76 LIGHT AND THE BEHAVIOR OF ORGANISMS 



ordinarily seen to extend on the opposite side for some dis- 

 tance, and stop, etc. 



The details in the process of orientation were observed 

 as follows: a specimen which had oriented in one beam of 

 light was selected, after which the light in this beam was 

 intercepted and that in the other simultaneously turned 

 on. The reaction of numerous specimens to a change in 

 the direction of the rays was thus observed and the move- 

 ments in several were recorded by means of camera sketches 

 made at short intervals. A typical record is presented in 

 Fig. 8, although a majority of the specimens observed 

 did not orient as precisely and definitely as did the one 

 represented in this record. By referring to Fig. 8 it will be 

 seen that the amoeba under observation gradually turned 

 from the side most highly illuminated, sending out pseudo- 

 pods only on the shaded side. What is the cause of this? 



If direct sunlight is thrown upon an amoeba which is 

 active in diffuse daylight, all movement stops instantly, 

 but there is ordinarily no immediate contraction of any of 

 the pseudopods. After a few moments of exposure new 

 pseudopods usually appear at the posterior end, and not 

 until these begin to form do the old ones begin to retract. 

 In changing the direction of the rays so that the amoebae 

 become strongly illuminated from the side, as described 

 above, the distribution of the light intensity on the different 

 pseudopods is changed since different surfaces become 

 exposed. Judging from our preceding statement it might 

 be expected that this change of light intensity would 

 inhibit the protoplasmic streaming in the pseudopods on 

 the illuminated side. I could, however, never be quite 

 certain that it did, although it often appeared so. The 

 difficulty in observation here lies in the fact that without 

 any change of illumination the pseudopods form, extend a 

 varying distance, then stop and retract while others form 

 elsewhere. When a pseudopod stops after the direction 

 of the rays is changed it is consequently impossible to be 

 certain that it would not have stopped had the light not 



