VITAL PHENOMENA 



89 



or only the surface, is impermeable to salts. The protoplasm 

 of most plant cells forms only a thin surface layer, the primoidal 

 utricle, the interior being filled with cell sap which is an aqueous 

 solution of many substances. Young plant cells, and all animal 

 cells, have only small or no vacuoles filled with cell sap, and 

 little or no nuclear sap, whereas mammalian erythrocytes are 

 entirely composed of a homogeneous gel. In the plant cell, at 

 least, only the surface layer can be the impermeable portion, and 

 Pfeffer found evidence that not all of the primoidal utricle, but 

 only the surface film, is impermeable. Hober showed that only 

 the surface of erythrocytes is impermeable to ions. His methods 

 are complicated, as will be seen from the following paragraphs. 



L ± 



Fig. 25. Scheme for estimating the electric conductivity of cell in- 

 teriors by the increase in capacity of a condenser into which the cells 

 are introduced (from Hober, 1914). 



It was shown by G. N. Stewart that erythrocytes do not con- 

 duct the electric current, and are therefore impermeable to ions, 

 unless they are laked. Hober showed that ions move freely in 

 the interior of the erythrocytes notwithstanding the fact that 

 they cannot get in or out. This was first done by the "capacity 

 method" (1910 a). The capacity of an electric condenser is in- 

 creased if a conducting body is introduced into the dielectric 

 between the two plates of the condenser, the increase being 

 greater the greater the electric conductivity of the body. The 

 apparatus consists of a primary coil (s, Fig. 25), through which 

 high frequency electric oscillations are passed, such as are used 



