THE PILOBOLUS GUN AND ITS PROJECTILE 137 



u 



protoplasm. Since, when maturing fruit-bodies are placed in 



the dark, the discharge of the sporangia is delayed for several 



hours, it may well be that the weakening of the wall along 



the splitting line is 



controlled by Hght, 



and it is possible that 



the shallow heap of red 



protoplasm which lies 



in the subsporangial 



swelling just below the 



sporangium and is 



fully exposed to the 



light(/inFig. 28,p. 70) 



is concerned with the 



control mechanism. 



The columella, 

 which is carried away 

 with the sporangium 

 when this is discharged, 

 has a wall which, except 

 for gelatinous swelling 

 at the peak,^ remains 

 very thin even after 

 the sporangium has 

 been discharged and it 

 is no longer pressed 

 against by the cell- sap 

 of the subsporangial 

 swelling. Evidently, 

 the wall of the colu- 

 mella differs from that 

 of the subsporangial swelling in being but slightly contractile. 



Occasionally, when a fruit-body of Pilobolus is placed in water on a 

 slide under the microscope, it explodes in such a way that the wall of the 

 subsporangial swelhng is torn into several fragments some of which 

 at once become curled up spirally like a roll of paper (Fig. 65, A-C). 



1 C/.p. 81. 



Fig. (55. — Pilobolus Kleinii. A, one of the fragments 

 of a subsporangial wall which burst when the 

 fruit-body was placed in water : when set free it 

 at once became spirally rolled. B, the same, 

 represented in transverse section. C, the same, 

 represented as flattened out. D, vertical section 

 of a fruit-body at the junction of the stipe and 

 subsporangial swelling, to show the perforate 

 protoplasmic septiun in situ. E, a protoplasmic 

 septum which was expelled from the spor- 

 angiophore as the sporangiophore contracted and 

 discharged its sporangium under water. F, a 

 similar septum. Magnification, A-C, 293 ; 

 D F, 67. 



