THE THEORY OF TROPISMS. 99 



theory. Yet the theory makes certain definite statements as to the cause 

 of the orientation and the way in which it is brought about. These 

 statements are open to observation and experiment. In most bilateral 

 animals it is indeed difficult to really test the theory. This is because 

 these animals may turn directly toward either side under the influence 

 of light, and it is difficult to tell whether this turning is due to the direct 

 action of the light on the motor organs or to a reaction of the organisms 

 as a whole induced by some change in physiological condition brought 

 about by the light. But in the ciliate infusoria we find a set of organisms 

 so constituted as to permit us to bring the theory to a direct test. These 

 organisms are unsymmetrical, and, as we have seen, the usual reaction 

 is by a motor reflex involving a turning toward a structurally defined 

 side. We can, therefore, arrange our experiments in such a way that 

 the turning demanded by the theory of tropisms shall be the opposite 

 of that usually produced in the reaction of the organism as a whole, and 

 observe the results. 



This is what was done with Stentor cczruleus, as described in the 

 second of these contributions. The result, as we have seen, is that the 

 organism turns toward a structurally defined side, without regard to 

 what is demanded by the theory of tropisms. The same result was 

 obtained with a number of flagellates and with a bilateral Metazoan 

 the rotifer Amtrcea cochlearis. 



Thus, in these cases, it is impossible to interpret the reactions as due 

 to the direct action of the light on the motor organs of the side on 

 which the light impinges. The response is as clearly a reaction of the 

 organism as a whole as is the reaction to mechanical stimuli. 



Now that it has been shown that orientation to light does occur in 

 some cases in a manner quite at variance with the postulates of the 

 theory of tropisms, and this in organisms widely separated in structure 

 and classification, it can no longer be held that orientation is, per sc, 

 a proof of the tropism theory. In other words, cases in which orien- 

 tation takes place, but in which the manner in which it is brought 

 about has not been observed, can not be assumed as cases of typical 

 tropism, clue to the direct action of the light on the motor organs 

 of the side affected. The reactions of flagellates and swarm-spores 

 to light, as described by Strasburger (1878), have long been con- 

 sidered types for the tropisms. In the second of these contributions 

 I have shown that in Euglena and Cryptomonas (the latter being one 

 of the organisms studied by Strasburger) the reactions do not take 

 place in accordance with the tropism schema. So far as can be judged 

 from Strasburger's account the reactions of the swarm-spores take place 

 in essentially the same manner as in the flagellates. As Rothert (1901) 

 has pointed out, there are many details in Strasburger's account which 



