THE PILOBOLUS GUN AND ITS PROJECTILE 115 



mass. In Pilobolus the absorption of light by the red pigment may 

 possibly increase the photosensitivity of the protoplasm in which 

 it lies, while in Volvox it seems clear that the red pigment functions 

 by shading the sensitive protoplasm of those eyes which happen 

 temporarily to be turned away from the light. 



The Ocellus of Pilobolus and the Human Eye. — The simple 

 ocellus of a Pilobolus and the complex eye of a human being, in 

 their heliotropic response to light, behave in essentially the same 

 manner ; for they are both in photic equilibrium only when the 

 spot of light formed in their interior by a luminous object, such as 

 a candle flame, is symmetrically placed in the middle of their back 

 wall or retina. If a man and a Pilobolus fruit-body were to be 

 kept for some time in a dark room and then a candle flame were to 

 be suddenly exposed in one corner of it : the man, immediately 

 and instinctively, would turn his head until he faced the candle 

 flame directly and, in so doing, he would unconsciously place the 

 spot of light in each eye in the middle of the retina, i.e. in the region 

 of the blind-spot ; while the Pilobolus fruit- body, very slowly (in 

 the course of an hour or so), would turn its sporangium and sub- 

 sporangial swelling until they faced the light directly and, in so 

 doing, it would automatically place the spot of light in the subspor- 

 angial swelling in the middle of the red protoplasm at the back of 

 the swelling, i.e. so that the centre of the spot of light would rest on 

 the centre of the protoplasmic septum at the top of the stipe ; and 

 the movement of the Pilobolus fruit-body, accomplished simply 

 by the unequal growth of the stipe, and the movement of the human 

 being, accomplished by means of a highly complex nerve-brain- 

 nerve-muscle mechanism, would both be caused by the same physio- 

 logical condition, namely, the asymmetry of a spot of light at the 

 back of a photically sensitive optical organ or eye. Thus, in 

 respect to their behaviour in the presence of incident rays of light, 

 the lowly cryptogam Pilobolus and Man, the lord and master of 

 the organic world, have much in common. 



A Heliotropic Experiment made on Pilobolus longipes. — The 

 heliotropic experiment about to be described was made on a fruit- 

 body of Pilobolus longipes for the following purposes : ( 1) to determine 

 how long it takes for a fruit-body which is illuminated by direct 



