I CYTOPLASM 205 



easily than at a p^ value below the I.E. P. Consequently, if one wants 

 to distinguish amidophiHc and amidophobic, or urea-permeahle and 

 glycerol-permahle protoplasts, the I. E. P. of the cytoplasm and the p^ 

 of the penetrating solution and the cell sap (Drawert, 1948) should 

 be known. Otherwise it cannot be decided whether the differences 

 observed are intrinsic properties of the protoplasm, as Hofler (1936 a, 

 1942) believes, or whether they have been induced temporarily by the 

 amphoteric cytoplasmic framework (Bogen, 1938; Rottenburg, 

 1945). It may be assumed that the relation between p^ and I.E. P. 

 plays a decisive part in comparative permeabiHty experiments, so that 

 in the end, like vital staining, they only represent new methods to 

 determine the state of ionization of the amphoteric cytoplasmic frame- 

 work. 



In the isoelectric state, i.e., in the case of a neutral framework, 

 A = o. Then the permeability formula reduces to n^/n^ — Uk/U^. 

 In this state, therefore, the cytoplasm is no longer selective in its 

 permeability to cations or anions. 



Since K. H. Meyer's theory is based on potentiometry, it allows 

 only of studying the ion permeability, which is of greater importance 

 to metaboHsm than the permeation of non-electrolytes studied so 

 often in plant cells. For the time being, however, its application to 

 cytoplasmic permeability is difficult (Meyer and Bernfeld, 1946), as, 

 of the many quantities which have to be accounted for, only very few 

 are known in the cytoplasm. Nevertheless, the morphological prin- 

 ciples of the considerations presented will doubtless bear fruit in 

 future theories of permeability. 



The tonoplast. Whereas the plasmalemma in plant cells probably 

 differs from the inner cytoplasm only by a protein framework of 

 greater density and a considerable lipid content, the vacuole skin, or 

 the so-called tonoplast membrane, must possess an essentially different 

 structure. It is this skin which impedes the entrance of hydrophilic 

 substances into the vacuole and on the contrary strongly furthers the 

 passage of lipophiUc substances (Plowe, 193 i). It must therefore 

 contain large quantities of lipids. Although this statement should not 

 be generalized without further criticism, it certainly applies to many 

 cases and especially to the classical example of ^////////epidermal cells. 

 In an interesting controversy Weber (1932) and Hofler (1932) dis- 

 cussed the question whether this lipid layer should be regarded as 



