BASIDIA AND THE DISCHARGE OF SPORES 33 



Otidea, Ascobolus, and Morchella, which are subaerial, the ascus- 

 gun is beautifully perfected : it is drawn out into a long tube with 

 the eight spore-projectiles arranged in a row just behind the gun's 

 apex, the ascus-wall is very elastic, the gun explodes at its apex 

 by the breaking away of a little lid, and the eight spores are shot 

 violently forward to a distance of some two or three centimetres. 

 The smoothness of the spore-walls permits of the projectiles passing 

 out of the end of the ascus-gun with the least possible friction. 1 On 

 the other hand, in some subterranean Tuberaceae, e.g. the well-known 

 Truffles of the genus Tuber, where a violent discharge of the spores 

 would no longer be of any biological advantage and does not take 

 place, the ascus has lost its tubular form and is now oval or rounded, 

 the spores are no longer arranged in a row one behind the other, 

 the number of spores in each ascus is less than eight usually two 

 to four, the ascus-wall is not so elastic and does not open apically 

 by a lid, and the spore- walls are no longer smooth. 



A comparison of gymnocarpous with cleistocarpous fruit-bodies 

 in Basidiomycetes and Ascomycetes thus compels us to conclude 

 that the typical forms of the basidium and of the ascus are 

 intimately associated with the efficiency of these cell-organs in the 

 performance of one of their chief functions, namely, the violent 

 discharge of the spores ; and, at the same time, it teaches us that 

 where the function of violent spore-discharge has become lost both 

 the basidium and the ascus tend to undergo a series of degenera- 

 tive modifications in structure. 



Balloons Falling from Rest or Fired from a Gun, Used to 

 Illustrate the Movements of Spores. It is impossible to observe 

 the minute spores of the Hymenomycetes whilst they are being 

 projected straight outwards from their sterigmata. In order to 

 determine the trajectory of a spore I therefore proceeded indirectly. 

 The maximum horizontal distance of propulsion and the terminal 

 rate of fall of a spore in still air were first measured, and then the 

 data so obtained were introduced into an equation based on Stokes' 

 Law. By means of a series of calculations made with this equation 

 the required trajectory was then plotted out. 2 This trajectory, 



1 For a discussion of the form and function of the tj^pical ascus fide vol. i, 

 pp. 240-247. 2 Vol. i, 1909, pp. 184-189, Figs. 64-66. 



VOL. n. r> 



