576 BIOLOGICAL EFFECTS OF RADIATION 



required increases as the salt concentration decreases, but in solutions 

 of NaCl there is no consistent correlation between the quantity of energy 

 and the concentration of the salt. In general, the quantity of energy 

 required appears to vary directly with the viscosity of the cytoplasm 

 (Mast and Hulpieu, 160). 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, 162). 



Increase in the illumination of any localized region of an amoeba 

 results in increase in the thickness of the plasmagel in this region. 

 Increase in the illumination of the entire amoeba results in increase in the 

 thickness of the plasmagel in the tip of the advancing pseudopods and 

 this causes cessation in movement (shock-reaction). 



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

 than the longer waves (Harrington and Leaming, 76, Mast, 139), and 

 ultra-violet is probably more efficient than visible light (Inman, Bovie, 

 and Barr, 111), but the distribution in the spectrum of stimulating 

 efficiency has not been precisely ascertained. Folger (64) maintains 

 that it is not closely correlated with temperature, but he did not thor- 

 oughly investigate the problem. 



In a beam of light Amoeba proteus orients fairly definitely and goes 

 from the light (Davenport, 47). If it is illuminated from one side, 

 pseudopods develop more freely on the shaded side than on the illumi- 

 nated side. This results in gradual turning away from the light. The 

 stronger the light the sharper the turn. Orientation is therefore brought 

 about by retardation in the formation of pseudopods on the more highly 

 illuminated side, owing to increase in the thickness of the plasmagel 

 on this side (Mast, 139; Luce, 130). There is some evidence which 

 indicates that Amoeba proteus is photopositive in very weak light 

 (Schaeffer, 195; Mast, 153), but no carefully controlled observations have 

 been made in reference to this. 



Folger (65, 67) maintains that mechanical stimulation causes cessation 

 in movement in Amoeba which appears, point for point, to involve the 

 same processes as cessation produced by increase in illumination, and that 

 if mechanical stimulation too weak to produce a perceptual effect is followed 

 by increase in illumination too slight to produce by itself a perceptual 

 effect, or vice versa, there is cessation in movement. He concludes that 

 light and mechanical agitation produce the same kind of changes, and that 

 since the latter cannot produce photochemical changes, response to 

 the former is not due to photochemical processes (Folger, 65, 67). 



It is well known that light produces photoelectric effects which may 

 consist in changes in the position of the valency electrons in the molecules 

 or in their total elimination, and that this results in the activation of the 

 molecules. It is consequently evident that light can produce changes 



