THE PILOBOLUS GUN AND ITS PROJECTILE 57 



distended by the osmotic (turgor) pressure exerted by the cell-sap 

 in the large central vacuole. This pressure, as will be shown 

 subsequently, is equal to that of about 5-5 atmospheres. Since 

 the inward pressure of the cell-wall must be equal to the outward 

 pressure of the cell-sap, we must suppose that, when a Pilobolus 

 gun is about to be discharged, the distended cell-wall of the 



OS 



mml 



Fig. 22. — To show the relative diameters of the black sporangium a, the subspor- 

 angial swelling 6, and the stipe c in two large wild fruit-bodies of two species 

 of Pilobolus. A, Pilobolus longipes (same fruit-body as that in Fig. 57). B, 

 P. Kleinii. In each drawing is represented an apical view of a fruit-body 

 facing the strongest rays of light and a cross-section of the stipe. In A the 

 radius of the sporangium is 56 per cent, of the radius of the subsporangial 

 swelling and in B 64 per cent. The light rays which strike the surface h are 

 refracted into the subsporangial swelling and form a spot of light which, when 

 the fruit-body is in heliotropic equilibrium, rests symmetrically upon the ring 

 of red protoplasm at the top of the stipe (c/. Fig. 50, D). As may be seen by 

 reference to the mm. scale, the diameters of the sporangium, subsporangial 

 swelling, and stipe in A are 0-58, 1-03, and 0-21 mm. respectively. 



sporangiophore is compressing the cell-sap with a pressure equal 

 to that of about 5-5 atmospheres. 



When discharge of a sporangium takes place, the neck of the 

 subsporangial swelling just beneath the sporangium is ruptured 

 transversely {vide the dotted line a in Fig. 28, p. 70), the wall of 

 the swelling and the stipe contracts elastically, and the cell-sap 

 is squirted out of the top of the swelling, with the result that a large 

 drop of sap carries the sporangium with it through the air. As 

 the projectile begins to describe its trajectory, the collapsing gun 

 (since action and reaction are equal and opposite) flies backwards 

 and immediately strikes the dung-ball to which it is attached. 



