REGULATION OF REACTIONS 265 



primary aim in this chapter to consider the factors involved 

 in producing these changes. 



I. Change in Sense of Reactions 



a. Effect of intensity of light. — Famintzin (1867, p. 20) 

 appears to have been the first to observe and record, 

 although not very definitely, that the sense of reaction in 

 organisms depends upon the intensity of the light. He 

 placed a shallow dish containing Chlamydomonas and Eu- 

 glena in diffuse daylight and a similar one in direct sunlight, 

 and covered about three-fourths of each at the room side 

 with an opaque screen. In the diffuse light the organisms 

 collected at the window side of the dish; in the direct sun- 

 light they collected in the shadow of the screen at the 

 opposite side. In 1872 Miiller recorded the observation 

 that seedlings of Lapidium, w^hich bend toward the source 

 of light if it is moderately strong, turn and bend in the oppo- 

 site direction if it is very intense, e.g. direct sunlight. 

 Strasburger (1878, p. 572) was perhaps the first to actually 

 see motile organisms turn about when the light intensity 

 was changed and swim in the opposite direction. He found 

 that various swarm-spores, which were strongly negative in 

 a given light intensity, became positive when the microscope 

 was moved farther from the window, but that they turned 

 about and swam in the opposite direction when the micro- 

 scope was again brought to the window. " Falls die 

 Schwarmer nicht zu grosse Neigung haben sich niederzu- 

 setzen, kann dies Spiel beliebig wiederholt werden." 



Reactions similar to those mentioned above were seen 

 by Stahl (1880, p. 412) in Vaucheria, by Berthold (1882) 

 in marine algae, by Verworn (1889, p. 50) in diatoms, by 

 Wiesner (1880, p. 38) in tendrils of Ampelopsis and Vitis, 

 by Oltmanns (1897, p. i) in Volvox, Phycomyces and vari- 

 ous seedlings, by Lubbock (1884) and Ostwald (1907) in 

 Daphnia, by Wilson (1891) in Hydra, by Frandsen (1901) 

 in Limax, by Radl (1901, p. 83) in Simocephalus sima, by 



