458 RESEARCHES ON FUNGI 



sporangium, when head on to a beam of parallel light rays, casts a shadow 

 which has an area 7-2 times that of a cross-section of the stipe. If there 

 were no subsporangial swelling, the shadow of the sporangium would cut 

 off the light from the top of the stipe before the ortho-hehotropic position 

 had been completely attained. This would prevent the gun from being 

 accurately directed toward the source of light. Evidently the difficulty 

 of supplying the stipe with its required delicate hehotropic stimulus has 

 been surmounted in the course of evolution by the intercalation between 

 the sporangium and the stipe of the large light-collecting subsporangial 

 swelling. That part of the swelling which bulges out beyond the black 

 sporangium receives the Hght and concentrates it by refraction upon the 

 base of the sweUing ; and, when the spot of light so produced is asym- 

 metrical in position, it provides the stimulus to which the motor region 

 of the stipe can react, 



A subsporangial sweUing is capable of producing not only a spot of 

 light but also a more or less well-defined image of the source of light or 

 of an illuminated object. Such images have been observed and photo- 

 graphed. 



The mechanism of heUotropic response in Pilobolus has been compared 

 with that of (1) the leaves of certain Flowering Plants, (2) a free-swimming 

 Volvox sphere, and (3) Man. 



In the heliotropic reaction of Pilobolus, which is caused by the asym- 

 metrical position of a spot of light in the subsporangial swelUng with a 

 consequent bending of the stipe, we have clear proof of the theory, first 

 suggested by Haberlandt, that heliotropic reactions may take place in 

 plants through oceUus action. 



The sporangiophore of Pilobolus is the only ortho-heliotropic organ 

 known which takes up its positively heliotropic position owing to the 

 possession of a special light-perceiving cell structure. 



Pilobolus may well be described as a fungus with an optical sense 

 organ or simple eye (ocellus) ; and, in using its eye for laying its gun, it is 

 unique in the plant world. 



In bright sunlight directed perpendicularly to the long axis of the 

 subsporangial swelling, a stipe of P. longipes was observed to turn the 

 swelling and the sporangium through an angle of 90'^, and thus to complete 

 its heliotropic reaction, in about one hour. 



A solution of the problem why it is that a Pilobolus fruit-body turns 

 so as to face directly one of two equal beams of light coming from suffi- 

 ciently different directions, and does not take up a position of resultant 

 reaction, has been given. It confirms Van der Wey's solution of the 

 same problem, published by him in 1929, and is based on the assumption 

 (1) that the subsporangial swelling acts as an oceUus and (2) that the 

 pigment zone at the base of the swelHng is the region of protoplasm where 

 perception of light takes place. Two spots of light are formed by the 

 two beams of light in the swelling : the spot of light nearer the base of 



