34 THE TROPISMS 



lus. Verworn coined the word thigmotaxis, which is practi- 

 cally synonymous with stereotropism, in order to designate 



"all those cases of barotaxis in which 



the phenomena are caused by the more or less strong contact 

 of living substance with solid bodies," the term barotaxis 

 embracing reactions which are "called forth by pressure 

 acting unequally on different sides" of the body. Rheotaxis 

 caused by currents of air or water, geotaxis which according 

 to Verworn is produced by differences in pressure on differ- 

 ent parts and thigmotaxis are all classed by this writer as 

 special cases of barotaxis. 



Thigmotaxis may be positive or negative, the difference 

 depending in many cases on the strength of the stimulus. 

 This is shown by the reaction of the rhizopod Orbitolites. 



FIG. 3. Showing the positive reaction of Amoeba to contact with a 

 solid object. (After Jennings.) 



When quiet this form sends out long and very fine filament- 

 ous pseudopodia. When these are brought in gentle con- 

 tact with another object the protoplasm flows out along the 

 filament and finally draws the whole body toward the object. 

 If the pseudopod is cut or pressed upon by a needle the 

 protoplasm flows centrally and the pseudopod is withdrawn. 

 Amoeba when floating in the water usually sends long pseu- 

 dopods out in various directions. If one of these strikes 

 the bottom it commonly adheres to it; the endoplasm 

 streams into it from the main body and finally the whole 

 Amoeba begins to crawl in the direction of the attached 



