136 J. F. MCCLENDOX. 



All stimuli producing positive tropism would then have to 

 penetrate to a greater or less distance into the Amoeba. But the 

 same stimulus thus acting on the interior might, in greater 

 intensity, affect also the plasma membrane, increasing its 

 permeability and changing the positive to negative tropism. 

 Such a change of the sign of tropism has been observed. 



Soap lowers the surface tension of fats and lipoids, and Ouincke, 

 Biitschli, Loeb, Robertson and others supposed that lowering 

 of the surface tension of living cells might be due to soap. How- 

 ever, I found that soap always causes negative tropism in Amoeba, 

 probably because it increases the permeability of the plasma 

 membrane. 



4. The Propagation of the Bio-electric Changes. 



On the hypothesis, that the electric phenomena in muscle and 

 nerve, as well as other animal and also plant tissues, is due to 

 change in permeability to ions, we might hope to explain the 

 wave-like propagation of these changes. Since extraneous 

 electric currents "stimulate" all tissues (presumably by in- 

 creasing permeability) thus causing them to produce additional 

 electric phenomena, it seems natural that these latter would be 

 self-propagating. It is probably the negative variation of nerve 

 which stimulates the muscle, and the negative variation of the 

 portion of the muscle fiber adjoining the nerve ending, which 

 stimulates the adjacent portions of the muscle. Ncrnst found 

 mathematical proof that electric stimulation is due to change in 

 ionic concentration at the semipermeable membranes. 



I have found evidence that the negative variation (current of 

 injury) in plants, may strongly affect adjacent cells. If an 

 electric current of suitable density is passed through plant or 

 animal tissue, negatively charged colloids in the protoplasm 

 migrate toward the anode. I have observed this movement in 

 living cells, and the resulting displaced bodies in histological 

 sections. In certain cases there may be some doubt whether 

 the colloids moved toward the anode, or water toward the 

 kathode, but in others, easily distinguishable bodies such as 

 chromatin granules or threads moved toward the anode. 



If the tip of a root be cut oil we observe a negative variation 



