580 BIOLOGICAL EFFECTS OF RADIATION 



(142) demonstrated, however, that if the evidence presented by Ban- 

 croft is vaHd, it proves that Bancroft's explanation of orientation in 

 Euglena is not correct. Moreover, the fact that, after Euglena is oriented, 

 the rate of locomotion is practically independent of the luminous intensity 

 (Mast and Gover, 159) also militates against Bancroft's explanation. 

 Bancroft's experiments should be repeated and extended under more 

 carefully controlled conditions. 



If Euglena is subjected for long periods to low illumination or to 

 darkness, it gradually becomes less active; and if the illumination is then 

 increased it gradually becomes more active again. The rate of change in 

 activity varies with the magnitude of the change in intensity, but this 

 response is never so sudden and abrupt as the shock-reaction. There 

 are therefore two types of responses to light in Euglena, one depending 

 primarily upon the rate of change in luminous intensity, the other pri- 

 marily upon change in the amount of light received. The one results in 

 orientation and aggregation, the other involves the degree of activity. 



In a field of light consisting of two horizontal beams crossing at right 

 angles, Euglena orients and goes toward or from a point between the 

 two beams. The location of this point is related to the relative intensity 

 of the two beams in such a way that the tangent of the angle between 

 the direction of locomotion and the rays in the stronger beam is approxi- 

 mately equal to the intensity of the weaker divided by that of the stronger 

 (Buder, 30; Mast and Johnson, 161). Buder judges this to show a quanti- 

 tative proportionality between the stimulus and the response. Mast and 

 Johnson conclude that "it has no bearing on the problem concerning the 

 quantitative relation between stimulus and response," but that it can be 

 explained on the assumptions that the eyespot is a photoreceptor and 

 that the stimulating efficiency of light varies with the angle of incidence. 



The shorter waves in the visible spectrum are more efficient than 

 the longer in stimulating Euglena and other flagellates. Strasburger 

 (202) concluded that stimulation is confined to violet, indigo, and blue in 

 the solar spectrum, with the maximum in the indigo. Engelmann (58) 

 maintains that for Euglena the maximum is in the blue between 4700 



o 



and 4900 A and Loeb and Maxwell (128) assert that in the carbon-arc 

 spectrum it is between 4600 and 5100 A. The unequal distribution of 

 energy in the spectrum was not considered in these conclusions. Mast 

 (144) made corrections for unequal distribution of energy and ascer- 

 tained the relative stimulating efficiency of negative and positive orienta- 

 tion at intervals of 100 A throughout the visible spectrum, and found 

 that as the wave-length increases the stimulating efficiency increases 

 very rapidly from zero at about 4100 A to a maximum of 21 arbitrary 

 units at 4850 A, and then decreases equally rapidly to zero at about 

 5400 A. He holds, however, that the limits of the stimulating region 

 depend upon the luminous intensity. 



