CHAPTER XVI 



THE EFFECT OF HUMIDITY ON THE RATE OF FALL OF SPORES 



It can be shown on mathematical grounds that, when bodies 

 the size of spores are allowed to fall freely in still air, they reach 

 their constant terminal velocity before they have gone their own 

 diameter or a distance of less than 10 /la. 1 It was expected 

 at first, therefore, that a spore would fall through the space in a 

 compressor cell, i.e. a distance of about 13 mm., at a uniform 

 speed. Accordingly, in order to test this supposition, the rate of 

 fall of spores through a field of 4-55 mm. at different distances 

 below the gills was measured. It was soon discovered that the 

 velocity of a spore gradually diminished as the spore fell after 

 emerging from the gills. It was suspected that this was due to 

 the gradual diminution in size of the spore owing to loss of water 

 from it by drying. Comparative experiments with the air in the 

 chamber in different states of humidity were then undertaken. 

 The air of the chamber was first made as moist as possible by 

 means of soaked blotting-paper, next the ordinary air of the 

 laboratory was employed, and finally the air was dried as far as 

 possible with crystals of calcium chloride. A different piece of 

 the same fruit-body was used in each case. The results of the 

 observations are recorded in the Tables on p. 180. The figures give 

 the velocities in millimetres per second. 



From these results we may conclude that, as a general rule, 

 the spores fall most rapidly on leaving the gills, and that the rate 

 of fall gradually diminishes. This appears to be so, even in 

 chambers which contain a free drop of water and soaked blot- 

 ting-paper (Fig. 58, p. 167), and in which the air must therefore 

 be saturated with moisture. The tiny oval spores seem to be 

 capable of giving off water vapour in an atmosphere saturated so 

 far as flat surfaces are concerned. 



1 Vide infra, Chap. XVII. 



