THE PILOBOLUS GUN AND ITS PROJECTILE ^z 



Shortly before the sporangium is to be discharged, the gelatinous 

 ring absorbs water, swells up considerably, presses strongly against 

 the sporangium-wall, and thus causes the wall to split transversely 

 into two very unequal parts : ( 1 ) a relatively small lower part in 

 the form of a narrow band which remains attached to the columella 

 and the subsporangial swelUng (Figs. 29, B, c, and 30, B) ; and (2) a 

 relatively large upper part in the form of a free convex cap. The 

 gelatinous ring pushes the sides of this cap outwards and somewhat 



Fig. 30. — Pilobolus Kleinii. Dehiscence and discharge of a sporangium, illustrated 

 semi-diagrammatically with median vertical sections. A, before dehiscence : 

 a, the top of a subsporangial swelling and b, the columella, both lined with a 

 thin layer of protoplasm and having the great central vacuole filled with 

 cell-sap ; c, the black sporangial wall ; d, the spores ; and e, a layer of jelly 

 between the spores and the wall of the sporangium and columella, now swelling 

 and about to cause the dehiscence of the sporangium. B, after dehiscence : 

 the layer of jelly e has swollen to such an extent that it has caused the 

 sporangium -wall to break transversely into two parts, an upper cap-like part 

 and a basal band (c/. Fig. 29, B) ; the jelly now protrudes through the gap in 

 the wall ; the broken transverse line at the top of the subsporangial swellmg 

 indicates where abscission will take place when the sporangium is discharged. 

 C, the sporangium just after it has been shot into the air : e, the protruding 

 jelly which will serve to attach the sporangium to some substratum ; s, part of 

 a globule of cell-sap which has been shot out of the subsporangial swelling and 

 is attached to the gelatinous, wettable, under side of the sporangium. 

 Magnification, 66. 



upwards and thus itself becomes exposed to view. After the 

 sporangium- wall has thus been pushed outwards and broken, the 

 sporangium has the appearance shown in Figs. 28, 29, B, and 30, B. 

 The breaking of the sporangium-wall into two parts a short 

 distance above its base and the exposure of the gelatinous ring has 

 been referred to by van Tieghem and other systematists as a process 

 of dehiscence. Usually, when a sporangium dehisces, e.g. in Mucor, 

 Saprolegnia, and Ascobolus, its spores are immediately Hberated, 

 but this is not so in Pilobolus ; for, when the sporangium- wall of 

 Pilobolus breaks, the spores are prevented from escaping from the 

 sporangium by the gelatinous ring which fills the gap between the 



