PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 279 



what differently from the outer protoplasmic membrane. He 

 believes the tonoplast to be less permeable to certain substances 

 than the outer membrane and more permeable to water. 



Permeability studies, micromanipulative work, and electric 

 potential measurements all indicate that the inner protoplasmic 

 (vacuolar) membrane is a distinct entity differing from the inner 

 protoplasm and possibly functioning otherwise than the outer 

 membrane. Thus viewed, the plant cell, as a permeability 

 system, consists of external solution/outer protoplasmic mem- 

 brane/protoplasm/inner protoplasmic (vacuolar) membrane/cell 

 sap. 



The cell membrane is formed of material that comes from the 

 protoplasm. This is particularly evident when the membrane 

 is repaired after being torn by a micro-needle. The newly 

 exposed surface is formed by readjustment of inner material. 

 That there is a readjustment of material, i.e., that the physico- 

 chemical and therefore permeable properties of the membrane 

 differ from those of the inner protoplasm, is shown by the work 

 of M. H. Jacobs. He demonstrated that a cell stained with 

 dyes (neutral red) gives the color of an acid condition when 

 ammonium chloride is injected into the cell but when the cell 

 is bathed in this salt, it loses its color because only ammonia 

 enters through the membrane, and hydrogen chloride remains 

 behind. When acid sodium carbonate is injected, an alkaline 

 reaction within the cell results; but when the cell is bathed in 

 this salt, it gives an acid reaction because of entrance of carbon 

 dioxide alone. 



The following conclusions may be drawn: There is a demon- 

 strable protoplasmic membrane. It is made of matter coming 

 from the inner protoplasm but differs from it in total make-up. 

 The membrane is capable of pronounced changes in its physical 

 properties. It is often optically indistinguishable from the inner 

 protoplasm of which it is an intimate part. It is reformed very 

 rapidly when torn. It exhibits permeability qualities which 

 differ from those of the inner protoplasm. It is rarely capable 

 of being lifted off except in the coagulated state. 



Chemical Constitution. — Many a theory or experiment is 

 condemned and buffeted about by criticism only to emerge in 

 the end as a more worthy contribution than any that followed it. 

 Such is the experimentally supported theory of Overton, whose 



