6 
Walter Stiles 
blue and night blue, while two were absorbed only slowly, namely, 
diazin green and Victoria blue R. 
The significance of these results in regard to the mechanism of 
cell permeability will be dealt with in a later chapter. 
This method is obviously chiefly useful as affording a qualitative 
test of the penetration of dyes into plant cells, and although it fur¬ 
nishes indisputable evidence of the entrance of a penetrating dye, it 
is not safe to use it as evidence of the non-penetration of a dye when 
the cell sap remains uncoloured. Nor is the method a very suitable 
one for quantitative measurements. Nevertheless, Collander has made 
it so by mounting sections of tissue immersed for a certain time in 
the dye in solutions of the dye of different concentrations and so com¬ 
paring the colour of the section with that of the surrounding solution. 
The solution the colour of which appeared of the same depth as that 
of the section is taken as having the same concentration as the dye 
in the cell sap. In this way Collander showed that after immersion 
in quite strong solutions of sulphonic acid dyes, the concentration of 
the cell sap in regard to dye is usually only a fraction of that of the 
external solution. 
(ii) Acids and alkalies. If the cell should contain a substance 
which acts as an indicator for acid or alkali, the passage of substances 
belonging to one or other of these groups can be detected by the change 
in colour of the sap. De Vries (1871 a , b) observed the permeability 
of the cells of the root of red beet to ammonia by the change in colour 
of the pigment in the cells from red to blue. 
Pfeffer (1877) extended this observation to other plant cells with 
coloured sap* such as the petals of flowers of Pulmonaria and the 
staminal hairs of Tradescantia, and showed that such cells are readily 
permeable to acids, including weak acids such as carbonic, acetic, 
tartaric and phosphoric acids, and to alkaline hydroxides as well as 
ammonia. If the reagent is used in dilute enough solution and the 
tissue well washed with water soon after treatment so that the acid 
or alkali is quickly removed, the original colour of the sap may return 
and the cell apparently suffer no injury by the treatment. 
Haas (1916 a) found a number of tissues suitable for examining 
the permeability of acids and alkalies by this method, these tissues 
including the petals of Browallia speciosa , Pelargonium and Hyacin- 
thus orientalis, var. Queen of the Blues , and the root of red radish. 
The rate of penetration of a number of acids and alkalies through 
these tissues was compared, the substances being used (1) in a concen¬ 
tration of o-oi N, and (2) in such a concentration that the hydrogen- 
