oo 



Tnnis. Acad. Sci. of iSt. Louis 



li^Hit. A keen eye will detect roaches riisliin<i^ in all 

 (lircctions to the cracks and cranni<'s. rndnnl)todly thoy 

 are rushinij^ away from the li^^hl. Is that i»chavior a 

 negative phototropism? ]x?t us analyze it experimentally. 

 Arrange a floorless rectangular glass pen. Have the 

 walls and coiling 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 slux-k may he 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 tlie cur- 

 rent. ]t dashes into the light. Tuni olT 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 olT. Shortly it will a])proach 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 i)hototropism has become a positive 

 holiotro])isTn. Let us examiiK^ 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 ])laco it in a ]>iece of 

 ordinary ])oard or cardboard. Place the roach in the 

 runway. Immediately it enters the darkened ]^ortion. 

 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 ncsponscs to I-i(?lit of the I\oaeh (Penplaneta orientalis). 

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



