Measurement of Electrical Conductivity. 229 
Another group of investigations is that in which the rate of a 
reaction is measured by means of changes brought about in the 
electrical conductivity of the liquid in which the reaction is proceed¬ 
ing. This method has been found useful in following enzyme 
actions in which one of the results of the action is to produce 
electrolytes where there were none before, or in which the reverse 
process, the using up of electrolytes to produce non-electrolytes, 
takes place. The method was introduced for this purpose by 
Sjoqvist (37) in 1895, and was early made much use of by Oke'r- 
Blom (28-31). A brief abstract of such work as has been done on 
the investigation of enzyme action by means of measurements of 
electrical conductivity is given by Bayliss (1) in his work on the 
nature of enzyme action. 
Somewhat similar in nature is the work of True and Bartlett 
(39) on the absorption and excretion of salts by the roots of peas 
growing in water-culture. These authors obtain a value for the 
absorption and excretion of salts by roots by measuring the change 
in electrical conductivity of the solutions in which the plants are 
growing. Their measurements are made by Kohlrausch’s method. 
Measurements in Living Plant Tissues. 
In measuring the conductivity of living tissues one is of course 
dealing with a much more complex and difficult matter than in the 
case of “in vitro” measurements. The first measurements of this 
kind of which mention may be made are those of Pollacci (36). 
This worker, in experimenting on the effect on photosynthesis of 
the passage of an electric current through an assimilating leaf, has 
incidentally measured the currents he used. The current was a 
direct one given by a battery of low E.M.F., e.g., two Daniell cells, 
and was measured by a D’Arsonval galvanometer. The current 
was passed through the leaf from the stalk to the tip, or in the 
reverse direction ; the electrodes used were presumably either those 
of Du Bois-Reymond or those described by Pollacci earlier in the 
same paper, as consisting of modifications of the so-called non- 
polarisable electrodes of D’Arsonval, which were held in contact 
with the leaf by means of wooden pincers. A number of leaves 
were experimented upon by Pollacci in this way, and in each case 
the resistance was measured at intervals for a varying number of 
hours, and curves plotted between current and time. With the 
alleged effect of the current we have not to deal at present; what 
we have to notice here is that in all cases there was observed a 
