i8o 
Walter Stiles 
and split longitudinally into four parts. Usually the pith elongates and 
the epidermis contracts owing to tensions in the tissue; consequently 
the strips of tissue curl with the pith occupying the convex side of 
the strip. If put into water the pith takes up water rapidly so that 
the whole rolls up into a spiral. If put in a strong enough salt solu¬ 
tion the pith gives up part of its water so that the pith contracts and 
the curvature is reduced or may even be reversed, that is, the pith 
occupies the concave side. A solution of any particular substance 
can thus be obtained by sufficiently exact grading of the solutions 
in which no change of curvature takes place at all. In this con¬ 
centration in which water is not taken up, the osmotic pressure of 
the external solution is equal to the osmotic pressure of the cells less 
the turgor pressure of the cells. Since these two last quantities may 
be assumed the same in similar strips of the same tissue, it follows 
that the solutions of different substances which just bring about no 
intake or excretion of water from the pith are isotonic with one 
another, though not with the cell sap. This method has two advan¬ 
tages. As a great number of cells are involved the values obtained 
are mean values. Also the experiment only takes a few minutes; 
indeed a long duration of the experiment must be avoided in order 
to prevent absorption of the salt or other dissolved substance by the 
tissue. According to de Vries it is a disadvantage of this method 
that the material for it can only be obtained in spring and summer, 
but there should be little difficulty in obviating this disadvantage 
where a warmed greenhouse is available. Details of the method in 
which scapes of dandelion (Taraxacum dens-leonis) are used as the 
experimental material in summer and hypocotyls of Ricinus seed¬ 
lings are used in winter, are given in Darwin and Acton’s Practical 
Physiology of Plants which is probably easily accessible to nearly all 
English readers 1 . 
1 Darwin and Acton and other writers assume that the solution which 
produces neither increase nor decrease in curvature equals the cell sap in 
osmotic “force” (i.e. pressure), but as we have already seen, and as de Vries 
pointed out in his own description of the method, the osmotic pressure of the 
external solution which produces no change in the tissue is not equal to the 
osmotic pressure of the cell, but to the osmotic pressure of the cell less the 
turgor pressure. In the tissues used for the tissue tension method it does not 
follow that the turgor pressure is small enough to be neglected. This is im¬ 
material in the determination of isotonic coefficients, for the osmotic pressures 
of the solutions of the different substances compared are in all cases presumed 
equal to the same osmotic pressure of cell sap less the same turgor pressure, 
and are therefore equal to one another. But the method does not give the 
osmotic strength of the cell sap, at any rate exactly. Thoday (19186) has 
accused Stiles and Jorgensen (1917 b) of falling into the same error when they 
speak of a solution in which disks of potato neither lose nor gain weight as 
