Mast, Orientation in Euglena with some Remarks on Tropisms. (J47 



energy in different positions of the spiral, least when the dorsal 

 and most when the ventral surface is exposed. Thus it is assumed 

 that the unoriented organism not only receives different quantities 

 of light energy in different positions, but that the amount received 

 changes continuously. Bancroft holds that orientation is depen- 

 dent upon the difference in the amount received in different posi- 

 tions of the spiral. Jenning's and the writer hold that it is 

 dependent upon the time rate of change in the amount received 

 (shock-reactions). This is the main point of difference in the 

 two theories which primarily concerns us. Both schools, let me 

 repeat, hold that during the process of orientation in positive 

 specimens there is an increase in deflection on the side of the 

 spiral toward the light, i. e., toward the surface bearing the 

 eye-spot. 



There is however another point of difference that may be 

 presented here. Bancroft holds that the degree of deflection 

 depends upon the amount of light energy received regardless 

 of the surface illuminated. He says ip. 421) that in negative 

 euglenae, after orientation, the spiral course is narrower, (the deflec- 

 tion less) in low than it is in high intensity, and that in positive 

 euglenae just the opposite is true. In positive euglenae then, accord- 

 ing to Bancroft, the deflection, as previously, stated, is greatest 

 when the energy received is least, i. e., when the unoriented 

 organism is in such a position in the spiral that the surface bearing 

 the eye-spot faces the light, and, he maintains, this results in orien- 

 tation. But after orientation the sensitive tissue still receives light 

 energy. Thus Bancroft holds that the degree of deflection con- 

 tinues to be regulated, after orientation as well as during the pro- 

 cess of orientation, that light acts continuously in accord with the 

 Bunsen-Roscoe law, and that thus the organisms are held upon 

 their course after orientation by the same factors which cause 

 orientation. The amount of energy received, however, as stated 

 above, does not, after orientation, vary in different positions of 

 the spiral. According to the change-of-intensity theory, therefore, 

 the condition of illumination which induced orientation no longer 

 exists; the stimulus which causes orientation acts only during the 

 process of orientation, not after the organism has become oriented. 

 Light may, however, according to this theory, continue to act on 

 the organism, much as heat does, making it more or less active, 

 causing changes in the sense of orientation, producing photosyn- 

 thesis, etc. Organisms in general tend to proceed in fairly direct 

 courses if there is nothing in the environment to prevent this. 

 Thus after orientation, according to our theory, they tend to remain 

 oriented owing to internal factors, i. e. structure, and physiological 

 processes, 



