THE PILOBOLUS GUN AND ITS PROJECTILE 117 



intervals of time. The mirror of the microscope was so turned that 

 it could not reflect any light upwards. From the beginning to the 



Fig. 56. — A glass-ring chamber, as used for observing the heliotropic curvature 

 of the stipe of Pilobolus longipes. A, a plan of the chamber with the 

 cover-glass removed, as seen from above : a, a glass slide covered with 

 black paper ; b, a glass ring covered with black paper c within and with- 

 out except for a slit in the front where, as indicated by the arrow, direct 

 sunlight is entering the chamber ; d, a piece of horse dimg bearing a 

 Pilobolus fruit-body which has just been arranged so that the axis of the 

 subsporangial .swelling is horizontal and perpendicular to the direction of 

 the rays of light which strike it ; the two other masses of horse dung assist 

 in keeping the air of the chamber moist. B, a median vertical section of 

 the chamber with the cover-glass on : a, the glass slide ; b, tlie glass ring ; 

 c c, black paper ; d, the horse dung bearing the Pilobolus fruit -body ; e, tlie 

 cover-glass, covered with black paper except at the very centre where there 

 is a window through which, as indicated by the arrow, the observer watched 

 with the low power of the microscope the heliotropic bending of the stipe of 

 the fruit-body ; sufficient light came up from below to enable these observa- 

 tions to be made without the use of the mirror. C, the whole apparatusseen 

 from in front where the direct rays of sunlight are entering through the slit 

 of glass not covered by black paper : the illuminated subsporangial swelling 

 in the middle of the chamber can readily be observed. Enlarged to 1 ' 33 

 the actual size. 



