The Response of Pilobolus to Light. 
BY 
ROSALIE PARR, 
With four Figures in the Text. 
Contents. 
PAGE 
Introduction .... 177 
I. Theories of Response .178 
II. Statement of Problem. .182 
III. Materials and Methods. 182 
IV. Experimental Data and Results. 187 
V. Conclusions and Theoretical Discussion. 200 
Summary. 203 
Introduction. 
M UCH work has been done on the response of organisms to light. 
Naturally all of the earlier and a large part of the later work was 
qualitative in nature. Up to the time of Wiesner’s classic work on helio- 
tropism, no attempt had been made to express the photic sensibility of 
plants in quantitative terms. Since the publication of his ‘ Die heliotropischen 
Erscheinungen im Pflanzenreiche ’ (1879), little advance was made along 
quantitative lines until 1909, when Blaauw published ‘ Die Perzeption des 
Lichtes In this important contribution modern physical methods are for 
the first time employed. The conclusions of Blaauw, however, are not 
in agreement with those of Wiesner, and both contradict without adequate 
explanation the results of the earlier investigators on phototropism, such as 
Gardner, Guillemin, Muller, and others. 
While considerable progress has been made in the study of the 
threshold of stimulation—more especially as related to duration and inten¬ 
sity of the light stimulus—we have no complete record of the response 
of a given organism to carefully graded and measured light energies in the 
different spectral regions. 
[Annals of Botany, Vol. XXXII. No. CXXVI. April, 1918.] , 
... N . 
