Permeability 
3 
i, Direct Test of Permeability by Observation 
of Visible Changes in the Cell 
If a substance on entering the cell should produce a visible change 
in it, it is obvious that such visible change is evidence of the per¬ 
meability of the cell to the substance. 
(i) Dyes. The simplest application of this method is the investiga¬ 
tion of the penetration of dyes into the cell, the permeability of which 
to a dye is shown by the coloration of the cell contents by the pene¬ 
trating dye. Using this method Pfeffer (1886) examined the per¬ 
meability of living cells of a considerable number of plant species to 
a number of dyes. He found that a passage of the dye into living 
cells took place with methylene blue 1 , methyl violet, cyanin, Bis¬ 
marck brown, fuchsin, safranin, methyl orange, tropseolin 000, 
methyl green, iodine green, Hoffmann’s violet, gentian violet and 
rosolic acid. These substances are taken up by the cell from very 
dilute solutions and accumulate in the cell sap so that the concentra¬ 
tion inside the cell must be much higher than that of the external 
solution. Thus methylene blue diffuses into the root hairs of Trianea 
bogotensis and the outer cells of the root of Lemna minor among others, 
so as to give a deep blue colour to the cell sap even when the solution 
of the dye presented to the cells is as dilute as 0-0008 per cent. In 
some cases some of the dye does not accumulate in the vacuole in 
solution, but in the form of a blue precipitate as in the root hairs of 
Azolla caroliniana and in Spirogyra communis. When this is the case, 
continued immersion in the dye may lead to accumulation of the dye 
in solution in the cell sap in addition to the precipitate, as observed 
by Pfeffer in Zygnema cruciatum. The blue precipitate appears to be 
a compound of the dye with tannin, and accumulation of the dye in 
solution in the vacuole apparently only takes place when all the tannin 
present has combined with the dye. Pfeffer supposes the accumulation 
of the dye in solution in the vacuole is due to the formation of an 
undetermined soluble compound of methylene blue with some sub¬ 
stance or substances present in the cell sap, and that the cell mem- 
1 In the English edition of Pfeffer’s Physiology of Plants, 1 (1900), pp. 94, 
96, 97, where these observations of Pfeffer are summarised, this dye is most 
unfortunately referred to as methyl blue, a dye which Pfeffer showed was 
incapable of penetrating into the cells he examined. In Pfeffer's original paper 
an unfortunate misprint occurs, methylene blue being included in the list of 
dyes which penetrate plant cells and also in the list of those which do not. 
The body of the paper makes it quite clear that in the latter case “methylen- 
blau” is a misprint for “methylblau.” 
