THE PILOBOT.US GUN AND ITS PROJECTILE 121 



to prevent each set of rays — or each individual light ray for that 

 matter — from setting up those changes in the protoplasm which 

 constitute the perception of a stimulus, and nothing to prevent 

 these simultaneous stimuli from acting together to produce a 

 resultant reaction. But this does not occur. The visible reaction 

 of each sporangiophore is to one and one only of the two possible 

 sources of stimulation." 



The fact that a Pilobolus fruit-body, when illuminated by two 

 equal beams of white light having sufficiently different directions, 

 reacts apparently to one source of light and not the other, which so 

 much puzzled Allen and Jolivette in 1914, can readily be explained 

 on the theory already set forth : that the spot of light formed by 

 the subsporangial swelling on the protoplasm on the wall of the 

 swelhng farthest away from the source of light gives the illuminated 

 patch of protoplasm a photochemical stimulus ; that the stimulus 

 (by diffusion of a growth-promoting substance through the proto- 

 plasm or otherwise) is conducted to the motor region of the stipe ; 

 and that the motor region of the stipe reacts to the stimulus by 

 growing faster on the side nearest to the source of the stimulus 

 than on the opposite side and thus bends heliotropically. An 

 endeavour will now be made to apply this theory to the problem 

 in hand.^ 



Let us suppose that a Pilobolus fruit-body, like that shown in 

 Fig. 47 (p. 92), is pointing vertically upwards and that it is illu- 

 minated from above by two equal beams of white light which cross 

 one another on their way to the fruit-body at an angle of 38° and 

 each of which makes an angle of 19° with the axis of the subsporangial 

 swelling. One of these beams is shown in Fig. 47 striking the 

 right-hand side of the swelling, and the other beam which strikes 

 the left-hand side of the swelling and is like the first but reversed 

 in direction can be readily imagined. 



By reference to Fig. 47 (p. 92), it can be seen that the right- 

 hand beam forms a spot of light st on the left-hand side of the sub- 

 sporangial swelling at a distance of about 0- 25 mm. above the median 



1 For an account ot Van der Wey's solution of this problem'and for the reason 

 which induced me to give here my own solution, which is in conformity with his, 

 vide Chapter I, pp. 44-45. 



