THE PILOBOLUS GUN AND ITS PROJECTILE 71 



the subsporangial swelling (Fig. 28, g), it is coloured red and heaped 

 up. The redness of these thicker masses of protoplasm, as Zopf ^ 

 has pointed out, is due to their containing carotin held within 

 minute oil-drops. 



The upper heap of protoplasm (Fig. 28, /) is relatively shallow 

 and pale red in colour. It is so situated that, when the Pilobolus 

 gun faces the light, it receives the full force of the incident rays. 

 It is probably photosensitive, and it is possible that its photochemical 

 reactions may serve to bring about chemical changes which increase 

 the osmotic pressure of the cell-sap or weaken the wall of the sub- 

 sporangial swelhng at the place {Riss-slelle) where it is to be ruptured 

 transversely when the projectile is shot away. Since Coemans 

 published his Monographic in 1861, it has been known that, when 

 maturing Pilobolus fruit-bodies are placed in the dark, the discharge 

 of their sporangia is delayed for several hours. ^ There must be 

 some photochemical mechanism by which the light of the sun is 

 employed in preparing the Pilobolus gun for discharge, and it is 

 possible that in the working of this mechanism the upper heap of 

 protoplasm plays an important part. 



The lower heap of protopjasm, shortly before the discharge of 

 the sporangium, usually has the form of a biconcave lens with a 

 rounded vacuolar passage-way in its centre, as shown at g in Fig. 28. 

 The passage-way varies in width but is never absent, so that the 

 protoplasTn at the top of the stipe never forms a complete septum 

 and the vacuole of the subsporangial swelling is always continuous 

 with that of the stipe. 



The lower heap of protoplasm contains numerous oil-drops, 

 with carotin dissolved in them, which, as indicated by shading at 

 g in Fig. 28, are : (1) very densely packed in its upper surface layer 

 which, in consequence, is coloured bright orange-red ; (2) are sparsely 

 distributed in its lower surface layer which, in consequence, is 

 coloured very pale orange-red ; and (3) are practically absent from 

 its inner core which, in consequence, is colourless. 



^ W. Zopf, " Zur Kenntniss der Farbungsursachen niederer Organismen, 

 No. Ill, Phycomyceten-Farbungen," Beitrdge zur Physiologic und Morphologie 

 niederer Organismen, Leipzig, Heft II, 1892, pp. 3-12. 



2 E. Coemans, "Monographic du genre Pilobolus," 1861, loc. cit., pp. 45-46. 



