244 RESEARCHES ON FUNGI 



division. The advantages accruing to a fungus from the separation 

 of the spores of an ascus after they have been cast up into the 

 air are: (1) The increase in the number of separate infecting 

 particles which the fungus can produce, and (2) the splitting up 

 of the mass of the ascus jet. The separate parts of the ascus jet 

 must each fall considerably more slowly than the ascus jet would 

 do if it remained undivided and contracted into a ball, for, 

 according to Stokes' Law, the terminal velocity of fall of a 

 microscopic sphere varies directly as the square of the radius. 

 The smaller the rate of fall of a particle, the further can it be 

 transported by the wind before settling. The separation of the 

 ascus spores from one another is therefore favourable to their 

 dispersal by air-currents. 



It has been shown that the spores of Hymenomycetes, when 

 falling in air unsaturated with water vapour, dry up within a 

 few seconds after leaving the hymenium, and that, in conse- 

 quence, their rate of fall often becomes considerably reduced. 1 In 

 some instances it was found that the initial terminal velocity 

 becomes reduced to one-half or one-third according to the degree 

 of humidity of the air. Doubtless, in Ascomycetes, the small 

 film of liquid on the exterior of each separate spore, and also the 

 spore itself, dry up in unsaturated air within a few seconds after 

 the spore has been discharged. Evaporation, by decreasing the 

 size of the falling particles, must indirectly decrease their rate of 

 fall, and therefore in the end be advantageous for the dissemination 

 of the spores by the wind. 



The Fixation of the Spores in the Ascus of Peziza R&panda. — 

 In order to permit of the efficient ejection of the ascospores, it is 

 necessary that they should be situated at the distal end of the 

 ascus ; for the ascus is an apparatus which squirts out only about 

 one-half of its contents — the half nearest the ascus mouth. Zopf 

 has shown that in many cases the spores are retained in the 

 expanded end of the ascus by a special apparatus of attachment : 

 the uppermost spore in some Sordariea3 is attached to an inwardly 

 directed process produced from the membrane at the ascus 



1 Part I., Chap. XVI. 



