June 6, 1913] 



SCIENCE 



87& 



moments it comes to lie in different positions, 

 in all of which the single photoreceptor is at 

 the same distance from the axis, and in some 

 of which it occupies positions that — if the for- 

 ward movement be for the moment disre- 

 garded — are symmetrically placed with refer- 

 ence to a plane passing through the axis of 

 locomotion and perpendicular to the sub- 

 stratum. Let us now compare two such sym- 

 metrical images of Euglena with a bilaterally 

 symmetrical animal, with paired eyes, moving 

 directly toward a source of light. In Euglena 

 the single photoreceptor is now on the left, 

 now on the right of the plane of symmetry. In 

 the other organisms, the two photoreceptors 

 are jjermanently fixed, and one on each side 

 of that plane. 



Now Euglena swerves toward the light only 

 when it is in such a position that the photo- 

 receptor when on one side of the plane of 

 symmetry receives light at an angle differing 

 from that at which it receives light from the 

 same source when occupying a symmetrical 

 position on the other side of the plane. Just 

 so, according to all varieties of the tropism 

 theory with which I am acquainted, Tarious 

 bilateral organisms swerve toward the light 

 when their eyes, symmetrically placed on 

 either side of the median plane of the body, 

 receive light from a single source at different 

 angles. Expressed in another way, this means 

 that the eyes under such conditions are sub- 

 jected to different effective light intensities. 

 The very mechanism, then, which has long 

 been held by advocates of the tropism theory 

 to account for the definite, errorless turning 

 movements of bilateral heliotropic organisms 

 toward or away from light, is the mechanism 

 that Mast has shovm to be accountable for the 

 heliotropic orientation of the non-symmetrical 

 Euglena. If this be the case, it seems evi- 

 dent that, whether or not the separate reac- 

 tions of Euglena whenever its photoreceptor 

 is effectively presented to the light are to be 

 regarded as overproduced movements that 

 may resemble trials, the definitely directive 

 reactions of bilateral animals to light have not 

 been developed by any process of selection 

 based on such movements. 



It remains to consider the possibilities of 

 such a mechanism for producing the deli- 

 cately accurate heliotropic adjustments of 

 some organisms; as well as the relation be- 

 tween pronounced shock reactions with no 

 obvious relation to the direction of locomo- 

 tion, and the definite errorless turning move- 

 ments ordinarily referred to as tropic reac- 

 tions. 



Mast* says of the orientation of Euglena in 

 light from two sources : 



When the light from the two glowers was equal 

 and the BuglencB positive, they moved in a general 

 way toward a point midway between the glowers. 

 But when it was unequal, they moved toward a 

 point nearer the source from which the more 

 intense light came. . . . This experiment is par- 

 ticularly striking if the glower on the track is 

 gradually moved from a position in which tha 

 light intensity from it is much lower than that 

 from the stationary glower to a position in which 

 it is much higher. Under such conditions one can 

 clearly see these organisms, especially the free 

 swimming forms, gradually change their direction 

 of motion through an angle of nearly 90°. 



I gather from this description that changes 

 in intensity are followed by reactions that 

 vary with the degree of change. If this be 

 true it may well account for the very slight 

 variations which bilaterally symmetrical heli- 

 otropic organisms make from a straight course 

 toward a source of light, and the precision 

 with which such variations are corrected. 



Further evidence of this sort is obtained 

 from Fig. 13, p. 96, where, if the reactions are 

 to be considered accurate in detail as drawn, 

 it is seen that the orienting contractions of 

 Euglena vary in magnitude as the path of the 

 organism inclines more and more toward the 

 source of light. 



Similarly, Jennings" describes the avoiding 

 reaction of the free swimming Euglena in the 

 following terms: 



The Euglenw are swimming about at random in 

 a diffuse light, when a stronger light is allowed to 

 fall upon them from one side. Thereupon the for- 



■* "Light and the Behavior of the Lower Organ- 

 isms," p. 86. 



'"Behavior of the Lower Organisms," p. 138. 



