PHOTOTROP1SM 



133 



This value decreases with a decrease in energy value of the spec- 

 tral regions. 



The method bv which Pilobolus aims and discharges its spo- 

 rangia toward the source of light was elucidated by Buller in a 

 series of observations that began in 1919. He discovered that 



c 



& 



50 



55 60 65 70 



Presentation time (minutes) 



75 



Fig. 18. Response of Pilobolus to light from two sources to show relation 

 of presentation time to frequency of light. Presentation time is regarded as 

 the time required for one-half of the sporangiophores to respond. The time 

 required for heliotropic response is seen to decrease progressively from red 

 light waves (those with low frequency) through yellow, green, blue, violet, 

 and indigo (those with high frequency). (After Parr.) 



