342 PHYSIOLOGY OF THE FUNGI 



sporangial swelling, acting as a lens, causes the light rays to converge on 

 the side of the swelling away from the source of light (Fig. 69A). Pre- 

 sumably, this causes a photochemical reaction in the protoplasm, and the 

 stimulus is transmitted downward to the motor region, the portion of the 

 sporangiophore just below the subsporangial swelling. The growth of 

 this region is more rapid on the side away from the light source, which 

 results in a bending of the sporangiophore until an equilibrium of light is 

 again reached; i.e., when the sporangium is pointing directly toward the 

 source of light (Fig. 695). 



There is a thicker layer of protoplasm near the upper portion of the 

 subsporangial swelling. This layer also contains some carotene. Buller 

 believes that it is photochemical ly reactive and may serve to bring about 

 chemical changes which result in the increase in the osmotic pressure of 

 the cell. When a culture with nearly mature sporangia is placed in the 

 dark, a much greater time is required for the discharge of the sporangia 

 than when it is left in the light. 



The weakest place in the wall of the Pilobolus structure is located just 

 below the sporangium, and it is here that the wall of the subsporangial 

 swelling breaks circularly as the sporangium is discharged. The increased 

 osmotic pressure becomes too great for the resisting elastic wall, and the 

 system is ruptured. A drop of cell sap is squirted out of the tip of the 

 subsporangial swelling as discharge occurs. The conical columella is also 

 carried away with the sporangium. 



In nature the sporangia adhere to the surface of vegetation, where they 

 may be ingested by herbivorous animals. The spores are released in the 

 digestive tract and pass out in the feces unharmed. It seems probable 

 that exposure to gastric juices helps to break dormancy of the spores and 

 favors immediate germination. The sporangium adheres to the vegeta- 

 tion by the lower gelatinous part, with the black, hemispherical, non- 

 wettable portion outward. Thus, the injurious ultraviolet rays are 

 screened out, and the spores remain viable, although they may not be 

 eaten for weeks or even months. 



Basidioholus ranarurti shows a great many characters similar to those of 

 Pilobolus, namely, the general structure of the sporangiophore, the 

 mechanism of discharge of the sporangia, the coprophilous habit, and the 

 general method of dissemination. The sporangiophore consists of a 

 slender lower portion and cylindrical enlarged upper portion, which 

 supports a spherical sporangium. The osmotic pressure in the spo- 

 rangiophore increases to the point that it exceeds the tensile strength of 

 the resisting wall, which is suddenly ruptured circularly near the base 

 of the enlarged portion (Ingold, 1934). At this instant, the upper portion 

 contracts and causes the cell sap to be squirted backward, giving a rocket- 

 like effect. The dissemination of spores is accomplished after the spo- 



