PROTOPLASMIC STRUCTURE 119 



measurements of permeability are those given Ijy 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 recover^' 

 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 CaCU 

 solutions have the opposite kind of effect and at first 

 decrease permeability; the antagonism between Na and 



' Osterhout, loc. cit., and Science, XXXVII (1912), 3; also "Quanti- 

 tative Researches in Permeability," The Plant World, XVI (1913). i-'Q- 



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



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



