PHOTOTROPISM 129 



species to react to light in the matter of sporulation, (2) staling 

 produced by mycelial growth, provided that the amount of stal- 

 ing does not in any way impede the extension of the mycelium, 

 and (3) the concentration of nutrients available, which should 

 not permit of sporulation so intense as to interfere with the pro- 

 duction of successive daily zones. 



PHOTOTROPISM 



The fact that fungi lack chlorophyll and that certain species 

 are capable of completing their entire developmental cycle in 

 the absence of light might at first thought incline the student to 

 the belief that they would not react phototropically. On the 

 other hand, it might be anticipated that radiant energy would 

 influence rate of growth, and that consequently species in which 

 growth or elongation is localized might respond to differences in 

 the intensity of light. Species with long sporangiophores, such 

 as Pilobolus or Phy corny ces nitens, or with long conidiophores, 

 such as Aspergillus clavatus, should be especially suited for studies 

 of this sort, since they respond to unilateral illumination. It 

 should be possible with such species to determine the minimal 

 amount of light required to stimulate a phototactic response and 

 to establish that quality, quantity, and duration of exposure are 

 functions of each other. In P. nitens it has been found that 

 response follows a change of % to % candlepower per meter 

 per second. Response may not occur immediately, so that re- 

 action time may be said to consist of an exposure and a latent 

 period. The duration of this latent period is constant for any 

 particular intensity unless the exposure time is reduced below the 

 minimum threshold; below this minimum the reaction time in- 

 creases progressively as the duration of exposure decreases. 



Among Phycomycetes. In response to light, Phy corny ces 

 nitens has long been known to bend in a zone just beneath the 

 sporangium. Blaauw (1914), who investigated the phototropic 

 response of this fungus by use of physical methods, regarded the 

 sporangiophore as a cylindrical lens which concentrates the light 

 on the cell wall opposite the source, causing greater photochemi- 

 cal activity in this area. The net result of this photochemical 

 action is bending of the sporangiophore, a response to unequal 



