Animal Instincts and Tropisms 257 



in which he pointed out that the so-called fondness of 

 animals for light and blue and for dark and red was 

 simply a case of an automatic orientation of animals 

 by the light comparable to the turning of the tips of a 

 plant towards the window of the room in which the 

 plant is raised.^ 



The phenomenon of a plant bending or growing to the 

 source of light is called positive heliotropism (while 

 we speak of negative heliotropism in all cases in which 

 the plant turns away from the light, as is observed 

 in many roots). The writer pointed out that animals 

 which go to the light are positively heliotropic (or 

 phototropic) and do so because they are compelled 

 automatically by the light to move in this direction, 

 while he called those animals which move away from 

 the light negatively heliotropic; they are automatically 

 compelled by the light to move away from it. What 

 the light does is to direct the motions of the animals 

 and to explain this the following theory was proposed. 

 Animals possess photosensitive elements on the surface 

 of their bodies, in the eyes, or occasionally also in 

 epitheHal cells of their skin. These photosensitive 

 elements are arranged symmetrically in the body and 

 through nerves are connected with symmetrical groups 

 of muscles. The Hght causes chemical changes in the 



^Loeb, J., Sitzungsber. d. physik.-med. Gesellsch. Wurzburg, 1888. 

 Ver Ileliotropismiis der Tiere und seine Uhereinstimmung mit dem IIcHo' 

 tropismus der Pflanzen. Wurzburg, 1889. Arch. /. d. ges. Physiol., 

 1897, Ixvi., 439. 

 17 



