CHAPTER XIV 



FACTORS INVOLVED IN REGULATING REACTIONS TO 



LIGHT — VARIABILITY AND MODIFIABILITY IN 



BEHAVIOR (continued) 



I. Changes in Sensitiveness, in the Optimum, and in Vari- 

 ous Other Features Regarding Reactions 



The sensitiveness and the optimum vary greatly in 

 different organisms and in the same organism under differ- 

 ent conditions. In some the optimum is nearly total 

 darkness, in others it is direct sunlight, 5000 ± ca. m. 

 Some are negative in extremely low intensities, others are 

 positive in equally low intensities. The flatworm Bipa- 

 lium kewense, e.g., avoids light so weak that it barely 

 affects the human eye, and responds to the slightest 

 changes in illumination; and the plants Lepidium sativum, 

 Amaranthus melancholicus ruber, Papaver paeoniflorum, 

 and Lunularia biennis bend toward the source of light 

 in an intensity as low as 0.00033 ca. m. (Figdor, 1893). 

 Some organisms are usually negative in direct sunlight, 

 and the intensity may be changed thousands of candle 

 meters without a response. Other organisms are positive 

 in equally high light intensity. What interests us here 

 chiefly is not the difference in response in different species, 

 but variability in response, and the changes in sensitive- 

 ness and in the optimum in given individuals and the 

 regulation of such changes. 



Strasburger (1878) found that if swarm-spores are kept 

 in light of relatively high intensity their optimum is much 

 higher than if they are kept in weak illumination. These 

 organisms, then, adapt themselves in some way to the 



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