THE PILOBOLUS GUN AND ITS PROJECTILE 127 



Fig. 59 at D until it was very considerable indeed. If the angle 

 were to be further decreased from 1° to 0°, the two spots finally 

 would coincide with one another. 



Allen and Jolivette ^ investigated the response of Pilobolus 

 sporangiophores to two equal sources of light which were very close 

 together and whose rays formed angles from about 13° to about 3° 

 with one another and, in discussing the results of their observations, 

 they 2 say : " When the two openings which serve as sources of 

 illumination are close together, there are, to be sure, a small number 

 01 sporanges which land about midway between the openings " 

 and " It is possible that the sporanges which fell between and below 

 the openings came from sporangiophores which perceived and 

 reacted to both lights at once, thus aiming at a point between the 

 tw^o openings." Evidently they suspected that with very small 

 angles at least some of the projectiles were aimed midway between 

 the two sources of light although, as they intimate, to admit this 

 as a fact would conflict with their results obtained with large angles 

 and make the explanation of the heliotropic reaction of Pilobolus 

 very difficult. "Why," they ^ ask, " should the resultant reaction 

 to two simultaneous stinmli appear only when the openings are 

 close together ? " 



In connexion with Fig. 50 it was shown above that, when the 

 angles between the incident rays of light from two equal sources of 

 light are very small, the two spots of light partly overlap and, as a 

 whole, can take up a symmetrical position on the protoplasmic 

 septum of the subsporangial swelling. Theoretically, therefore, 

 when the angle between the two beams of incident light is very 

 small, it is possible for the two stimuli produced by the two beams 

 of light to act together and produce a resultant reaction in the 

 motor region of the stipe. It seems to me probable that, under 

 test conditions, such a resultant reaction would be often observed 

 or, in other words, that with small angles between equal or almost 

 equal beams of light the sporangiophores would be found to point 

 more or less between the sources of light rather than at one or the 



1 Ruth F. Allen and Hally D. M. Jolivette, he. cit., pp. 566-569. 



2 Ibid., p. 593. 



3 Ibid., pp. 593-594. 



