134 EFFECTS OF RADIATION ON FUNGI 



the subsporangial swelling is an ocellus which acts as a lens. 

 When the fruiting structures are in heliotropic equilibrium, the 

 light is focussed on a red perforate septum at the base of each 

 subsporangium. The red color is imparted by a carotinoid pig- 

 ment. The sporangium itself is black and hence casts a shadow 

 on some part of the subsporangium beneath. According to Buller 

 (1934), the sequence of events is somewhat as follows: Light 

 strikes the upper end of the fruiting body, and the incident rays 

 enter the part of the upper wall of the subsporangium that bulges 

 around the sporangium. They are focussed on the wall below, 

 where a region of greater photochemical activity is thereby pro- 

 duced. The stimulus is thus transmitted to the motor region of 

 the stipe (stalk of sporangiophore), and in response differential 

 growth occurs. .Most rapid growth, as has been discussed in 

 Phy corny ces mteiis, occurs on the side of the stipe nearest the 

 area where the light is focussed. As a consequence the ocellar 

 mechanism is tilted until the rays fall symmetrically upon the 

 red perforate ring at the base of the subsporangium. In this posi- 

 tion a state of physiological equilibrium becomes established, and 

 the sporangium is directed head on toward the light. Some ap- 

 preciation of the rate of response may be gained from Buller's 

 observation on P. longipes, in which he found the stipe capable of 

 turning through an angle of 90° and of completely orienting the 

 sporangium in about an hour. 



The discharge of the sporangium is also the result of photic 

 effects. When the rays are properly centered, the photochemical 

 reactions on the protoplasmic content of the subsporangium re- 

 sult in increased osmotic pressure. Eventually the pressure is 

 sufficient to separate the subsporangial and sporangial walls, the 

 rupture beginning as a collar around the periphery of their zone 

 of contact. When this release of tension occurs, the subsporangial 

 wall, being weakest beneath the sporangium, bursts. The spo- 

 rangium is thus carried away by a squirting process. 



Among the striking observations on Pilobolus made by Allen 

 and Jolivette (1914), as has been mentioned, was that, if two 

 equal beams of white light are converged upon the fruiting bodies, 

 with an angle of convergence greater than 10°, the sporangio- 

 phores direct the sporangia toward one or the other of the two 

 sources of light. Contrary to expectations, the aim of the spo- 

 rangiophores is therefore not in the direction of the resultant of 



