Measurement of Electrical Conductivity. 2 $\ 
The flow of sap appears to exert no influence on electrical 
resistance of trees ; nor, other than temperature, do meteorological 
factors, such as relative humidity, pressure, or winds. 
It should be noted that in these observations a wide range of 
resistance was noted; for instance the resistance measured on the 
south side of the tree varied from 18,000 ohms to 53,900 ohms. 
Taking this variation into consideration it seems very doubtful 
whether the differences of the mean resistance of different sides of 
the tree, which were only 5 per cent, between north and south, and 
8 per cent, between east and west, have any meaning. 
By measurement of the resistances of ringed and unringed cut 
branches it would appear that the cambium offers the least 
electrical resistance ; this is followed by the phloem and sap wood. 
The same writers also made some measurements with herbaceous 
plants ; the species used was the Tobacco (Nicotianum tabacum L.). 
The platinum electrodes were 14 inches apart. Here again rather 
wide variations in resistance were noted which could not generally 
be correlated with temperature. Thus at 60 n F. two measurements 
in the same experiment were 108,000 ohms and 150,000 ohms, a 
difference of more than 30 per cent, of the mean of the two 
readings. 
The most noteworthy work that has yet been done on the 
living plant in which conductivity measurements are made is that 
of Osterhout (32, 35) on the permeability of protoplasm to ions. 
In this work discs of Laminaria were cut out with a cork borer, and 
100 to 200 of them were packed closely together so as to form a 
cylinder. This cylinder was immersed in sea water and other 
liquids and was kept in place by means of glass rods. At each end 
of the cylinder was a block of hard rubber containing a platinum 
electrode. The measurements were made by Kohlrausch’s method. 
The resistance of the cylinder of tissue in sea water was 1,110 ohms 
and when killed it fell to 320 ohms. 
When this cylinder of tissue was placed in various solutions it 
was possible to show the rate at which the ions in sodium chloride 
or other electrolytic solution entered into the tissue by the rate at 
which the resistance fell from 1,110 to 320 ohms. When solutions 
of sodium chloride and calcium chloride were mixed in a certain 
proportion it was found that the resistance remained unaltered for 
twenty-four hours. This was attributed to the antagonistic action 
of the salts on one another, both sodium and calcium ions being 
prevented from entering. Sea water is also such a “ balanced ” 
solution. 
