the spores in some way is capable of accel- 

 erating their rate of fall, for the present it would seem as though 

 the spore-fall method of testing Stokes' Law shows that the 

 actual velocity of fall of spheres about 10 i^ in diameter is some 

 50 per cent, greater than the Law demands. 



The appended Table, giving the results of observations upon 

 the rates of fall of spores of various species, was compiled before 

 the tests for Stokes' Law were made. The air in the chamber of 

 the compressor cell was not saturated and sim])ly contained the 

 required piece of fungus. The field of the microscope was 

 usually near the gills. The rates of fall of spores of a single 

 fruit-body, as the spores dry up, gradually decrease after the spores 

 have left the gills. The figures given in the Table serve merely 



