264 Animal Instincts and Tropisms 



a plant bend to the light and is a special case of the 

 same law of Bunsen and Roscoe which also explains 

 the photochemical effects in inanimate nature; or in 

 other words, the will or tendency of an animal to move 

 towards the light can be expressed in terms of the 

 Bunsen-Roscoe law of photochemical reactions. 



The writer had shown in his early publications on 

 light effects that aside from the heliotropic reaction of 

 animals, which as we now know depends upon the 

 product of the intensity and duration of illumination, 

 there is a second reaction which depends upon the 

 sudden changes in the intensity of illumination. These 

 latter therefore obey a law of the form : Effect = f (^j) . ^ 

 Jennings has maintained that the heliotropic reactions 

 of unicellular organisms are all of this kind, but in- 

 vestigations by Torrey and by Bancroft'' on Euglena 

 have shown that Jennings's statements were based on 

 incomplete observations. 



4. In these experiments only one source of light was 

 applied. ''When two sources of light of equal intensity 

 and distance act simultaneously upon a heliotropic 

 animal, the latter puts its median plane at right angles 

 to the line connecting the two sources of light. "^ 

 This fact has been amply verified by Bohn, by Parker 

 and his pupils, and especially by Bradley Patten, who 



* Loeb, J., Arch. J. d. ges. Physiol., 1893, liv., 81; Jour, Exper. ZooL, 

 1907, Iv., 151. 



' Bancroft, F. W., Jour. Exper. ZooL, 1913, xv., 383. 

 sLoeb, J., Studies in General Physiology, Chicago, 1905, p. 2. 



