4 CONDUCTIVITY AND PERMEABILITY 



the points X and Y in the wire P may be called Cp ; that in the other 

 wire Cw The total current, C, flowing between X and Y will be 

 the sum of the partial currents, or, 



C = Cp + Cw 



We may consider the current (conductance) as equal to the recip- 

 rocal of the resistance and write 



1-1 L 

 R ~ Rp Rw 



in which R is the total resistance between X and Y, Rpis the resis- 

 tance of the wire P, and Rw, that of W. Applying this equation to 

 Laminaria* (and expressing the resistance in the usual way as the 

 per cent of the normal) we may calculate the values of Cw, Cp, 

 Riy, and Rp. 



Under normal conditions in sea water, the resistance is taken as 

 100 and therefore C = 1 -r- 100 but in certain solutions (having the 

 same conductivity as sea water) the resistance may rise to 300 or 

 more; and in this case C would equal 1 h- 300 = .0033 (or less), and 

 since some of it must flow in the protoplasm the amount which trav- 

 erses the cell wall must be less than this. We are therefore safe in 

 putting it as low as 1 -f- 350 = .002857. 



All the experiments hitherto made indicate that the conductivity 

 of cell the wall remains unaltered in spite of changes in the chemical 



that of the protoplasm and is in series with it. We maj'^ therefore consider the 

 protoplasm to be replaced by a single wire having a resistance equal to that of 

 the two layers of protoplasm which are traversed by the current in a direction at 

 right angles to their planes. 



^ So far we have considered only the simplest case, when the plant is only one 

 cell thick. But it is evident that these considerations also apply when several 

 membranes are placed together, forming a mass comparable to the tissue of 

 Laminaria. The only difference is in that case the current would traverse a very 

 thin layer of cell waU in passing from one protoplasmic mass to the next, so that 

 what we have spoken of as the resistance of the protoplasm would be composed 

 in part of the resistance of these cell walls. When the protoplasm is dead the 

 total resistance is only 10.29 and the resistance of these cell walls must be only a 

 small fraction of this. Consequently their resistance in the living tissue of 

 Laminaria is undoubtedly less than 1 when that of the protoplasm is 140. The 

 resistance of these cell walls may therefore be neglected. 



