MACROSCOPIC OBSERVATIONS 93 



fall by their own weight in the air. Since the rate of fall in quite 

 still air is uniform and only about 1 mm. per second, 1 whilst air- 

 currents and convection currents beneath the fruit-bodies have 

 proportionately a much greater speed, which amounts, even in a 

 quiet greenhouse, to several feet per minute, it is not astonishing 

 that the spore-clouds appear to float away from the fruit-bodies 

 as if they were not sinking at all. 



1 Vide infra, Chaps. XV. and XVI. 



