THE HATE OF FALL OF SPOKES 



i/3 



.4 manttopsis I 'aginata 



Chamber, containing soaked blotting-paper or cotton-wool and free water below, 

 closed for half-an-hour before observations for velocity were taken. Field of 

 microscope close under gills. 



It is difficult to explain why the observed velocities of fall 

 should be nearly 50 per cent, greater than that demanded by 

 Stokes' Law. However, perhaps the explanation has some con- 

 nection with the fact (to be discussed more fully in the next 

 chapter) that, even in an apparently saturated chamber, the 

 spores in falling even such a small distance as 5 mm. lose a 

 certain amount of water. It was found that after leaving the 

 gills, the rate of fall of the spores slightly decreases. Further 

 experiment showed that this was due to the contraction in the 

 volume of the spores consequent on drying up. 1 It has been 

 suggested to me by Professor Poynting, that the loss of water 

 by a spore during its fall might lead to an evaporation pressure 

 of such a kind that the spore would be forced more quickly down- 

 wards than would be the case if no loss of water were taking 

 place. Owing to the impossibility of employing perfectly dry 

 spores in my experiments, there seems to be no way at present 

 to test this hypothesis. 



That the speed of fall on the average was found to be 46 per 

 cent, greater than that given by Stokes' Law may possibly be 

 accounted for by surface slip. With very minute particles Stokes 



Vide infra, Chap. XVI. 



