THE PILOBOLUS GUN AND ITS PROJECTILE 65 



the chamber, the lower part of the chamber was provided with a 

 removable panel shown at B. 



To catch any Pilobolus projectiles that might be shot vertically 

 upwards, the hole h in the top of the chamber was covered either 

 with a circular sheet of glass (like that shown at D) or with a sheet 

 of beaver-board (like that shown at E) having a slotted rectangular 

 aperture in its centre into which could be inserted a sheet of glass 

 of the same size as a photographic half-plate. In the critical 

 experiments the cover E was always used in preference to D. 



To ensure that bright light rays should pass vertically down- 

 wards through the sheet of glass on the top of the chamber, a 

 four-sided box G without top or bottom was set over the sheet of 

 glass as shown at/ in F, and then light coming from the sky through 

 a window a was reflected by means of a mirror d vertically down- 

 wards through the sheet of glass as indicated by the arrow e. 



About 10 A.M. on the day on which an experiment was to be 

 made, the air of the experimental chamber was first moistened by 

 spraying it with water and by enclosing in the chamber dishes of 

 water and wet sheets of blotting paper. As soon as the air of the 

 chamber had become sufficiently laden with water-vapour, the 

 panel B was removed, a culture dish (A, c) was set in the middle of 

 the floor of the chamber, and then the panel B was put back in its 

 place. The Pilobolus guns growing on the dung-balls in the culture 

 dish were then illuminated solely by rays of light coming vertically 

 downwards to them through the sheet of glass at the top of the 

 chamber. Under these conditions the guns adjusted themselves 

 heliotropically and soon came to look vertically upwards in the 

 manner shown at C. Later in the morning the guns discharged 

 their sporangia vertically upwards toward the glass window at the 

 top of the chamber. Since every sporangium sticks to glass where 

 it comes into contact with it, it was always possible to tell whether 



Fig. 2() — cont. 



at (ji) ; g, the top of the case A ; h, the beaver-board cover containing in its 

 slot a slieet of glass i. H, a cros.s-section of the lower part of the case A seen 

 from aVjove when the upper part a lias been removed : a, beaver-board ; b, wood ; 

 c, air. In A the roof of the upper part a rests on four sticks of equal length 

 (c/. a in 1) which in turn rest on the four wooden corners cf the lower part b 

 (cf. b in H). To enable one to change four sticks of the length a (in I) to 

 four of the length b, and so raise the height of the case, one takes away the 

 panel d in A. 

 VOL. VI. F 



