THE PILOBOLUS GUN AND ITS PROJECTILE 135 



by imprisoning air and thus preventing the water from coming 

 directly into contact with its surface. 



The level at which the wall of the subsporangial sweUing splits 

 across when the sporangium is discharged (called by de Bary the 

 Riss-stelle) is indicated by a dotted hne in Figs. 28 (p. 70) and 32 

 (p. 76), and by the upper dotted line in Fig. ^62. The splitting is 

 perfectly accomphshed, so that the edge of the mouth of the con- 

 tracted subsporangial swelling is always smooth and circular (Fig. 62, 

 B-G). Doubtless, therefore, the wall at the abscission level is 



Fig. 63. — Pilobolus Kleinii. Vertical median section through the top part of a sub- 

 sporangial swelling : A, just before the discharge of the sporangium ; and B, 

 a fraction of a second after discharge when the cell-wall has contracted. A : the 

 upper broken line indicates the level of absci-ssion just beneath the sporangium 

 (c/. Fig. 32) ; a, the cell-wall stretched by the pressure of the cell-contents b. 

 B, the wall has contracted and that part between the two broken lines in A 

 has thickened and formed the lip or collar c c (c/. c in Fig. 62). Magnification, 

 300. 



prepared for breaking whilst the wall is being formed and long 

 before the discharge of the sporangium. Some evidence in support 

 of this view will now be given. If one strokes a ripe sporangium off 

 its columella (c/. Fig. 31, p. 75) and examines the wall of the turgid 

 subsporangial swelling and columella with the microscope, the wall 

 appears everywhere about equally thin (c/. Fig. 30, B, p. 73). If 

 now one ruptures the stipe, so that the turgidity of the whole 

 sporangiophore is lost, the wall of the subsporangial swelling and 

 columella contract. It is then seen that the wall is fully twice as 

 thick immediately under the abscission level as it is immediately 

 above. Also, when a subsporangial swelUng and attached columella 

 are treated with chlor-zinc iodine, the upper part of the wall of the 

 subsporangial swelhng swells greatly except at the abscission level. 



