214 J. H. Priestley and R. M. Tupper-Carey 
a temperature which did not damage the root. The tip of the root 
protruded into a salt solution into which a second platinum elec¬ 
trode dipped. The resistance between the platinum wire and upper 
platinum electrode was then determined by the normal Kohlrausch 
method, using the alternating current from the secondary of a small 
induction coil with a high frequency note, and determining the ratio 
between the root resistance and a known high resistance by finding 
the null point on a potentiometer wire when connected with these 
resistances in the ordinary Wheatstone bridge method. 
Electrode 
Salt Solution 
Bean Root 
Platinum wire 
Mercury 
Fig. i 
The roots used were always young root tips of Broad Bean, 
because their relatively large size permitted their insertion on the 
platinum wire with the possibility that the wire is entirely within 
the endodermal cylinder. The resistance of the root tip was then 
compared with the resistance of the killed root tip and with the 
resistance of the solution after the root tip has been withdrawn. 
The resistance of the living roots thus mounted proved very high, 
from about 2000 to 10,000 ohms in different experiments. After 
the roots were killed the resistance fell to a few hundred ohms. 
If root tips were used, taken direct from the dry bean seed and then 
soaked for an hour in water, the resistance of these roots is still 
higher, averaging some 12-13 thousand ohms. The reason for this 
will be apparent later (see p. 221). 
