156 RESEARCHES ON FUNGI 



in a test-tube one inch in diameter. They behaved like paraffined 

 beads, i.e. they at once moved away from the sides of the test-tube 

 and collected in the middle of the surface film of water (Fig. 76, B). 



If a small glass vessel is over-filled with water so that the surface 

 film of water bends downwards to the glass rim, tiny wettable 

 floating objects, such as unparaffined hollow glass beads, move from 

 the sides of the vessel to the centre of the film of water, whereas 

 tiny un wettable floating objects, such as paraffined hollow glass 

 beads, move from the centre of the water film to the sides of the 

 vessel. 1 Some sporangia of Pilobolus were placed in water which 

 overfilled a wide test-tube one inch in diameter. They behaved 

 like paraffined glass beads, i.e. they at once moved away from the 

 centre of the surface film of water and came into contact with the 

 rim of the test-tube (Fig. 76, C). 



The two physical experiments just described afford further 

 evidence that the black convex sporangial wall of Pilobolus is 

 unwettable. 



In the light of the observations just recorded, an attempt will 

 now be made to explain how it is that a discharged sporangium 

 always settles on any object it strikes so that its basal gelatinous 

 side is turned toward the surface of the object. 



The Pilobolus projectile consists not of a sporangium only as 

 Grove and others have supposed but, as we have seen, of a sporangium 

 and a large drop of cell-sap. Owing to the fact that the gelatinous 

 under side of the sporangium is wettable while the black convex 

 upper side is unwettable, it is clear that, as the sporangium is 

 travelling through the air, the drop of sap must be attached to the 

 gelatinous side of the drop, as shown in Fig. 74, A (p. 151) ; and 

 this view is supported by the pipette-drop observations described 

 above. It may well be, as Grove supposed, that the projectile 

 rotates as it travels forward, but whether it rotates or not does not 

 make any difference to the explanation of the mode of settling of 

 the sporangium, which is now about to be given. 



Let us suppose that a Pilobolus projectile is travelling vertically 

 upwards toward a sheet of glass. As it is about to strike the surface 



1 C. V. Boys, Soap Bubbles and the Forces which Mould them, The Romance 

 of Science Series, S.P.C.K., London, 1895, pp. 33-34. 



