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PERMEABILITY 
By WALTER STILES 
CHAPTER XII 
QUANTITATIVE RELATIONS IN THE PENETRATION 
OF DISSOLVED SUBSTANCES INTO PLANT CELLS 
hile qualitative tests of the penetration of dissolved substances 
vv into plant cells have yielded results of considerable interest, 
we can hardly expect to be able to formulate the laws governing the 
passage of substances into and out from the cells without adequate 
quantitative data. These data we most certainly do not yet possess; 
nevertheless what information we have is of great interest and is 
sufficient to show that the simple osmotic view of the plant cell is 
a very inadequate hypothesis and is incapable of affording a complete 
explanation of the cell in regard to its relations to dissolved substances. 
Without further preface various aspects of the quantitative relations 
of plant cells will be dealt with in the following sections of this 
chapter. 
The Unequal Absorption of the Ions of a Salt 
by Plant Tissue 
In earlier work on the absorption of salts by plant tissue it was 
assumed that salts were absorbed as such by plants. Of late years, 
however, it has come to be recognised that there may be an unequal 
absorption by plant tissue of the two ions of a single salt. In the 
light of recent knowledge certain observations of long standing 
become easily explicable on this ground, such as the acidity or 
alkalinity developed in some water culture solutions in which plants 
have been growing for a time. 
If unequal absorption of ions takes place there are necessary 
consequences of such a phenomenon. The penetration of an excess 
of one ion into the tissue cannot, on account of the attraction of the 
oppositely charged ions, take place without the replacement of this 
excess by an equal quantity of another ion carrying the same charge. 
This replacement can take place in two ways. In the first way 
an equivalent quantity of hydrogen-ion or hydroxyl-ion, as the case 
