338 PROTOPLASM 



using the customary type of galvanometer, which is, therefore, 

 replaced by a telephone receiver; minimum buzzing of the latter 

 indicates the point on the wire (the "null" point) where 

 equilibrium is reached. A conductivity cell or vessel having 

 platinum electrodes which are immersed in the volume of liquid 

 to be measured is used. Its cell constant, or capacity factor, must 

 be determined. This factor takes account of all physical 

 properties peculiar to the vessel, such as the size of and distance 

 between the platinum electrodes. 



Electrolytic conductance measurements serve as a means of 

 determining the salt concentrations of soil water, the sea, and 

 the body fluids of plants and animals. Obviously, the amount 

 of nonelectrolytes (sugars, etc.) present is not indicated by con- 

 ductivity determinations. Osmotic pressure is an index of the 

 combined concentrations of electrolytes and nonelectrolytes in 

 solutions. The conductivity of plant juices (sap) has been used 

 as an indicator of the amount of electrolytes that have entered 

 or left the tissue. It is thus an indicator of changes in the 

 permeability of the living cell. The green alga Valonia encloses 

 a large vacuole or sac which can be tapped and the sap within 

 collected. The amount of sap obtained is rather small for con- 

 venient quantitative chemical analysis but sufficient for quick 

 conductivity determinations which indicate the total electrolytic 

 concentration. Juices may be extracted from plant tissues by 

 pressure, and values of total concentration of electrolytes 

 determined by conductivity measurements. The juice from an 

 onion extracted by pressure has a conductivity of 6 X 10~* 

 mhos, which is equivalent to about 0.05A^ potassium chloride. 



Conductivity has been used to indicate the electrolytic con- 

 centration of solutions (water cultures) in which plants are 

 experimentally grown. In this convenient way, the amount of 

 salts taken out of solution or the salts given off by plants when 

 grown in distilled water can be determined. 



Lecomte du Noiiy has determined the conductivity of horse 

 blood and found it to be 0.012168 = 12 X 10"^ (mhos), which is 

 just twice that of the plant juice given above. 



A conductivity value of protoplasm has been obtained by S. C. 

 Brooks, who found it to have a resistance of 19,000 ohms, which 

 represents a very low conductance, equivalent to 0.00145 M 

 sodium chloride. Gelfan found a higher conductance for the 



