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 . . . ... . . . . im-which 
the phenomena are caused by the more or Slee 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. 

a b c 
Fic. 3.—Showing the positive reaction of Amceba 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protoplasm flows centrally and the pseudopod is withdrawn. 
Ameeba 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 
Ameeba begins to crawl in the direction of the attached 
