40 THE BEHAVIOR OF LOWER ORGANISMS. 



region lighted directly from the window, that they react by turning 

 toward the right aboral side and passing back into the shadow. It is 

 thus clear that it is the light coming from the ivindotv to which they 

 react, not the light reflected from the sides of the dish. We have here 

 realized the condition concerning which there has been so much dis- 

 cussion, and which has been considered impossible and unrealizable by 

 various authors — a negative organism reaching the darker region by 

 swimming toward the source of strongest light. 



This would of course be quite inexplicable on the tropism theory as 

 set forth by Holt & Lee. What does it indicate as to the real nature 

 of the reaction? To this inquiry there can be but one answer. The 

 organism reacts on passing from a darker to a lighter area, without 

 regard to the direction from which the light comes. It reacts to the 

 increase in the amount of light falling upon it as compared with the 

 condition an instant before it had passed into the lighted area. The 

 reaction takes the usual form — a backing and turning toward the right 

 aboral side, followed by a forward motion. The organism, therefore, 

 is directed again toward the shaded area, which it enters. 



In all our experiments thus far there have been marked differences 

 in the illumination of different areas. Let us now arrange the condi- 

 tions so that light comes from one side, and all parts of the vessel are 

 equally illuminated. This may be done by placing the Stentors in a 

 glass vessel with plane walls at one side of a source of light, such as a 

 window or the bulb of an incandescent electric light. The Stentors, 

 after a very short interval in which the reaction seems indefinite, swim 

 away from the source of light, thus gathering at the side away from 

 the window, where they move about in a disordered way. During the 

 reaction the Stentors are oriented, with the longitudinal axis in the 

 general direction of the light rays and with the anterior end away from 

 the source of light. 



Thus while it is true that the direction of the rays of light has little 

 if any effect on the reaction when the animals are at the same time 

 subjected to a sudden change from dark to light, it does determine the 

 direction of movement when acting alone. In order to discover just 

 how the reaction occurs it is necessary to observe the animals at the 

 moment when they change from their former undirected swimming to 

 the movement away from the source of light. 



For determining this a large number of Stentors are placed in the 

 dish next the window on a dark background. The light comes from 

 one side and a little from above. The direct rays of the sun were not 

 employed. 



Above the glass vessel are focused the lenses of the Braus-Driiner 



