THE EFFECT OF HUMIDITY 



181 



after leaving the gills, seems to be that the spores diminish in 

 size owing to the loss of water. According to Stokes' Law the 

 velocity varies as the square of the radius of a sphere. One must 

 remember that a spore has an enormous surface compared with 

 its mass, and therefore, when falling, can readily and quickly part 

 with some of its contained water. In falling 5 mm. in ordinary 

 air, when leaving the gills, spores of Collybia dryophila (the 

 smallest with which I have yet worked) were found to lose 20 P 4 

 per cent, of their velocity ; in falling 6 mm. the spores of 

 Polyporus squamosus lost 12' 7 per cent., and those of Psalliota 

 campestris 6 -3 per cent. The results are collected in the following 

 Table : 



Diminution in Velocity of Falling Spores 



Further observations were then made upon decrease in velocity 

 when spores were allowed to fall through a distance of 15 cm. 

 For this purpose a brass chamber (Fig. 62) was constructed. 

 The space within it was 16 cm. long, 1*1 cm. wide, and 0*6 cm. 

 deep. To one side of the chamber a glass plate was fixed with 

 cement, and to the other side a long cover-glass could be affixed 

 with vaseline. A piece of the fungus fruit-body, which included 

 parts of three or four gills, was placed in the chamber at one 

 end. When the latter was set in the upright position, spores 

 fell from top to bottom. With the horizontal microscope, obser- 

 vations on the velocity of the spores were made at different 

 distances from the gills. 



The retardation in the velocity of fall was found to be most 

 rapid immediately after the spores had left the gills, and to 



