NEUROBIOTAXIS 273 
Like Gassner, Schellenberg also seems to be inclined to ascribe 
the anodic tropism to the greater amount of electricity running 
through the water, or rather to the weakness of the current 
that runs through the root-tip and which should be too weak to 
cause a kathodic tropism, a supposition that seems to be ac- 
cepted by Rothert,2> though he admits that it has not been 
proved. If only a weaker current were sufficient to cause the 
anodo-tropic phenomenon, a smaller amount of electricity 
would have to do the same! These authors, moreover, do not 
explain biochemically why a weak current should cause an 
anodic tropism and a stronger current a kathodic one. 
Coehn and Barratt (loc. cit.) tried to explain biochemically 
this phenomenon of reversal of the galvano-tropism in the fol- 
lowing way. They assumed that on the boundary between the 
object used for the experiment and the surrounding medium 
(the water) a semipermeable membrane is present that possesses 
a different permeability for positive and negative ions. This 
assumption is quite legitimate, since the occurrence of such 
semipermeable membranes is a very common phenomenon in 
nature. 
If we now assume that the permeability for negative” ions is 
greater in this membrane than that for positive ions, of the 
ionized NaCl or KCl (provided the concentration thereof in 
the surrounding fluid be greater than in the protoplasm) a 
larger quantity of negative ions will be transferred into the cell 
than of positive ions, and the cell will then be overcharged with 
negative ions and pass to the positive pole on the transmission 
of the constant current. 
On the other hand, if the concentration of KCI or NaCl in 
the medium be less than in the cells, a larger quantity of negative 
ions than positive will leave the cell, and the cell-bodies, charged 
2> Rothert. Die neuen Untersuchungen iiber den Galvano-tropismus der 
Pflanzenwurzeln. Zeitschrift fiir allgemeine Physiologie, Bd. 7, 1907, p. 192. 
26 Such a special permeability for negative ions (anions) has been proved to 
exist in the case of blood corpuscles by Hamburger (Zeits. f. Biologie, Bd. 28, p. 
405, 1891). Compare also Hamburger und Van Lier, Durchlissigkeit der rothen 
Blutkérperchen fiir die Anionen von Natriumsalzen. Arch. f. Anat. u. Physiol., 
Physiol. Abt., 1902, p. 492. 
