J. F. MCCLEXDOX. 



increased permeability of part of a tissue surface, may cause 

 electric currents to flow through cells some distance from the 

 wound. These currents probably stimulate the cells through 

 which they pass, which in turn become permeable and produce 

 electric currents. This explains the propagation of stimuli 



4- 



Anions represented by minus sign, kations represented by plus sign. Arrows 

 denote the direction of migration of ions. The large, circle represents the plasma 

 membrane, the dotted line denoting the permeable and the continuous line, the 

 impermeable portion. 



through loose tissues, and the structural changes, as observed 

 by Nemec. 



The rate of propagation of the "wound stimulus" is very slow, 

 whereas that of propagation of the "stimulus" (negative vari- 

 ation) in sensitive plants is more rapid, and that of the nerve 

 impulse still more rapid. We have not, however, sufficient data 

 to show whether this is a mathematical objection to the hy- 

 pothesis. 



The streaming movements in plants may be stopped by a 

 strong stimulus or "shock." This stimulus is usually propagated 

 in one or more directions. Ewart 1 states that the rate of propa- 

 gation at 1 8 in a single elongated cell of Nitella is 1-20 mm. 



1 Loc. cit. 



