A. C. Halket. 
170 
After the drops are measured the tubes are set aside for some 
time. After an interval the drops are re-measured and the difference 
in length found. If the tubes have been carefully filled it will be 
found either, that in one tube of the series no change takes place in 
the size of the drops, or, as most frequently happens, that the drops 
of the cell sap increase in size when in tubes with solutions above a 
certain strength, while they decrease in size when with solutions 
below that strength. In the first case the solution with which no 
change takes place in the size of the drops is taken as isotonic with 
that of the cell sap. In the second case the mean between the 
known solutions, in the two tubes, is regarded as isotonic. 
Reference must be made to Barger’s paper for further discussion of 
the method. 
I first estimated the osmotic pressure of the cell sap of plants 
in this way while with Professor P. W. Oliver’s expeditions to the 
Bouche d’ Erquy in Brittany. During the course of the work there 
it was thought desirable to investigate the value of the osmotic 
pressure of the cell sap of some of the characteristic plants. The 
The greater part of the work was done on Salicornia. This is a 
very succulent plant, with a jointed stem and small opposite scale 
leaves. Underneath the epidermis there are two or three layers of 
small-celled palisade tissue, then a zone of aqueous tissue before 
the central strand with the vascular tissue is reached. The osmotic 
pressure of the cell sap was first estimated by plasmolysis, but this 
method was found to be satisfactorily applicable only to the cells of 
the epidermis and not to the cells of the aqueous tissue which are 
large and have no easily visible protoplasmic lining. It was there¬ 
fore suggested by Mr. F. Baker, a chemist attached to the party, 
that it might be possible to use Barger’s method for determining 
molecular weights in order to find the values of the osmotic pressure. 
The method was tried and found to be quite applicable, and many 
estimations were made. 
Efficiency of Method. It was necessary to test the delicacy of 
the method, so a series of experiments was made to find whether 
the difference in the vapour pressures of solutions of sodium chloride 
in water, differing only slightly in strength, was sufficient to make 
the drops in the capillary tubes show any change in length. The 
solutions of salts generally used for the plasmolytic method vary in 
strength by ’01 of a gram-molecular solution. It was found that 
similar differences in strength could be detected by Barger’s method. 
This is shown by the figures given below :— 
