BASIDIA AND THE DISCHARGE OF SPORES 41 



one five-hundredth part of a millimetre in a second and, to fall 

 2 5 cm. or 1 inch, require 3 hours and 5 minutes. 



The ideas of most people concerning the speed of falling bodies 

 are gained by observing the fall of comparatively large objects, 

 and it is therefore a little difficult to realise that there are living 

 organisms which, when falling in still air, require upwards of three 



Rates of Fall of Spherical Bacteria in Still Air. 



hours to fall a single inch ; but such an organism is Streptococcus 

 gracilis. The individual cells of this bacterium, under natural con- 

 ditions, probably fall even more slowly than our calculation has 

 shown owing to the fact that, when isolated in the air, they would 

 be dried up and therefore have a smaller diameter than 2 ^ which 

 was measured when they were immersed in a culture medium. 



The rates of fall per second and the length of time required 

 to fall an inch for (1) thistle-down, (2) moist basidiospores, 2 and 

 (3) bacteria may be tabulated as shown in the Table on page 42. 



From a comparison of the data given in the Table it is evident 

 that the rates of fall of basidiospores are intermediate between the 

 rates of fall of Thistle fruits and of the cells of bacteria. Thus 

 the rate of fall of a Thistle fruit is about one hundred times that of 

 a Mushroom spore, but the rate of fall of a Mushroom spore is about 

 twelve times that of the cells of Micrococcus Freudenreichii and 

 about seven hundred times that of the cells of Streptococcus gracilis. 



The minute size of wind-disseminated cells, whether they be 



1 The velocity of fall here given for Streptococcus gracilis is 82 per cent, greater 

 than that given by the uncorrected equation for Stokes' Law (first equation on 

 p. 21), while that for Micrococcus Freudenreichii is only 8-2 per cent, greater. 



2 Vol. i, 1909, p. 175. 



