THE PILOBOLUS GUN AND ITS PROJECTILE 



89 



from the sporangiophores of other Mucorineae which do not dis- 

 charge their sporangia, e.g. Mucor Mucedo ^ and Sporodinia grandis. 

 The wall of a Pilobolus fruit-body bears numerous very minute 

 crystals of calcium oxalate. In P. longipes these crystals are 

 arranged : most densely on the sporangium- wall, less densely on 

 the wall of the subsporangial swelling, and far less densely on the 

 wall of the stipe (Fig. 44, A, C, and D). They can be most readily 

 observed on the translucent 

 fringe (c/. Fig. 33, No. 3, 

 p. 77) of a discharged spor- 

 angium mounted in chloral 

 hydrate. On the wall of a 

 piece of fringe, in face view 

 (Fig. 44, A), the crystals can 

 be seen to be of two sizes : 

 (1) less numerous larger 

 crystals of various shapes, 

 and (2) far more numerous 

 smaller columnar crystals. 

 Some of these crystals are 

 shown in profile in Fig. 44, 

 B. The larger crystals are 

 2-3 [J, high and the smaller 

 ones about 1 y. high. Van 

 Tieghem, Brefeld, Zopf , and 

 others, in their somewhat 

 diagrammatic drawings of 

 Pilobolus sporangia (Figs. 100, 101, 106, 107, and 110, pp. 203-219), 

 have represented the crystals projecting from the sporangium-wall 

 as being much longer than they actually are. It may well be that 

 the unwettability of the sporangium- wall is due to the numerous, 

 closely-set crystals imprisoning air and thus preventing water 

 from coming into contact with the wall's surface (Fig. 45, A). It 

 is also probable that the drops excreted from the sporangium, 



1 O. Brefeld, Untersuchungen uber Pike, Heft I, Leipzig, 1872, p. 12. Brefeld 

 remarks that the numerous drops excreted from the surface of the young sporangio- 

 phore of Mucor Mucedo have a weak acid reaction. 



Fig. 45. — Pilobolus longipes. Water and the 

 cell-wall. A, diagram showing : a, the 

 black sporangial wall ; b, crystals of 

 calcium oxalate protruding from the wall ; 

 and c, water in which the sporangium has 

 been immersed. Owing to the presence of 

 the crystals and the surface tension of the 

 .water, it is supposed, as here represented, 

 that a layer of air is held between the water 

 and the cell -wall so that the latter cannot 

 readily be wetted. B, diagram .showing : 

 d, the wall of a subsporangial swelling ; e, 

 crystals protruding from the wall ; and 

 /, a drop of water, still very small, which 

 is being excreted. The drop has a very 

 small base and it is supposed that it touches 

 the crystals in the manner shown. Mag- 

 nification, about 2250. 



