THE PILOBOLUS GUN AND ITS PROJECTILE 129 



(1) To imitate the condition of the Pilobolus fruit-body when in 

 a state of heUotropic eqiiihbrium. Hold 

 the flask by its neck in the beam of Hght 

 with its long axis parallel to the direction 

 of the incident rays and its black basal 

 ball facing the rays. It will now be found 

 that the rays of light falling on that part 

 of the flask's bulb which bulges out beyond 

 the black ball are refracted so that they 

 converge within the bulb and brilliantly 

 illuminate the cotton-wool plug at the 

 junction of the flask's bulb and neck. 



(2) To imitate the condition of the 

 Pilobolus fruit-body when not in a state 

 of heliotropic equilibrium. Hold the 

 flask in the beam of light M'ith its long 

 axis making a considerable angle with 

 the direction of the incident rays, so that 

 the rays fall obliquely on the flask's bulb 

 and basal ball. It will now be found 

 that the cotton- wool plug is no longer 

 brilliantly illuminated, but that a spot of 

 light is formed by the refracted light rays 

 upon the side of the bulb. The spot of 

 light can be made evident by j^lacing a 

 sheet of white paper against it or near it. 



(3) To imitate the movement of the 

 spot of light down the side of the sub- 

 sporangial swelling of the Pilobolus fruit- 

 body when the stipe, responding to a 

 heliotropic stimulus, is turning the sub- 

 sporangial swelling and sporangium 

 through an angle. Hold the flask as 

 just described in (2), so that the spot 

 of light is upon one side of the bulb. 

 Now turn the flask so that its axis 

 gradually assumes a direction parallel 



VOL. VI. 



Fig. 



60. — Diagrammatic 

 longitudinal median sec- 

 tion through a model used 

 to illustrate the ocellus 

 function of the subspor- 

 angial swelling of Pilo- 

 bolus Kleinii : a, a 

 Florence flask ; b, mould- 

 ing clay adhering to the 

 base of the flask ; c, black 

 tissue paper covering the 

 ball of clay ; d d, water ; 

 e, cotton wool ; /, a cork. 

 When the model is pointed 

 toward the sun or toward 

 an artificial source of light 

 a few feet away, the raya 

 of light are refracted 

 through the bulb of the 

 flask on to the plug of 

 cotton wool, so that this 

 becomes brightly lighted. 

 About one-half the natural 

 size. 



