BENDING OF HIGHER PLANTS TOWARD THE LIGHT 67 



illuminated side, and 7 did not definitely deflect toward 

 either side. 



These results seem to indicate that it is difi"erence in light 

 intensity on the organism which regulates the direction of 

 movement. The lateral deflections are, however, as indi- 

 cated in Fig. 6, relatively small. The maximum is scarcely 

 more than 2 mm. in a movement of 10 mm. tow^ard the 

 source of light. Considering the conditions of the experi- 

 ments superficially one would expect a much greater deflec- 

 tion if the direction is regulated by the relation in light 

 intensity on different parts of the surface. A corn plumule 

 frequently has a diameter of over one millimeter at a point 

 not more than one millimeter from the tip, well within the 

 sensitive zone. In such a plumule placed in contact with 

 the edge of the beam of light having the lowest intensity, 

 the difference of intensity between the surface facing the 

 glower and that facing in the opposite direction is appar- 

 ently not as great as the difference of intensity between 

 the two sides. Consequently one might conclude that if 

 the movement is regulated by difference of intensity, the 

 plumule should bend at least as far toward the highly 

 illuminated edge of the beam as toward the glower. 



There are however serious objections to such a conclusion. 

 In the first place it is not known whether or not the sensitive 

 tissue extends to the surface. It may be that it is restricted 

 to the central portion of the plumule and that it is very 

 narrow, so that the intensity difference on opposite sides 

 of this tissue is relatively slight under the conditions of the 

 experiment. In the second place It is evident that light 

 can affect the tissue only by penetrating it, and since the 

 rays strike the surface facing the glower nearly at right 

 angles, and the more highly Illuminated side at a very small 

 angle, much more light will penetrate the former than the 

 latter. And in the third place, under the conditions of 

 the experiment, the Illumination of the two sides will be 

 equalized by the movement of the plumules much sooner 

 than will that of the two surfaces. 



