22 Trans. Acad. Sci. of St. Louis 



light. A keen eye will detect roaches rushing in all 

 directions to the cracks and crannies. Undoubtedly they 

 are rushing away from the light. Is that behavior a 

 negative phototropism ? Let us analyze it experimentally. 

 Arrange a floorless rectangular glass pen. Have the 

 walls and ceiling of one-half of this enclosure opaque and 

 of the other transparent. Place this pen on an opaque 

 floor so constructed that, on the touching of a key, an 

 electric shock may be given to any creature standing on 

 it. Thus we have a run-way, one-half of which is in dark- 

 ness and the floor of which is an electric shocking board. 

 Place a roach in the lighted portion of the run-way. Im- 

 mediately it rushes into the darkness. Turn on the cur- 

 rent. It dashes into the light. Turn off the current and 

 it will soon re-enter the darkened portion of the run. 

 Every time it enters the darkness switch on the current; 

 as soon as it leaves the darkened portion turn the cur- 

 rent off. Shortly it will approach the entrance to the 

 darkened run-way very cautiously; later it will halt at 

 the dividing line and refuse to enter the darkness and 

 will often retreat to the other end of the run-way.* Per- 

 haps you think we have simply reversed the tropism; 

 what was a negative phototropism has become a positive 

 heliotropism. Let us examine this behavior a little 

 more. As soon as the roach has thoroughly formed the 

 habit of refusing to enter the darkness, remove the run- 

 way from the shocking board and place it in a piece of 

 ordinary board or cardboard. Place the roach in the 

 runway. Immediately it enters the darkened portion. 

 Restore the run to the shocking board and place the roach 

 therein. It now refuses to enter the darkness. Evi- 



< Turner, C. H.; An Experimental Investigation of an apparent Re- 

 versal of the Responses to Light of the Hoach (I'ertplaneta orientaliM). 

 Biol. Dull., 1912, Vol. XXIII, pp. 371-386. 



