GEOTROPISM OF PARAMECIUM AND SPIROSTOMUM. 3 



in that position." On such a "purely physical ground," he 

 criticized Aderhold and believed it self-evident that "in com- 

 plete quiescence of the flagellum the posterior end of the protist 

 should be directed downward and not the flagellated anterior 

 end." ''With its flagellum directed upward and moving it, 

 such an oriented individual must move toward the surface of 

 the water" (24, p. 122). Further, Verworn believed that the 

 force of gravity does not act upon the organism as a stimulus. 

 This view has been called by Davenport the "mechanical 

 theory" (2, p. 121). 



After experiments on bacteria, flagellata, and ciliata, Massart 

 in 1891 arrived at the same conclusion as Schwarz and Aderhold; 

 and he stated that his "experiments do not agree with Verworn's 

 theory" (18, p. 166). 



After a more extensive and careful study than any of his 

 predecessors had made, Jensen in 1893 "disproved" the mechani- 

 cal theory of Verworn and proposed his pressure theory instead. 

 But Jensen's experiments were not beyond criticism for his 

 methods were unsatisfactory. He "killed" 50 paramecia in a 

 "filtered alcoholic iodine solution" and watched the individuals 

 fall in the solution to determine which end, anterior or posterior, 

 was directed downward. More than half of them, according 

 to him, sank without any definite orientation of the axis or in 

 "beliebiger Querstellung," while a greater part of the rest sank 

 with their posterior ends directed downward, and the other part 

 with their anterior end directed downward. 



From these results and others, he concluded that "the ori- 

 entation of the Paramecium with the long axis vertical and in 

 the direction of gravity would not be brought about by a purely 

 physical effect" (n, p. 455). Jensen also tried similar experi- 

 ments with Euglena viridis, and found that the majority of 

 them sink with their anterior end downward (u, p. 457). 



Recently Lyon rightly criticized Jensen's experiments in killing^ 

 the animals in the alcoholic iodine solution, saying that "the 

 centers of buoyancy and gravity might have been changed 

 through distortion or through localized changes of density in 

 the protoplasm" (17, p. 423). Lyon having questioned Jensen's 

 method, "made use of the fact that the animals when centrifuged 



