154 PHYSICAL CHEMISTRY 



Galvanotropism cannot be explained away as electric convection 

 since the galvanotropism of the ameba is cathodic, whereas the 

 electric convection of the ameba that is unable to reach and catch 

 hold of the substratum is anodic. Ludloff (1895) showed that 

 the galvanotropism of Paramecium is a forced movement, the 

 cilia toward the cathode being reversed, and causing a turning 

 toward the cathode. Since any strong stimulus causes reversal 

 of cilia, this observation shows that the Paramecium is stimu- 

 lated at the cathode, even by a constant current. Dale (1911) 

 has studied galvanotropism in a number of species of infusoria. 



The direction of tropisms has been reversed by change in 

 viscosity, reaction, or temperature of the medium, and by the 

 addition of neutral salts, acids and anesthetics. Some reagents 

 employed, such as C0 2 and alkaloids, change the reaction but 

 their action may not be due to H ions. Wo. Ostwald supposes 

 that a change in viscosity that reduces the efficiency of the motor 

 organs changes negative geotropism into positive geotropism. 

 If this is true, the head of the animal must already be pointed 

 away from the earth, in order that positive geotropism take 

 place. Furthermore, negative geotropism would be analogous 

 to the effect of a strong constant electric current which carries 

 the Paramecium to the anode while its head is toward the cathode. 

 Such a change from positive to negative geotropism is not what 

 is usually meant by a reversal of tropism, which is made to 

 imply a reversal of orientation. Some writers use the word 

 taxis since it means orientation, but tropism, meaning a growing 

 and borrowed from plant physiology, has come into general use. 



Bancroft (1906) attempted to identify reversal of tropisms 

 of Paramecium with the antagonistic actions of Ca to Na, claim- 

 ing that the direction of tropism is determined by the ratio of 

 Ca to Na. 



We have attempted to explain ameboid motion, and conse- 

 quently tropisms of the ameba as due to local increase in per- 

 meability. The ameba recedes from the side on which the 

 permeability is increased. The agent causing positive tropism 

 must therefore shield the ameba from stimuli. The movements 

 of plants are caused by increase of permeability, and Trondle 

 (1910) and Lepeschkin (1908) have shown that light increases 

 the permeability of plants to salts and dextrose. 



