MOTOR RESPONSES 287 



or from the light; that this induces shock reactions which result in 

 orientation; and that the organisms remain oriented and proceed directly 

 toward or away from the light, because, after they have attained either 

 of these two axial positions, rotation no longer produces changes in the 

 illumination of the photosensitive substance in the eyespot, and they 

 therefore continue in the direction assumed. In other words, the orienting 

 stimulus ceases after the organism has become oriented. The organism 

 then continues directly toward or away from the light because (1) 

 owing to internal factors, it tends to take a straight course, and because 

 ( 2 ) if for any reason it is turned from this course, the orienting stimulus 

 immediately acts, and induces shock reactions which bring it back on 

 its course. 



Bancroft (1913) presented evidence against the contention that 

 photic orientation in Euglena is due to shock reactions and concluded 

 that it is due to tonus effects brought about by "the continuous action 

 of the light," in accord with his conception of Loeb's tropism theory. 

 Mast (1914) demonstrated, however, that if the evidence presented 

 by Bancroft is valid, it proves that his 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 in- 

 tensity (Mast and Cover, 1922) also militates against his explanation. 

 Orientation in light frotn two sources. — In a field of light consisting 

 of two horizontal beams crossing at right angles, Ei/glena orients and 

 goes toward or away 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 approximately equal to the intensity 

 of the weaker divided by that of the stronger (Fig. 101) (Buder, 1917; 

 Mast and Johnson, 1932). Buder maintains that this demonstrates that 

 there is a quantitative proportionality between the stimulus and the re- 

 sponse. Mast and Johnson conclude that "it has no bearing on the prob- 

 lem 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. 



Wave length and stimulating efficiency. — The shorter waves in the 

 visible spectrum are more efficient than the longer in stimulating Euglena 



