182 SPORE DISSEMINATION 



servations on Peronospora, Fhytophthora infestans, and Botrytis, 

 "The slightest change in the humidity of the surrounding air, such 

 for instance as may be caused by the breath of the observer, at 

 once produces changes in their turgescence and torsion; the lat- 

 ter give a twirling motion to the extremity of the gonidiophore 

 and the ripe spores are thereby thrown in every direction." 



Link, in 1809, was among the earliest observers to consider the 

 problem of discharge of sporangia bv Pilobolus. Since then many 

 others have recorded their studies of this phenomenon, and grad- 

 uallv a clear conception of the mechanism involved has evolved. 

 The ingenious experimentation bv Buller is especially pertinent 

 and illuminating. Members of this genus are coprophilous and 

 can best be studied by cultivation on fresh dung of herbivorous 

 animals, collected and placed in the laboratory in moist chambers. 

 After a few davs a crop of sporangia should have formed, and 

 new crops may form each day for several successive days. Each 

 sporangiophore consists of a hat-shaped, black sporangium that 

 surmounts a bulbous subsporangial swelling, the upper portion of 

 the stipe. This subsporangial swelling functions both as an 

 ocellus that causes the stipe to direct its free end toward the source 

 of light and as a part of the squirting apparatus that propels the 

 sporangium. 



A laver containing bright red pigment, carotene, is formed in 

 the basal wall of the subsporangial swelling. This layer extends 

 partly across the stipe and forms a centrally perforate, biconcave 

 septum. Immediately beneath this perforate septum is the motor 

 region, which responds in such fashion as to direct the sporangium 

 head on toward the light, when heliotropic equilibrium is estab- 

 lished. In this position the incident light is centered on the per- 

 foration of the septum. Bending is a photochemical response, as 

 is also the increased pressure of turgor in the subsporangium that 

 follows when the sporangium faces the light. At the time of 

 expulsion this pressure in Pilobolus longipes may be equivalent 

 to approximately 5.5 atm. 



While these phototropic reactions are taking place, the spo- 

 rangium wall splits into two layers, the inner of which remains 

 intact to enclose the spores. The expansion of the columella, 

 which presses upward against the sporangium, together with the 

 liquefaction of the outer wall circumferentially around the base 

 of the sporangium, results in Assuring of the outer wall. The 



