CYTOPLASM 



197 



This phenomenon should not be confused with the well-known 

 fact that frozen plants can often be kept alive if thawed slowly. In 

 these objects the imbibition water, indispensable to life, has not yet 

 crystallized and it is only necessary to avoid inundation of, and 

 damage to, the protoplasmic structure by water from ice melting too 

 suddenly. 



g. Morphological Principles of the Permeability Problem 



Like all physiological questions, the problem of physiological 

 permeability is founded on morphological assumptions. The lipid 

 theory of Overton (1899), the :iltrafilter theory of Ruhland (191 2, 

 1950), the mosaic theory of Nathanson (1904) and the modern, com- 

 bined lipid filter theory of Collander (1932, 1937a) are all based on 

 certain morphological concepts which, it is true, have not been gained 

 directly, but via physiological experiments or reasoning (Davson and 

 Danielli; 1943). Before going into these questions of the submicro- 

 scopic structure of protoplasmic boundaries, a more accurate micro- 

 scopic description of the cell boundaries must be given. 



Problem of the boundary layers. The phenomenon of the cap-plasmo- 

 lysis (German: Kappen-Plasmolyse) proves that certain plasmolytic 

 agents are capable of penetrating into the cytoplasm, though not into 

 the cell vacuole. For this reason Hofler (193 i) distinguished between 

 permeability, i.e., the passage from the outside through the cytoplasm 

 into the cell sap, and intrability, in which only the cytoplasm is reached. 

 In addition one must in certain cases take into account a membrane 



Fig. 114. Cell with cap-plasmolysis to demonstrate the various types of permeability 



(from Hofler, 1932). a) Membrane permeability (p plasmolysis forecourt); Z') intrability 



(2 cytoplasm); c) permeability (v vacuole). 



permeability, i.e., a resistance of the cell wall to penetration (Fig. 114). 



In cellulose cell walls the membrane permeability can be neglected; 



they are permeable to all plasmolytic agents and therefore also to 



nutrients. Cutinized cell walls show a different behaviour Tmoss leaves. 



