MOTOR RESPONSES 277 



does not account for the increase in the amount required (with increase 

 in intensity) if the observations are made in strong light. 



The quantity of Hght energy required to induce cessation of movement 

 depends upon the chemical composition of the surrounding medium. In- 

 crease in HCL, for example, causes an increase in the quantity of light 

 required. On the other hand, an increase in CO, causes decrease in the 

 quantity required. In solutions of KCl, CaCl,, and MgClg, respectively, 

 the quantity of energy required increases as the salt concentration de- 

 creases, but in solutions of NaCl there is no consistent correlation be- 

 tween the quantity of energy and the concentration of the salt. In gen- 

 eral, the quantity of energy required appears to vary directly with the 

 viscosity of the cytoplasm (Mast and Hulpieu, 1930). The observations 

 made by these authors extended, however, over only a very limited range 

 of environmental variation. The conclusions reached are therefore not 

 applicable to wide ranges of variations in the environment (Mast and 

 Prosser, 1932). 



Increase in the illumination of any localized region of an amoeba re- 

 sults in an increase of the thickness of the plasmagel in this region (Fig. 

 95). Increase in the illumination of the entire amoeba results in an 

 increase in the thickness of the plasmagel at the tip of the advancing 

 pseudopods. In turn, this causes a cessation of movement (shock-reac- 

 tion). 



The shorter waves of light are more efficient in inducing this response 

 than the longer waves (Harrington and Teaming, 1900; Mast, 1910). 

 According to Inman, Bovie, and Barr (1926), ultra-violet light is prob- 

 ably more efficient than visible light. Although the distribution in the 

 spectrum of stimulating efficiency has not been precisely ascertained, 

 Folger (1925) maintains that it is not closely correlated with tempera- 

 ture. He did not thoroughly investigate the problem, however. 



Kinetic responses. — If an amoeba is kept for some time in very weak 

 light it becomes inactive; if the light is then increased, the organism 

 gradually becomes 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 Difjlugia (Mast, 1931c), but 



