THE PILOBOLUS GUN AND ITS PROJECTIT.E 125 



fruit-body. Each beam, after refraction of its rays at the surface 

 of the subsporangial swelUng, will foriu a spot of light on the proto- 

 plasm lining the back wall of the swelling, and the two spots will 

 overlap one another to some extent, as shown diagrammatically 

 at D in Fig. 59. The two heho tropic stimuli produced by the over- 

 lapping spots of light must travel down one and the same side of 

 the swelling to the motor region of the stipe and there combine in 

 causing the stipe to bend heliotropically in the direction of the two 

 sources of light. As the bending of the stipe takes place, the two 

 overlapping spots of light will descend the wall of the swelhng and 

 take up a position on the protoplasmic septum where there is room 

 for both of them. There are two possibilities with respect to the 

 exact position of the two spots relatively to the septum : either 

 (1) the spot which first comes into contact with the septum takes 

 up an exactly symmetrical position over the septum, its centre 



Fig. 59 — cont. 



of equal intensity came and struck a Pilobolus fruit-body 42 cm. distant. The 

 fruit-body reacted in such a way that it pointed its gun at a spot midway between 

 the two sources of light, as indicated at B where the two outer arrows a and b 

 show the direction of the two beams of hght and the middle arrow x the final direc- 

 tion of the axis of the subsporangial swelling and sporangium. C shows the 

 final position of the fruit-body. The rays a and 6 in C have all come through 

 the window a in A and are therefore drawn parallel to a in B. They are refracted 

 into the subsporangial swelling and fall on the protoplasmic septum at p where 

 they form a spot of light {cf. 6 in D) slightly to the right of the centre of the 

 septum. The corresponding rays of light coming through the window b in 

 A, and therefore in a direction parallel with b in B, ha^'e not been represented 

 in constructing C ; but, if they had, it would be seen that they would form 

 a spot of light slightly to the left of the centre of the septum p, as shown at 

 a in D. The two spots of light (cf. a and fe in D) overlap and they can and do 

 both rest on the septmn at the same time, so that the septum is symmetrically 

 illuminated. Hence, finally, the axis of the gim is directed, as shown at x in B, 

 at a spot midway between the two sources of light. 



When the two windows in A are far apart, so that the beams of light striking 

 the Pilobolus fruit-body are inclined to one another at a large angle, say 40°, 

 instead of a small one, say 5°, as in the case already discussed, if the fruit-body 

 is at first pointing at a spot midway between the two sources of light, the two 

 spots of light formed by the two beams on the wall of the subsporangial swelling 

 must be far apart (cf. a and 6 in E) and cannot overlap, one spot being on one 

 side of the swelling and the other opposite to it on the other side of the swelling. 

 Unstable physiological equilibrium must result : one spot of light will stimulate 

 the motor region of the stipe more than the other with the result that the fruit- 

 body will turn towacd the source of light which gives the stronger stimulus, 

 i.e. it will come to jyoint toward one of the windows and not to a spot midway 

 between them. At the end of the reaction, one spot of light will have moved 

 downwards on the wall of the swelling and will have taken up a symmetrical 

 position on the septum whilst the other will have moved upwards and have 

 become much farther removed from the septum than it was originally. The 

 scale F indicates the size of each part of C, D, and E : magnification, 69. In A 

 the vertical lines divide the wall into centimetres. 



