40 RESEARCHES ON FUNGI 



From these data a simple calculation shows that the Thistle fruits 

 fall with a steady terminal velocity which is : 



39 times that of the spores of Amanitopsis vaginata, 

 104 ,, ,, ,, the Wild Mushroom, 



162 ,, ,, ,, Polyporus squamosus, and 



342 ,, ,, Colly bia dryophila. 



These figures serve to indicate how very much more slowly the 

 spores of Hymenomycetes fall than the parachute-like fruits of 

 a typical Thistle. On a windy day, one often sees thistle-down 

 blowing for very long distances over hills and plains ; but, on such 

 a day, the invisible spores of the Hymenomycetes which, as we 

 have seen, fall so very much more slowly than thistle-down, must 

 be transported by the wind with even much greater ease than this 

 delicate material and must, therefore, often be carried for many 

 miles before they finally come to earth. 



A Comparison of the Rates of Fall of Spores and Bacteria.- 

 In still air, the spores of the Hymenomycetes fall much more slowly 

 than thistle-down or the winged seeds and fruits of Flowering 

 Plants but, nevertheless, they fall very much faster than Bacteria. 

 Among organic bodies adapted for passive dispersal by the wind, 

 they therefore occupy only an intermediate position. 



Spherical species of Bacteria Micrococci, Streptococci, etc. 

 are much smaller than the spores of the Hymenomycetes. Their 

 diameters vary from about 2 to 2 /A. The rate of fall of individual 

 bacteria may be calculated by using the second equation given on 

 p. 21. Now, for our purposes, p may be taken as 1, o- as negligibly 

 small, //, as 1*8 X 10" 4 , g as 1000, and p as 76 cm. of mercury. 

 The following Table gives the diameter, radius, terminal velocity of 

 fall in still air, and time required to fall 2*5 cm. (approx. 1 inch) 

 for the individual cells of certain species of spherical bacteria. 



The cells of the largest species, Micrococcus Freudenreichii, 

 which have a diameter of 2 //, and a radius of 1 //,, fall only about 

 one-eighth of a millimetre in a second and, to fall 2 '5 cm. or 1 inch, 

 require 3 minutes and 8 seconds. 



The cells of the smallest species, Streptococcus gracilis, which 

 have a diameter of 2 ^ and a radius of 1 /n, fall only about 



