178 RESEARCHES ON FUNGI 



of the centre of the cloud. They came to the conclusion that for lycopodium 

 spores the formula gives velocities 50 per cent, in excess of those observed. 



My method for testing Stokes' formula appears to have various advantages 

 over that used by Zeleny and M'Keehan for the following reasons. Amani- 

 topsis spores have smooth walls and are practically truly spherical, whereas 

 lycopodium spores have sculptured walls and are four-sided. Amanitopsis 

 spores have a diameter only about one-third as great as lycopodium spores. 

 In the tube method convection currents cannot be eliminated, and it must 

 surely be somewhat difficult to decide the exact centre of the spore clouds. 

 By my method of using a very small chamber, the difficulty of convection 

 currents was reduced so as to be negligible, and the velocities of the individual 

 spores could be measured with considerable accuracy. Amanitopsis spores are 

 liberated spontaneously by the fungus, whereas lycopodium powder requires to 

 be set in motion by artificial means. 1 



1 The substance of this Appendix is contained in a letter to Nature on " The 

 Rate of Fall of Fungus Spores in Air," April 14, 1909. 



