GENERAL SUMMARY 457 



by violently expelling cell-sap, shoots away the sporangium from the 

 sporangiophore. 



A subsporangial swelling is transparent and refracts light like the bulb 

 of a Florence flask filled with water ; and its diameter is always greater 

 than that of the black sporangium which it supports. The lens action 

 of a subsporangial swelling has been investigated directly and also by 

 means of construction diagrams. 



When the sunhght strikes upon one side of a subsporangial swelling, 

 the light rays are refracted through it and converge so as to form a spot 

 of light on the other side. When the incident rays of light strike the 

 sporangium head on and are exactly parallel to the long axis of the swelling, 

 the spot of light which is formed by the rays entering that part of the 

 swelling which bulges out beyond and around the sporangium is symmetri- 

 cally placed at the base of the swelling. Under these conditions there is 

 physiological equilibrium and no heliotropic response takes place. When, 

 however, the incident rays of light strike a swelling obliquely, the spot of 

 light is placed on one side of the wall of the swelling in a manner which is 

 asymmetrical for the swelling as a whole. Under these conditions, the 

 protoplasm which is lighted by the spot of light sends a heliotropic 

 stimulus down to the protoplasm at the top of the stipe just beneath the 

 base of the swelling. The top of the stipe then reacts by growing in 

 length most rapidly on the side nearest to the spot of light and thus 

 bending as a whole. As a result of this reaction, the swelling is moved 

 about its base through an angle and the spot of light passes downwards 

 on the wall of the swelling until it comes to be symmetrically placed at 

 the base of the swelling. As soon as the spot of light arrives at this 

 terminal position, a physiological state of equilibrium becomes established 

 in the sporangiophore and the heliotropic reaction ceases. At the end 

 of the turning movement the gun is directed toward the source of the 

 brightest light. 



In Pilobolus Kleinii and other Piloboli the protoplasm in the lower 

 part of the subsporangial swelling and at the top of the stipe contains a 

 red pigment (carotin) ; and, at the top of the stipe just above the stipe's 

 motor region, the protoplasm is heaped up so as to form a strongly bi- 

 concave, very red, centrally perforated septum. As shown by direct 

 observations and by theoretical diagrams, this protoplasmic septum is 

 admirably shaped and situated for receiving the light rays converging 

 on it when the gun is in a position of complete or almost complete physio- 

 logical equilibrium. Its concave upper surface, its position in respect 

 to the subsporangial lens, and its strong pigmentation suggest a com- 

 parison in function with the retina of the eyes (ocelli) of certain Mollusca. 



The diameters of the sporangium, the subsporangial swelling, and the 

 motor region of the stipe below the swelling in a well-grown fruit-body of 

 P. Kleinii were observed to be 0-43 mm., 0-76 mm., and 0-16 mm. 

 respectively. A simple calculation based on these data shows that the 



