THE PILOBOLUS GUN AND ITS PROJECTILE 153 



it. The assumptions are not justified : for there is no evidence to 

 show that a sporangium ever strikes an object without sticking to 

 it, and there is plenty of evidence to show that a sporangium will 

 stick to the vertical surface of any object when the surface is 

 perfectly dry. 



Ingold ^ has supposed that, from the moment the sporangium 

 is discharged, it trails behind the drop which carries it forward, so 

 that the drop always strikes the obstacle first and causes the spor- 

 angium to settle with its gelatinous side toward the obstacle. As 

 already pointed out in Chapter 1,2 it seems most unlikely that the 

 projectile, on leaving the sporangiophore, should rotate through 

 exactly 180° and no further. 



A satisfactory explanation of the mode of landing of a sporan- 

 gium, as we shall see, can be made if one takes into account the 

 fact — hitherto noticed only by Ingold — that water can adhere to 

 the under side but not to the upper side of the sporangium or, in 

 other words, that the lower gelatinous side of the sporangium can he 

 wetted by water, whereas the upper convex side covered by the sporangial 

 wall is unwettable. The resistance to being wetted offered by the 

 sporangial wall may be due in part to the crystals of calcium oxalate 

 which protrude in such large numbers from its surface. Some 

 evidence to show that the sporangial wall actually is unwettable 

 will now be brought forward. 



If one brings some freshly-discharged sporangia into contact 

 with water contained in a crystallising dish, they at once float (c/. 

 Fig. 76, B) at the surface wdth their lower gelatinous side immersed 

 in the water and their upper black convex side standing out of the 

 water, looking upwards, and appearing to be perfectly dry. The 

 sporangia will float in this way for several days. If one tries to 

 submerge the floating sporangia with a glass rod, it is difficult to 

 drive any of them below the surface of the water, and one perceives 

 that this is due to the resistance of the black sporangial wall to 

 being wetted and the consequent action of the force of surface 

 tension. When a sporangium has been submerged, one can see 



1 C. T. Ingold, "The Sporangiophore of Pilobolus," The New Phylologist, 

 Vol. XXXI, 1932, pp. 58-63. 



2 This volume, p. 46. 



