SEDIMENTATION IN STILL AIR 



*(i) Buller, 1909. (2) Zeleny & McKeehan, 1910. (3) McCubbin, 1918. (4) Ukkelberg, 

 ^933- (5) Stepanov, 1935. (6) J. J. Christensen, 1942. (7) Yarwood & Hazen, 1942. (8) Weinhold, 

 1955. (9) Bodmer, 1922. (10) F. Knoll ex Rempe, 1937. (11) J. Dyakowska ex Erdtman, 1943. 



Two methods have been used for measuring terminal velocity. The 

 simpler method is to time the fall over a short, measured distance in a 

 small chamber of still air by direct observation with a horizontal micro- 

 scope. It was used in the pioneer work of Buller (1909), and by Yarwood 

 & Hazen (1942). So far this method has been used only for small, slowly- 

 falling spores, because large ones travel too fast to be timed by direct 

 observation. The method could no doubt be extended to fast-moving 

 spores by photographing with a flash of known duration. The technique 

 most generally used, however, has been to release spores or pollen at the 

 top of a column of still air in a vertical cylinder and find the time they 

 take to arrive at the bottom. This is the method used by Zeleny & McKee- 

 han (1910), McCubbin (1918), Ukkelberg (1933), Stepanov (1935), and 

 Weinhold (1955). 



McCubbin and Ukkelberg report results of similar type. The number 

 of wheat-rust spores reaching the bottom of the tube in successive inter- 

 vals of time showed a negative skew distribution. Ukkelberg was able to 

 B 17 



