642 Mast, Orientation in Euglena with some Remarks on Tropisms. 



Concerning merely the nature of the stimulus, there are three 

 essentially different theories of orientation. 



These may be designated as follows: (a) Relative-intensity 

 theory; (b) Ray -direction theory; and (c) Change- of -intensity 

 theory. 



(a) Th e Relative-intensity Theory: The relative-intensity 

 theory teaches that orientation is regulated by the relation in the 

 intensity of the stimulating agent on opposite sides of the reacting 

 organ or organism. If the intensity is unequal it is supposed that the 

 two sides move at different rates until such a change in position 

 is brought about that it becomes equal i. e. until the organism 

 becomes oriented. The stimulating agent is supposed to act con- 

 tinuously after orientation, as well as during the process of 

 orientation, the only difference being that in the former case it 

 acts equally on both sides and in the latter unequally. 



This theory may be divided into two sub-theories, which are, 

 however essentially the same, (1) the local-action theory and (2) the 

 reflex-action theory. 



(1) The local-action theory was formulated by Ray in 1693. 

 It is as far as I have been able to learn the first theory of orien- 

 tation recorded. Ray maintained that plants turn toward the 

 window because the surface facing the window is cooler than the 

 opposite surface, and consequently the cells on that side grow more 

 slowly than those on the other side, thereby causing the structure 

 to bend toward the light. Each element in the process of 

 orientation was supposed to be directly stimulated. This 

 theory demands no separation of tissues into sensory and motor, 

 and no transmission of impulses, at least to any appreciable ex- 

 tent. De Candolle in 1832 made use of essentially the same 

 theory to account for the orientation of plants in light. He held 

 in accord with Ray that the bending of the plant toward the light 

 is due to difference in the rate of grow r th on opposite sides, but 

 he maintained that this difference is due to the action of light, 

 not to that of heat as Ray asserted. The explanation of orientation 

 given by Verworn (1899, p. 499), Holt and Lee (1901) and Daven- 

 port (1897, p. 209), and some of those given by Loeb (1906, 

 p. H8) 1 ), are in full accord with De Candolle's theory, the opinion 



1) "How can light bring about heliotropic curvatures? Let us suppose that 

 light strikes a plant on one side only, or more strongly on one side than on the 

 opposite side, and that it be absorbed in the superficial layers of tissue of that 

 side. In this case we assume that on that side certain chemical reactions occur 

 with greater velocity than on the opposite side. What these reactions are is un- 

 known ; we may think provisionally of oxidations. This change in the velocity of 

 chemical reactions either produces a tendency of soft] elements on that side to con- 

 tract a little more than on the opposite side, or creates otherwise a greater resistance 



