PROTOPLASMIC STRUCTURE 119 



measurements of permeability are those given by elec- 

 trical conductivity. The conductivity of a living tissue 

 is a measure of its permeability to ions; and while it is 

 conceivable that the permeability to other substances, 

 such as non-electrolytes like sugar, may vary inde- 

 pendently (within certain limits) of the permeability to 

 ions, the advantages of estimating permeability quan- 

 titatively are such that the conductivity method must 

 be regarded as the one to be preferred wherever it can 

 be applied. 



Osterhout has shown that by means of the con- 

 ductivity method the permeability of plant tissues 

 under varying external conditions can be readily and 

 accurately determined; also that permeability can be 

 varied at will, reversibly, in either direction, especially 

 under the influence of neutral salts and lipoid-solvent 

 compounds.^ In pure NaCl solutions the permeability 

 of Laminaria fronds is increased, to a degree depending 

 on the duration of exposure and the temperature; on 

 replacing the tissue in sea water the permeability returns 

 to or toward the normal, the degree of possible recovery 

 depending upon the extent of the change produced by 

 the NaCl solution.^ If the permeability has been 

 increased beyond a certain limit, its reversal is impossible 

 and the plant is dead. He has suggested, therefore, 

 that the property of "vitality" may be measured by 

 determining the electrical resistance.^ Isotonic CaClz 

 solutions have the opposite kind of effect and at first 

 decrease permeability; the antagonism between Na and 



' Osterhout, loc. cii., and Science, XXXVII (191 2), 3; also "Quanti- 

 tative Researches in Permeability," The Plant World, XVI (1913), 129. 



2 Osterhout, Jour. Gen. Physiol., Ill (1920), 145. 



3 Osterhout, Science, XL (1914), 488. 



