Stiles and J ergensen. — Studies in Permeability. IV. 57 
Pyridine , m. 
Time. 
Conductivity. 
Colour. 
hrs. mins. 
1.0 
133 6 
2.0 
284*2 
3-05 
379*8 
3-30 
397*6 
4.50 
400*9 
5*50 
448*8 
6.35 
465*3 
no general yellowing 
Isobutyl alcohol , 0*5 m. 
Time. 
Conductivity . 
Colour. 
hrs. mins. 
0.30 
33*8 
1.15 
72-1 
leaves turning yellow 
i -35 
119*8 
leaves strongly yellow 
2-35 
127*3 
3-15 
309*0 
S.o 
412*6 
Isoamyl alcohol o*2 m. 
Time. 
Conductivity. 
Colour. 
hrs. mins. 
0.30 
108*3 
leaves strongly yellow 
1.0 
198*4 
1.25 
242*4 
2.20 
298*8 
3 -o 
500-0 
4. 10 
548*0 
We have also tried leaves of Pelargonium , in which a change to olive- 
green occurs with similar results. But it will be obvious from the examples 
with Oxalis , which gives a very marked colour change, that the method of 
electrical conductivity measurement is a much more accurate one for 
following permeability changes than the one depending on colour changes 
in the leaves. Moreover, it is independent of the nature of the substances 
in the external solution, whereas whether the colour change takes place in 
the leaf or not may depend on the chemical constitution of the substance 
in solution and have no relation to permeability. 
Czapek’s method, which consists in testing for the presence or absence of 
tannin in the leaf by means of caffeine, is very similar to the method based 
on the colour change in leaves. It has all the disadvantages of that method, 
and the additional one that it involves the use of a microchemical test in 
place of a very clear colour change easily observable by the naked eye. 
For studying permeability changes in the cell in general, the electrical 
conductivity method here described is to be recommended in preference to 
earlier methods on account of its accuracy and its general applicability to 
all plant tissue and all non-electrolytic external solutions, and because it 
enables the changes taking place to be investigated in a manner impossible 
with the older methods. It must, however, be clearly understood that the 
results obtained by its means have direct reference to the permeability of 
the cell only as regards electrolytes ; the results do not necessarily hold, and 
probably do not hold, for all substances. 
