230 Walter Stiles and Ingvar jorgensert. 
gradual diminution of the current, although generally there was a 
rise for a short time early in the second day of observation. Except 
for this the fall appeared quite regular. Thus in one case the current 
decreased from 2-38 microamperes to 0-10 microamperes in 56J 
hours. Similar results were obtained in all other cases. 
Some direct attempts at measuring the conductivity of living 
plant tissues have been made by Mameli (23). Her method consists 
in immersing platinum electrodes of various shapes in the tissues 
of the plant, and measuring the resistance between them by means 
of an alternating current and Hartmann and Braun’s modification 
of the telephone-bridge arrangement of Kohlrausch. Although no 
distinct minimum of vibration could be made out in the telephone 
for the first fifteen minutes, yet after that time a distinct minimum 
could be discerned and the resistance measured. By this method 
the author was able to make out that as leaves detached from the 
plant undergo slow drying the specific conductivity at first diminishes, 
then it rises again, and in some cases passes the value of the specific 
conductivity of the attached leaves. She found also that the specific 
conductivity of a plant organ decreases as the organ gets older. 
This result she imagines can be explained by the increase in quantity 
of ash constituents in the leaves as they get older, and consequently 
in the decrease of ionisation. It is necessary on this view, she adds, 
to imagine that outside the ash constituents, all other substances, 
both ionised and non-ionised remain constant. Finally in two cases 
examined, she found the specific conductivity at the base of the 
leaf was greater than at the apex. 
Stone and Chapman (38) have published some measurements 
of the electrical resistance of trees. Their method consisted in 
driving galvanised iron nails about three inches long into the wood 
of the trees. These served as the electrodes and were in some 
cases 10 feet, in other cases 8 feet apart. 
The trees experimented upon were the sugar-maple (Acer 
saccharum Marsh) and the American elm (Ulmus americana L.). 
The current used was apparently a direct one as it was measured 
by a galvanometer. It was only used for measuring the resistance 
and did not flow in between the measurements. Stone and 
Chapman interpret their results as showing that the resistance 
depends on the temperature—the higher the temperature the less 
the resistance. The average resistance is highest on the north side 
of the tree, followed by the west, south and east sides. This they 
suppose is correlated with the temperature, which is least on the 
north side, followed by the west, south and east. 
