172 RESEARCHES ON FUNGI 



The Table on the next page gives a summary of the data obtained 

 for testing Stokes' Law with falling spores of Amanitopsis vaginata. 

 The velocities given are the average velocities of 200 spores in 

 Specimen I., of 100 in Specimen II., and of 50 in Specimen III. The 

 densities are certainly correct to within 1 per cent. 1 The diameters 

 are the average diameters for at least 50 spores, these being spherical. 

 The measurements were made with a Poynting Plate Micrometer in 

 the manner already described. The spores of Specimen I. were 

 those collected at the bottom of the compressor cell whilst observa- 

 tions on the fall of some of them were being made. The spores of 

 Specimen II. were obtained from another part of the fruit-body from 

 which the piece had been dissected out for velocity observations in 

 the compressor cell. The spores of Specimen III. 2 were those col- 

 lected from the piece of fungus used for the velocity records, but 

 collected from it immediately after these had been taken. It is 

 clear, therefore, that I was unable to measure the diameters of 

 exactly those spores for which the velocity of fall had been recorded. 

 This is a defect in my method. However, the defect seems to me 

 of little importance, for it was found that any 50 spores, taken at 

 random from any part of a single fruit-body, have the same average 

 size. In order to obtain the average size of the spores of which 

 the velocity had been recorded, it was therefore only necessary to 

 measure the average size of any 50 spores obtained from the fruit- 

 body. 



In making calculations with Stokes' equation, the viscosity of 

 air 3 was assumed to be 1*8 x 10^ 4 and its density negligible compared 

 with that of a spore. The value of g was taken as 981. 



From the Table it is clear that the figures obtained by 

 observation for the rate of fall of the spores are of the same 

 order of magnitude as those demanded by Stokes' Law. How- 

 ever, the Law is not confirmed in detail, for as an average for the 

 three cases it was found that the actual velocity of fall of the 

 spores was 46 per cent, greater than the calculated. 



1 Vide Chap. XIII. 



2 For actual measurements, vide Chap. XIV. 



8 l - 8 x 10~ 4 is the value usually taken for dry air at room temperatures. The 

 effect of moisture is to very slightly reduce the viscosity. 



