150 PHYSICAL CHEMISTRY 



That the surface of the ameba is electrically polarized is shown 

 by the fact that the ameba moves toward the anode in an electric 

 field, but this fact does not prove whether the electrical polariza- 

 tion is due to semipermeability or to the electrical polarization 

 of the colloidal particles of which the surface of the ameba is 

 composed. The movement in the electric field (electric convec- 

 tion) • may be abolished, or reversed, by adding H ions or certain 

 heavy metal ions to the medium. These ions influence the per- 

 meability as well as the electric polarization of colloidal particles. 

 The former is certain, and it is simplest to assume that the 

 change in polarization is the result of the change in permeability 

 alone, and not the combined effect of change in permeability and 

 adsorption. The permeability hypothesis explains a number of 

 physiological processes and is in harmony with a larger number 

 of facts than any other. 



All of those agents which stimulate muscle or nerve cause a 

 rounding up of the ameba. We may interpret this approach to 

 the spherical form as due to increase in the tension of the plasma 

 membrane, following decrease in polarization, which is the result 

 of increase in permeability or the disappearance of the semi- 

 permeability to certain ions or classes of ions. We should, there- 

 fore, expect most of the tropisms of the ameba to be negative, 

 as is found to be the case. The ameba avoids the region from 

 which a strong stimulus comes because the permeability and 

 surface tension of the plasma membrane on that side is increased 

 and the protoplasm flows in the opposite direction. 



It is more difficult to explain positive tropisms, since decrease 

 in permeability has been less often measured. But permeability 

 being a relative term, decrease in permeability in one region has 

 the same effect as increase in permeability in all other regions. 

 Osterhout found that the permeability of kelp in calcium salt 

 solutions is less than in sea water. This may be due to the fact 

 that the sodium of the sea water increases the permeability and 

 that the excess of Ca antagonizes the action of the sodium in 

 the sea water, preventing this increase and making it appear 

 that the calcium decreases it. We may assume that salts or 

 other substances in the water tend to increase the permeability 

 and that substances which antagonize their action cause positive 

 tropisms. 



