THE FALL OF ASCOSPORES 



255 



The large mass of a group of eight ascospores, of the sporangium 

 of Pilobolus, or of the conidium of Empusa tnusc^, is unfavourable 

 to the dispersion of these structures by the wind owing to the fact 

 that it causes them to fall with comparative rapidity. Let us 

 compare the terminal vertical velocity of an Ascobolus immersus 

 ascospore group with that of a basidiospore of Amatiitopsis 



Fig. 83. — Empusa muscn: The house-fly has been killed by the fungus and 

 is now fixed by its proboscis to a window-pane. The halo around the fly's 

 body consists of discharged conidia, many of which have been shot to a 

 distance of 2 cm. and some to about 3 cm. Photographed by C. W. Lowe. 

 I natural size. 



vaginata. Assuming Stokes' Law and equal densities for the 

 falling particles, it may be shown that 



where V =the terminal vertical velocity of the basidiospore, 



Vj = the terminal vertical velocity of the ascospore group, 

 a =the radius of the basidiospore, 



«! = the radius of a sphere with a volume equal to that of the ascospore 

 group. 



Since, from measurements made, we may take V = 0'5 cm. per 

 second, a = 0'0005 cm., and ai = 0*005 cm., we may calculate that 



