THE THEORY OF TROPISMS. 97 



the method of reaction by a " motor reflex" had been described the 

 reactions to chemicals had been referred in a general way to the tropism 

 schema, but critical observations, which would differentiate between 

 the possibilities, have been lacking. It is necessary to use the greatest 

 caution in this matter, as is shown by the case of Chilomonas. Garrey 

 (1900), although he stated that ' a study of the mechanics by which 

 the organism is oriented or by which it is prevented from moving 

 from the ring into the stronger acid of the clear area, or the weaker 

 acid surrounding the ring, proved fruitless," nevertheless concluded 

 that the reaction in this animal was a case of typical tropism. In a 

 paper published in the same number of the same journal (Jennings, 

 1900, a), I showed that when the mechanism of the reaction is worked 

 out, this conclusion does not hold, but that the reaction takes place 

 through a motor reflex, similar to that in the Ciliata. In cases, there- 

 fore, where the mechanism of the reaction (that is, the exact movements 

 which the organism performs) has not been worked out, conclusions as 

 to the nature of the reaction are of little value. The only case of which 

 I know in which an author acquainted with the method of response by 

 a " motor reflex " maintains, on the basis of observation, a reaction of 

 unicellular organisms to chemicals in accordance with the theory 

 of cheinotropism, is the case of Saprolegnia swarm-spores, as described 

 by Rothert (1901). In this case we are dealing with very minute 

 organisms, and Rothert has made no attempt to give an analysis of the 

 relation of the direction of turning to the differentiations in the body 

 of the organism, such as we found to be necessary above for Chilomonas 

 before the real nature of the reaction could be determined. 



Thus it is clear that cases of true cheinotropism, in accordance with 

 the general tropism schema, are exceedingly rare, if they exist at all. 

 In almost all the lower organisms in which this matter has been care- 

 fully studied it has been demonstrated that the reaction to chemicals is 

 of a different type from that demanded by the tropism theory. 



In the discussion so far we have devoted attention particularly to the 

 question of orientation. When we examine the second question pro- 

 posed, as to whether the stimulus acts directly upon the motor organs 

 of that part of the body on which it impinges, we find the answer 

 somewhat less clear than it was in the case of mechanical stimuli. It 

 is true that in the Infusoria and Rotifera the direction of turning is, as 

 in the case of mechanical stimuli, always toward a structurally defined 

 side, without regard to the direction from which the chemical is diffus- 

 ing, so that at first view it seems beyond question that the reaction is 

 not due to the direct action of the stimulus on the motor organs of the 

 region on which it impinges. While this conclusion is highly probable, 

 the observed facts do not demonstrate it for chemical stimuli as they 



