48 TKOPISMS 



unequal. In this condition the equal impulses of locomo- 

 tion will result in an unequal contraction of the muscles 

 on both sides of the animal. As a consequence the animal 

 will turn automatically until its plane of symmetry is in 

 the direction of the rays of light. As soon as this happens 

 the illumination of both eyes and the tension of sym- 

 metrical muscles become equal again and the animal will 

 now move in a straight line either to or from the source 

 of light. What appeared to the older authors as the 

 expression of fondness for light or for darkness was 

 according to the writer's theory the expression of an 

 influence of light upon the relative tension of symmetrical 

 muscles. g 



Animals which are compelled to turn and move to the 

 source of light the writer called positively heliotropic, 

 those which are compelled to turn and move in the oppo- 

 site direction he called negatively heliotropic. The 

 designation heliotropism (or phototropism) was chosen 

 to indicate that these reactions of animals are of the 

 same nature as the turning of plants to the light ; and the 

 writer was indeed able to show that sessile animals bend 

 to the light as do plants which are raised near a win- 

 dow; 288 while motile animals move to (or from) a source 

 of light as do the motile swarmspores of algae or motile 

 algae themselves. 



We will first discuss positively heliotropic motile ani- 

 mals. The positively heliotropic caterpillars of Porthesia 

 chrysorrhcea 288 or the winged plant lice of Cineraria 288 

 or the newly hatched larvae of the barnacle 183 were used 

 by the writer in his earliest experiments and they may 

 serve as examples. The larvae of Porthesia must be used 

 after hibernation before they have taken food. When 

 about 50 or 100 of such larvae are put into a test tube and 



