REACTIONS TO LIGHT IN CILIATES AND 

 FLAGELLATES. 



In the reactions to light we are dealing with a stimulating agent 

 which differs in one very important respect from chemicals and from 

 heat or cold. The distribution of the agent with which we are con- 

 cerned is not affected by the currents of water produced by the organism ; 

 hence there is no tendency for one side or part of the organism to be 

 more strongly affected than the rest, as was found to be the case for 

 chemicals and for heat and cold. This peculiarity light shares with 

 the electric current and with radiant heat. The conditions demanded 

 for immediate orientation through direct action of the agent on the 

 locomotor organs, in the manner required by the general theory of 

 tropisms as set forth in the foregoing paper (p. 7), are therefore present. 

 In a recent paper Holt & Lee (1901) have attempted to show that 

 the reactions of organisms to light actually take place in accordance 

 with this theory. 



We shall examine the reactions to light in Stentor cceruleus and in 

 certain flagellates, in order to determine whether they take place in 

 accordance with the tropism schema, and, if not, just how they do occur 

 and on what factors they depend. 



THE CILIATA. 

 STENTOR C^RULEUS. 



As is well known, very few of the ciliate infusoria react to light 

 Light reactions have been described by Engelmann (1882, a) for several 

 chlorophyllaceous ciliates ; by Verworn (1889, Nachschrift) for Pleuro- 

 ncma chrysalis; and by Davenport (1897) and Holt & Lee (1901) for 

 Stentor cceruleus. In none of the ciliates have the reactions been 

 described in sufficient detail to enable us to determine their exact 

 nature. 



In Stentor cceruleus the reaction to light manifests itself in the 

 culture dish by the usual aggregation of the organisms at the side away 

 from the window. If a number of Stentors are removed to a watch 

 glass or trough, and this is placed near a window or other source of 

 light, most of the Stentors are soon found on the side of the vessel away 

 from the light. If one-half of the glass is shaded by a screen, most of 



