OBSERVATIONS ON UNICELLULAR FORMS 105 



clearly seen that a majority responded by definitely in- 

 creasing the width of the spiral The point at which such 

 response was given varied much with different individuals 

 under different conditions and could therefore not be accu- 

 rately ascertained. It was however discovered that if the 

 organisms were oriented in 61 ca. m. the intensity had to 

 be decreased to 17 ca. m. before unquestionable response 

 resulted. This shows that under the conditions of the 

 experiment it requires a decrease of 44 ca. m., or over 66 

 per cent of the total intensity, to induce the avoiding 

 reaction when the light strikes the anterior end. 



By changing the position of the movable glower, the re- 

 lation between the intensities of light from the two glowers, 

 here arranged as in many of the preceding experiments, 

 was so adjusted that when the Euglenae were suddenly 

 exposed in the stronger light after they had oriented in the 

 weaker, nearly all responded at once, regardless of the sur- 

 face illuminated. Those in which the ventral surface was 

 exposed turned away from the source of light; those with 

 the dorsal surface illuminated turned toward it, and the 

 rest turned in various other directions. The reaction is 

 very striking under these conditions, although of course it 

 was possible to ascertain only approximately the change 

 of intensity necessary to produce it. It was found after 

 many trials that the least reduction of light intensity with 

 a simultaneous change in the direction of the rays, which 

 caused this reaction in a majority of the specimens, was 

 32 ca. m., the intensity of the light from the stationary 

 glower being 61 ca. m. and that from the movable glower 

 29 ca. m. It will of course be understood that individuals 

 frequently respond to much smaller changes of intensity, 

 depending upon their physiological state. With the an- 

 terior end exposed then, a reduction of 44 ca. m. without a 

 change in the direction of the rays is sufficient to cause the 

 avoiding reaction. With the ventral surface exposed a 

 reduction of 32 ca. m. together with a simultaneous change 

 in the direction of the rays causes the avoiding reaction, 



