PHOTIC REACTIONS OF TADPOLES 237 



energy involved is small in amount, a critical quantitative study 

 of the responses may enable us to substitute for the terms 

 'strongly phototropic' and 'slightly phototropic, ' heretofore 

 used in describing degrees of sensitivity, a numerical value for 

 the intensity-time product. A series of such studies on various 

 organisms, if successful, should enable us to arrange them in the 

 order of their photosensitivities. This in turn may throw some 

 light on the organization of the animal and on the extent to 

 which this state serves it in its life. 



Quantitative studies have already been made by Hecht 

 ('18, '19a, b, '20) on the dynamic nature of the responses of 

 Ciona and of Mya. The anunals used in the present study 

 were tadpoles of Rana clamitans. These were especially appro- 

 priate in such a study, since they remain undisturbed in the dark 

 for long periods, and in sudden illumination produce clear-cut 

 responses by forward swunming or by movements of the tail. 



My thanks are due Prof. G. H. Parker, who suggested this 

 problem and supervised the work and for whom, both as teacher 

 and as adviser, I have the most sincere respect. His character- 

 istic way of making suggestions and offering criticisms has 

 served as an inspiration during the time of my association with 

 him. 



II. HISTORICAL REVIEW 



Interest in the photodynamic behavior of animals and plants 

 and the extent of agreement with the Bunsen-Roscoe law has 

 been shown only comparatively recently. Blaauw ('09) was 

 one of the first to undertake an investigation of this subject. 

 In the production of a curvature in the seedlings of Avena sativa 

 when exposed to light of various strengths, he found that the 

 value of the product of the intensity of light and the time re- 

 quired to cause the same number of the plants to bend toward the 

 illuminated side wete nearly constant. Similar observations 

 have been recorded by Froschel ( '09) with work on other plants. 



Loeb and Northrop ('17) used the larvae of barnacles. These 

 animals were found to be strongly phototropic. When lights 



