276 



MOTOR RESPONSES 



amount of light energy required to induce cessation of movement is due, 

 at least in part, to adaptation (Mast, 1931a). For if the light is rapidly 

 increased and then held, streaming soon begins again, i.e., the organism 

 recovers from the effect of the increase in light. In other words, it be- 

 comes adapted (Mast, 1939; Folger, 1925). This shows that the effect 

 of rapid increase in light is eliminated while the organism is continuously 



Figure 95. Camera drawings of Amoeba sp. illustrating the response to localized 

 illumination. Rectangular areas, regions of high illumination ; arrows, direction of proto- 

 plasmic streaming; dotted lines in B, C and D, positions and forms shortly after the 

 illumination of the parts indicated ; n, nucleus ; r, contractile vacuole. E and F, same 

 specimen ; F, form and direction of streaming assumed by E after the anterior end had 

 been illuminated for a few minutes. (After Mast, 1932.) 



exposed to the light. It also indicates that there are two opposing processes 

 involved, i.e., that increase in light induces certain changes in the or- 

 ganism and that internal factors tend continuously to oppose and to 

 eliminate these changes. If this is true, the more rapidly a given amount 

 of light is received, the less time there is for recovery, and consequently 

 the greater will be the effect of a given quantity of light. This probably 

 accounts for the increase in the amount of light energy required (with 

 decrease in intensity) when observations are made in weak light; but it 



