Permeability 179 
leaves of Tradescantia discolor (Rhceo discolor ), and (3) red cells of 
the upper epidermis of the scales of the leaf stalk of Begonia mani- 
cata y together with cells of the red spots in the epidermis of the leaf 
stalk in the neighbourhood of the scales. The last is not to be recom¬ 
mended so much as the other two but is particularly useful in the 
case of acids. The chief advantage of these particular cells is that 
they contain coloured sap in the vacuole which renders the observa¬ 
tion of the slightest sign of plasmolysis much easier than in the case 
of cells with colourless sap. 
It is a disadvantage of this method that individual cells are 
observed, and if different cells of the same tissue differ in their 
osmotic pressure observation has to be made on a number of cells. 
In order to reduce differences in osmotic pressure of different cells 
to a minimum, it is necessary to use cells as near together as possible. 
If the cells are not all of the same osmotic pressure, the solution in 
which half the cells become plasmolysed may be taken as the solution 
which is osmotically equivalent to the mean of the cell saps (cf. 
Darwin and Acton, 1901). A slight disadvantage is that the method 
requires time as the observer has to wait for equilibrium to result 
which is not likely to take less than an hour. An advantage of the 
method is that material for its performance can be obtained at all 
seasons of the year. The exactness of the determination depends on 
the degree of agreement between the different cells employed, on the 
number examined to determine the mean osmotic pressure of the 
cells and on the differences in concentration between the solutions 
of any of the substances examined. 
Plasmolytic transport method. In this variant of the plasmolytic 
method cells are weakly plasmolysed in a solution of the salt under 
investigation and when plasmolysis is ended the cells are transferred 
to potassium nitrate solutions of different concentrations. That solu¬ 
tion in which the volume of the plasmolysed protoplast remains 
unchanged is isotonic with the solution first used. This method has 
the advantage as compared with the comparative plasmolytic 
method that the same cell is used for the comparison of the experi¬ 
mental solution with the standard ones. It has the disadvantage 
that the long immersion of the cells in salt solutions may result in 
damage to the cells. De Vries regarded the method as useful chiefly 
in the investigation of the influence of concentration on isotonic co¬ 
efficients, as the method allows the use of a range of concentrations. 
Method of tissue tension. For the practice of this method* a piece 
of the growing stem or other elongated organ is cut out from a plan! 
