78 LIGHT AND THE BEHAVIOR OF ORGANISMS 



been changed. If light acts directly on the protoplasm 

 it might also be expected that in a pseudopod laterally 

 illuminated, the flow on one side would be retarded, thus 

 causing it to curve. But no evidence of this could be seen. 

 How then does orientation take place if the pseudopods 

 which are present continue and do not turn from the source 

 of light? There is but one way that I can see, and that is 

 by the inhibition of the formation of new ones on the more 

 highly illuminated side of the organism. 



Since we know that an increase of intensity inhibits 

 streaming in the pseudopods of Amoeba it seems strange 

 that no one has thus far been able to see any reaction in an 

 amoeba in passing from a region of one intensity to that of 

 another. Davenport (1897, p. 186) studied their move- 

 ments in a field " separated by a sharp line into a light and 

 dark half," but could detect '* no effect resulting from the 

 change from light to dark or the reverse." I made ob- 

 servations much like those of Davenport, and found that 

 when the amoebae came in contact with the light area 

 they usually stopped and proceeded in a different direction, 

 as represented in Fig. 9. The light area used in these 

 experiments was about 0.5 mm. square and had very 

 definite edges and a high intensity. It was produced by 

 focusing a limited area of a luminous Welsbach mantle on 

 the slide by means of the mirror and an Abbe condenser. 

 These observations were made in a dark room and no light 

 except the small beam from the Welsbach rrantle reached 

 the microscope. 



By referring to Fig. 9 it will be seen that after one 

 pseudopod came in contact with the illumination and was 

 stopped, the amoeba did not at once proceed in the opposite 

 direction so as to avoid the light, but sent out other pseu- 

 dopods at only a slight angle with the first, apparently 

 trying to get around the obstacle in this way. The char- 

 acter of the response did not change after the first pseudopod 

 came in contact with the light, or after the second and the 

 third came in contact with it. But after the fourth became 



