644 Mast, Orientation in Euglena with some Eemarks on Tropisms. 



law, that is, all orienting reactions which bear a definite relation 

 to the product of the intensity of the stimulation agent and the 

 time it acts, no matter whether it acts continuously or intermit- 

 tently. Stated in other words this means that the reaction is pro- 

 portional to, or at least bears some definite relation to the absolute 

 amount of energy receive by the sensitive tissue from the stimu- 

 lating agent. Thus according to Loeb there are two factors in- 

 volved in the process of orientation, bilateral symmetry and photo- 

 chemical changes which bear some specific relation to the absolute 

 amount of energy received by the sensitive tissue. 



Bancroft says that all of the evidence accumulated favors 

 Loeb's theory, but as a matter of fact his arguments from begin- 

 ning to end show that he is considering only one feature of this 

 theory, namely, that which refers to the nature of the stimulating 

 agent. This new theory of orientation advocated by Bancroft 

 which in reality is merely a segment of Loeb's theories, we shall 

 call the continuous-action theory. 



All processes of orientation which bear a specific relation to 

 the absolute amount of energy received by the sensitive tissue from 

 the stimulating agent are in accord with this theory, regardless as 

 to whether it is received continuously or intermittently; regardless 

 as to whether the organism is symmetrical or asymmetrical ; regard- 

 less as to whether in the process of orientation both or only one 

 of two symmetrically located sensitive areas are functional. This 

 theory refers only to the process of orientation. It has nothing 

 to do with tropism as defined by Bancroft, Loeb, or any 

 one else. 



(b) The Ray-direction Theory. In accord with the ray- 

 direction theory formulated by Sachs in 1876 orientation in some 

 unknown way is regulated by the direction in which the lines of 

 force from the stimulating agent penetrate the tissue of the reac- 

 ting organism. The stimulating agent is supposed to act continu- 

 ously during the process of orientation, but there is no necessary 

 relation implied between the reaction and the amount of energy 

 received; and the theory expressly states that orientation is not the 

 result of difference in the intensity of stimulation on opposite sides 

 of the reacting organ or organism. This was the first theory ad- 

 vocated by Loeb. He says (1888, p. 2), ,,Die Orientierung der 

 Tiere gegen eine Lichtquelle wird wie bei den Pflanzen (J.v. Sachs) 

 bedingt durch die Richtung, in welcher die Lichtstrahlen die tieri- 

 schen Gewebe durchsetzen, und nicht durch die Unterschiede in 

 der Lichtintensitat auf den verschiedenen Seiten des Tieres". Later 

 however he gave up the idea that the direction of penetration is 

 vital in the process of orientation and substituted the notion that 

 the angle between the sensitive tissue and the direction of force 



