270 PROTOPLASM 



leaf — indeed, adjoining cells — permit alcohol to enter at quite 

 different rates. A block of cells within a certain area may be 

 killed by 10 per cent alcohol within a few seconds, while immedi- 

 ately adjoining cells survive for some minutes (Fig. 141). 



Many studies have been made on the rate of entrance of 

 salts of the common metals into cells. These all lead to the 

 conclusion that a definite series of cations exists, of which the 

 following is an example: 



Rb >K > Na > Li > Mg > Ba > Sr > Ca 



in which rubidium enters the cell most rapidly, and calcium the 

 least rapidly. Among anions, it seems that the nitrate ion enters 

 faster than the chlorine ion, which is somewhat faster than the 

 sulphate ion. While there are theories, to be taken up shortly, 

 which satisfy certain permeability phenomena very well, there 

 is no adequate explanation for the selective permeability of ions. 

 In the above series, the ions fall into two groups, the members of 

 each group being alike as to magnitude of charge and valence. 

 One can do little more than list the observed facts. 



A number of so-called permeability phenomena may not, in 

 truth, be such, at least in their entirety. We must first distin- 

 guish between passive and active permeability. Strictly 

 speaking, only the former is true permeability; the latter is 

 cellular activity. It is known that certain living (the apple skin) 

 and nonliving (celloidin) membranes are more permeable to 

 the cation potassium than to the anion chlorine. This may 

 be a strict permeability phenomenon, or it may be one of elec- 

 trokinetics in which electric potentials are involved, as in a 

 galvanic cell, where anions travel in one direction and cations 

 in another. A porous membrane inserted between the two 

 need not affect the results. 



The lining of the human intestine is readily permeable to 

 water and other substances, while the stomach absorbs little 

 or nothing of either. We may here be dealing not with perme- 

 ability processes as ordinarily interpreted but with the adsorp- 

 tion of water by tissues. Tissue permeability is quite a different 

 thing from cellular permeability, because in the former case 

 much is taken in as intracellular matter. Tissue, like that of 

 the intestines, is permeable to all electrolytes; i.e., it is not 

 semi- or selectively permeable. Certain other types of animal 



