574 BIOLOGICAL EFFECTS OF RADIATION 



(a) If an amoeba is kept for some time in very weak light it becomes 

 relatively inactive; and if the light is now increased, it very gradually 

 becomes more active again. This response is similar to the response to 

 change in temperature. It is primarily correlated with the magnitude 

 of the change, not with the rate of change in intensity. It is probably 

 due to the effect of light on the rate of transformation of gel to sol and 

 vice versa. This type of response occurs also in Difflugia (Mast, 1536), 

 but the observations on it should be repeated and extended under 

 carefully controlled conditions. 



(6) If the intensity of light on an active amoeba is rapidly increased, 

 movement stops suddenly (Engelmann, 57, et al.). If it is slowly 

 increased this does not occur. This response, therefore, depends upon 

 the rate of change in luminous intensity (Mast, 139). Pfeffer called 

 responses of this sort ''Schreckbewegung," shock-reaction. 



The character and the magnitude of the response to rapid increase in 

 light vary greatly in Amoeba. It may consist merely of momentary 

 retardation in streaming in a localized region in a pseudopod, of total 

 cessation throughout the entire animal with reversal in direction of 

 streaming after recovery, or of any one of an endless number of modifica- 

 tions between these extremes. The character of the response is corre- 

 lated with the amount of light received, as well as with the rate of 

 reception. There is no fixed threshold and the ''all-or-none law" does 

 not apply (Mast, 153). 



If an amoeba is intensely illuminated for only a very short time, 

 movement does not cease until some time after the light has been cut off. 

 The period between the beginning of illumination and the response is 

 known as the "reaction time"; the time illumination must continue, the 

 "stimulation period"; and the time it need not continue, the "latent 

 period" (Folger, 64). There are therefore two processes involved in 

 producing this response. The one occurs only in light, the other in light 

 or in darkness. The action of light probably results in the formation of a 

 substance which by its action produces, independent of light, another 

 substance which induces the response. 



After an amoeba has responded to rapid increase in illumination, 

 some time must elapse before it will again respond to the same increase 

 in illumination. There is therefore a refractory period, a period during 

 which the amoeba recovers from the effect of the stimulation. During 

 a part of this period the amoeba may remain either in light of the same 

 intensity as that which induced the response, in light of lower intensity, 

 or in darkness, but during the rest of the period it must be in light of 

 lower intensity or in darkness. There are therefore two processes which 

 occur during the refractory period, one (1 to 2 min.) which proceeds with 

 or without any change in luminous intensity, and one (10 to 20 sec.) 

 which proceeds only if the intensity is decreased. These processes result 



