REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 147 



in the swarm spores as in Euglena. As set forth on page 139, the move- 

 ment toward or from the source of light, in a field of which all parts 

 are equally lighted, is due to the fact that in the unoriented individuals 

 the sensitive anterior end is subjected to frequent changes in the inten- 

 sity of illumination. It is first directly lighted, then shaded. These 

 changes induce reaction. By the method of trial the organism then 

 comes into a position such that these changes cease. Such a position 

 is found only in orientation. All these relations evidently hold equally 

 well for the swarm spores ; for details the reader may refer to the ac- 

 count of the behavior of Euglena. 



What happens if the field containing the organism is lighted from 

 one side, and there are at the same time variations in the intensity of 

 light in different parts. of the field? Strasburger devised certain ex- 

 periments to answer this question. These experiments have becqme 

 celebrated, and an immense amount of ingenuity has been expended 

 in endeavoring to interpret them in one way or another. Strasburger 's 

 experiments involved the use of the wedge-shaped prism shown in 

 Fig. 99. This prism was placed over the drop containing the swarm 

 spores, in such a way that the light came obliquely from the direction 

 of the thick end of the wedge, as in Fig. 99, Y. Now the intensity of 

 illumination is greater on the side farthest away from the source of 

 light, and decreases as we pass toward the source of light. Will the 

 positive swarm spores move toward the source of hght, and thus into a 

 region of less illumination, or will they rather move into the region of 

 greater illumination, and thus away from the source of light ? 



Strasburger found that the positive swarm spores move toward the 

 source of light, and hence into the region, of less illumination. It is 

 extraordinary that this result should have occasioned the surprise and 

 comment which have been bestowed upon it. Strasburger's previous 

 experiment with perpendicular light (Fig. 99, X) had shown that the 

 variations in intensity of illumination in different parts of a drop under 

 this prism were too slight to cause reaction, the organisms remaining 

 scattered throughout the drop. Evidently so far as the organisms were 

 concerned these sUght variations did not exist ; they were not perceived. 

 Therefore, when the light comes from one side, the organisms react 

 exactly as they do when such variations do not exist. They swim 

 toward the source of light for the same reason that they do when the prism 

 is not present. The experiment consists essentially in making the differ- 

 ences in the intensity in neighboring regions so slight that they are un- 

 perceived. We need not, therefore, be surprised that the organisms fail 

 to react to them. 



The experiments show, what they were designed to show, that the 



