124 GRAVITY AND PROTOPLASM [Cn. V 



Geotaxis in the higher organisms, especially Vertebrates, 

 cannot here be discussed at length. It is sufficient to state 

 that as LOEB ('91, p. 189) concludes, it is probably dependent 

 upon the internal ear. Miss PLATT has, at my suggestion, sub- 

 jected young negatively geotactic tadpoles to solutions of gum 

 arabic of the same specific gravity as themselves, and has found 

 that they still migrate upwards. This result makes it probable 

 that here also orientation is effected by the internal ear, and 

 hence is independent of the action of gravity upon the entire 

 body. 



Finally must be mentioned the phenomenon of acclimatiza- 

 tion to a central pressure. This has been observed by JENSEN 

 ('93, p. 470), who says, when Paramecram or Urostyla has been 

 strongly " centrifugated ' towards the peripheral end of the 

 tube, where it is subjected to a high pressure, it shows, when 

 the tube is then placed vertically, a much livelier geotaxis 

 than it would have done without " centrif ugating." Clearly 

 the temporary action of the high pressure has increased the 

 irritability to gravity. 



To recapitulate : Gravity affects the structure of protoplasm 

 by separating the lighter and heavier substances. It may 

 determine the direction of locomotion by determining the ver- 

 ticality of the axis of the body. Varying the intensity of 

 gravity's attraction diminishes the precision with which this 

 determination takes place. The determination of the vertical 

 position is, in the lower organisms, probably due to difference 

 in ease of movement when going up and going down. 



LITERATURE 



ADERHOLD, R. '88. (See Chapter I, Literature.) 



DEHNECKE, C. '80. Ueber nicht assimilirende Chlorophyllkorper. Inaug. Diss. 



Koln. Abstr. in Bot. Ztg. XXXVIII, 795-798. Also in Bot. Centralbl. 



I, 1537. 

 HERRICK, F. H. '95. Movements of the Nucleolus through the Action of 



Gravity. Anat. Anz.' X, 337-340. 8 Jan. 1895. 

 JENSEN, P. '93. Ueber den Geotropismus niederer Organismen. Arch. f. 



d. ges. Physiol. LIU, 428-480. 5 Jan. 1893. 

 '93 a . Die absolute Kraft einer Flimmerzelle. Arch. f. d. ges. Physiol. 



LIV, 537-551. 24 June, 1893. 



