PUFFING IN THE DISCOMYCETES 261 



with a speed of several centimetres a second and carries many of 

 the spores passively forward with it. The rate of fall of the spores 

 is much less than the speed of the air. Hence it is possible for the 

 blast of air to carry the spores through the side-pipe 6 and even 

 some 1-3 inches beyond its mouth. 



The forward movement of the air in the test-tube caused by the 

 bombardment of the air by the projectiles does not create a partial 

 vacuum in the tube, because the mouth of the cup of the fruit-body 

 is somewhat smaller than the mouth of the tube to which it is 

 applied ; so that, as the air is driven forward in the body of the tube 

 (Fig. 124, a), more air enters at the mouth of the tube (Fig. 124, 

 near h). 



The baUistic phenomenon under discussion may be treated 

 mathematically as follows : 



Let Mj = the mass of the projectiles (the spores and the cell- 

 sap) shot out from all the ascus-guns, 

 v^ = the velocity of projection, 

 M2 = the mass of the air set in motion by the impact of the 



projectiles, and 

 Vc, = the common velocity of the air and projectiles after 

 impact. 



Then, by the principle of the conservation of energy, 



M, v^ = (Ml + M2) v^ 



i.e. the momentum of the blast of air and of the projectiles which 

 the air carries along with it is equal to the momentum of the pro- 

 jectiles at the moment when they left the asci. It therefore follows 

 that the blast of air is caused by the air being bombarded by the 

 projectiles. 



After a blast of air has been started, its velocity is progressively 

 diminished by friction with the still air through which it moves- 

 Hence the blast of air produced by the apothecium, like a smoke-ring 

 blown from the mouth, does not travel very far. 



Since action and reaction are equal and opposite, when puffing 

 takes place the stalked apothecium of Sarcoscypha protracta must 

 tend to recoil with a momentum equal to that of the projectiles 



