Permeability 197 
is undoubtedly a very real difficulty here to which it will be necessary 
to return later. It is an attractive theory to suppose that whereas 
the protoplasm is readily permeable to inorganic salts, it is imper¬ 
meable to organic substances with larger molecules such as carbo¬ 
hydrates and organic acids, and that the osmotic pressure of the cell 
sap is to be attributed to these. This will, however, not explain the 
results of de Vries and Copeland recorded above. 
Calculations of the molecular weight of the solutes in the ex¬ 
pressed sap of seedling peas made by Maquenne (1896) by means of 
determinations of the freezing point, suggest an average molecular 
weight of 239, considerably higher than that of glucose, namely 180. 
On the other hand, the molecular weight of the solutes in the sap of 
Helianthus was found to be on the average only 136, and as glucose 
was present in the sap, it would appear that the osmotic pressure 
must be ascribed in large part to substances with lower molecular 
weights than that of glucose. Rather similar values were obtained 
by Dixon (1914) for the mean molecular weight of the solutes in the 
leaf sap of Wistaria sinensis and Ulmus campestris , the values in the 
former varying from 149 to 169, and in the latter from 148 to 165. 
Comparison of the electrical conductivity of saps, which gives an 
approximate measure of the content of electrolytes, with the osmotic 
pressure, shows that there is no constant relation in different species 
between the content of electrolytes and total osmotically active 
solutes (Harris, Gortner and Lawrence, 1920, 1921 a, 1921 b). 
The Suction Pressure 
Although the amount of water which a cell is capable of absorbing 
must depend on the osmotic concentration of the cell sap and the 
impermeability of the protoplasmic membrane, it is suction pressure 
which gives a measure of the pressure with which water is absorbed 
by the cell. Attention has recently been directed to the suction 
pressure in plant cells by Ursprung and Blum, who describe methods 
for determining the full suction pressure (1916 d) and who have 
published the values they have determined for it in a number of cases. 
Determination of the full Suction Pressure 
Ursprung and Blum (1916 d) describe two methods for the deter¬ 
mination of the full suction pressure. These may be described as the 
indirect plasmolytic method and the equilibrium method. 
Indirect plasmolytic method . It will be recalled that the full suction 
pressure equals the osmotic pressure of the cell sap less the wall 
