THE PILOBOI.US GUN AND ITS PROJECTILE 123 



right-hand side of the motor region of the stipe from which it is 

 retreating. Thus, in the end, the stimulus given by the descending 

 left-hand spot of light prevails over and completely dominates the 

 stimulus given by the ascending right-hand spot of light, with the 

 inevitable result that the motor region of the stipe bends the sub- 

 sporangial swelling round to the right until it comes to be directed 

 head on to the right-hand beam of light. When this position of 

 heliotropic equilibrium has been established, what was the left-hand 

 spot of light now rests symmetrically on the protoplasmic septum 

 at the top of the stipe, while the right-hand spot of light rests on 

 the right-hand side of the swelling at a distance of about 0*5 mm. 

 from the median plane of the protoplasmic septum which is about 

 equal to twice its original distance from the septum. If the right- 

 hand spot of light when in its fixed position 0-5 mm. from the 

 septum continues to cause a stimulus to be sent down to the right- 

 hand side of the motor region of the stipe, we must suppose that 

 this stimulus is so feeble, relatively to the powerful stimulus sent 

 down equally to all sides of the motor region of the stipe from the 

 septum illuminated by the other spot of light, that its effect on the 

 stipe, so far as causing it to bend is concerned, is negligible. 



Thus, by taking into account the special structure and optical 

 behaviour of a typical Pilobolus fruit-body, it has been possible to 

 explain why it is that a Pilobolus fruit-body, when stimulated by 

 two beams of light coming from sufficiently different directions, 

 reacts by turning to one of the sources of light and pointing to it 

 with as much precision as it would exhibit were the other source 

 of hght non-existent. 



So far we have considered the heliotropic response of the Pilo- 

 bolus fruit-body to two sources of light the rays of which make a 

 relatively large angle, 10°-36° or more, where they meet at the 

 fruit-body's surface. To complete our enquiry, we shall now 

 consider the heliotropic response of the fruit-body to two sources 

 of hght the rays of which make a relatively small angle, say 5° or 

 less, where they meet at the fruit-body's surface. 



Let us assume that the angle between the two equal beams of 

 white light which fall on a Pilobolus fruit-body is 5°, as shown at B 

 in Fig. 59, and that at first the two beams fall upon one side of the 



