THE PILOBOLUS GUN AND ITS PROJECTILE 151 



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Fig. 74. — Diagram to illustrate tlie manner in which a projectile of Pilobolus Kleinii 

 strikes and adheres to an object in the path of its trajectory. A, a projectile 

 which is being shot upwards in the direction shown by the arrow and is about 

 to strike the cover-glass d. The projectile consists of : (1) a sporangium, filled 

 with spores, covered in part by a black unwettable sporangium-wall a and in 

 part by a basal gelatinous ring b and the wall of the columella (not here shown), 

 both of which are readily wetted ; and (2) a large drop of cell-sap c which has 

 been shot out of the subsporangial swelling of the Pilobolus gun concerned and 

 which adheres to the wettable structures just mentioned. A point on the left 

 side of the sporangium just below the arrow will strike the cover-glass first. 

 There will be a moment of momentum about tliis point and the drop of cell-sap will 

 quickly move upwards about the point, strike the cover-glass, and flatten out 

 upon it. As the drop flattens, it must inevitably push the unwettable black 

 wall of the sporangium away from itself and the cover-glass and drag the wettable 

 gelatinovis cell-wall and the wall of the columella toward itself and the cover- 

 glass. The result of all this, when the projectile has come to rest, is represented 

 at B. In B there are shown: a, the unwettable black sporangium-wall; b, the 

 wettable gelatinous ring surrounding the spores e ; and the drop of cell-sap c, which 

 is now flattened out on the cover-glass d. As the whole dries up, the sporangium 

 shrinks imtil the broken edge of the black wall touches the cover-glass, the drop c 

 becomes a halo of precipitated particles, and the gelatinous layer causes the 

 sporangium to adhere very firmly to the cover-glass. Magnification, G3. 



