i8o COMPARATIVE PHYSIOLOGY OF THE BRAIN 



ensues and a greater resistance is thus offered to the 

 increase in length on this side than on the opposite 

 side. The result is that the stem bends and becomes 

 concave on the side toward the light. As soon, how- 

 ever, as the bending has progressed so far that the 

 stem comes into the direction of the rays of light, all 

 the symmetrical elements are struck by the light at 

 the same angle. The intensity of light is thus equal 

 at symmetrical points, and there is no longer occasion 

 for the stem to leave this direction. It thus con- 

 tinues to grow in the direction of the rays of light. 

 Negatively heliotropic elements, roots, for instance, 

 differ from positively heliotropic elements in that the 

 light produces a relaxation of the protoplasm. Hence 

 when the light comes from one side, the resistance to 

 the growth on that side will be less than on the op- 

 posite side, and the tip will bend away from the 

 source of light. As soon as the tip comes into the 

 direction of the rays of light and the symmetrical 

 points are all struck by them at the same angle, the 

 intensity of the light on both sides is the same, and 

 every cause for leaving this direction is removed. It 

 has been known for a long time that many animals 

 are " attracted " by the light and fly into the flame. 

 This was considered a special instinct. It was said 

 that these animals loved the light, that curiosity 

 drove them into it. I have shown in a series of 

 articles, the first of which appeared in January, 1888, 

 that all these actions are only instances of those phe- 

 nomena which were known in plants as heliotropism. 



