Measurement of Electrical Conductivity. 227 
Measurements Outside Living Tissues. 
An early attempt to use phenomena of electrical conductivity 
in the solution of problems connected with plant physiology was 
made by Heald (16) in 1902. This worker expressed the sap from 
various plant organs and made measurements of the electrical 
conductivity of the plant juices so obtained by means of Kohlrausch’s 
well known method. His conclusions were that plant juices are 
good conductors and that generally the specific conductivity of the 
sap increases progressively from the root upwards to the shoot. The 
juice was afterwards incinerated and the ash so obtained diluted to 
the original volume of the juice; the conductivity of the resulting 
liquid was then measured. By this means Heald was able to show 
that the specific conductivity is a rough measure of the relative 
amounts of mineral substances present in the juice. 
Nicolosi-Roncati (27) in 1907, using a method similar to Heald’s, 
also showed the gradual increase of specific conductivity of the 
extracted juice as one passes from the root upwards. He also 
measured the freezing point of the juices and showed that there 
was a rough parallelism between the electrical conductivity and the 
lowering of the freezing point due to the presence of dissolved 
substances. 
This author also showed that there is a decrease of conductivity 
of the extracted juice during the ripening of the fruits in the cases 
of Arum italicum and Solatium laciniatum. He also tried in one or 
two cases determining the conductivities of the cortex and stele 
separately, but no generalisation could be deduced from the results 
obtained. 
The work of Heald and Nicolosi-Roncati has been criticised by 
Mameli (23), chiefly on two grounds. In the first place she 
emphasises the wide departure of the conditions of experiment from 
those in the actual living plant; in this connection she calls 
attention to the easy alteration in chemical composition to which 
the extracted juice would be subjected on exposure to air. Secondly 
she shows that the specific conductivity of the juices will depend 
largely on the pressure used in extracting the juice. With the rest 
of this writer’s observations we shall deal later on ; in regard to the 
present question she gives figures showing that as the pressure to 
which the tissues are subjected is increased, so the electrical 
conductivity of the extracted juice increases. Thus in the case of 
the apex of the phyllode of Opuntia Ficus-Indica the specific 
conductivity of the expressed sap varied from 108-6 X 10' 3 to 312-5 
x 10- 3 according to the pressure used in the juice extraction. 
