Permeability 93 
to give a quantitative measure of absorption. For these latter 
methods are based on the assumption that after entering the cell 
the absorbed substance remains in solution in the same condition 
as it previously existed outside, so that if it were capable of entering 
the cell it would do so until there were equality of concentration 
inside and outside the cell, unless the permeability of the cell mem¬ 
branes to the substance became reduced to approximately zero. But 
as a matter of fact this assumption cannot be correct, for the position 
of equilibrium depends on the nature and concentration of the 
substance. Consequently the rate of deplasmolysis does not necessarily 
give a measure of the total absorption of solute, but only of the 
increase in the osmotic concentration of the cell sap, which may be 
a very different thing if the whole of the absorbed solute does not 
remain in solution and so retain its osmotic activity. 
That such a complication is possible was indeed recognised by 
Fitting (1919), and also by Hofler and Steigler (1921) who investigated 
the intake of urea by means of Holler’s plasmometric method. Some 
of the results obtained by Hofler and Steigler may be quoted. They 
found a tissue, the red-violet epidermis of the stem of Gentiana 
Stuvmiana, which absorbs urea with remarkable rapidity. The mean 
osmotic concentration of these cells lies between 0*4 M and 0-55 M 
sucrose. When these cells, after a preliminary washing in water for 
about twenty minutes, are placed in a gram-molecular solution of urea, 
the intake of urea is such that the concentration of this substance 
inside the cells increases by about 0*02 to 0-07 gram-molecule per 
minute. This compares with about 0*05 and 0-06 gram-molecule per day 
found for the intake of this substance by the epidermal cells of the 
underside of the midrib of the leaf of Rhceo discolor by de Vries (1889 b); 
o*oo8 to 0-016 gram-molecule per hour found for the same tissue by 
Fitting (1919); o-oi to 0*03 gram-molecule per hour by the parenchy¬ 
matous cells of the internodes of Tradescantia elongata immersed in a 
plasmolysing solution of concentration 0*50 M found by Hofler; and 
0-04 to o-11 gram-molecule per hour found by the same author for the 
intake of urea by the bulb scales of Allium Cepa. If the concentration 
gradient is taken into account, the absorption of urea by the epidermal 
cells of the stem of Gentiana Stuvmiana is thirty times as rapid as in 
the bulb scales of Allium Cepa , forty-five times as rapid as in the case 
of Rhceo discolor and sixty times as rapid as in the case of Tradescantia 
elongata. 
This rapid intake is not shown by other substances, for the 
same cells of Gentiana Stuvmiana immersed in o-6o M potassium 
