Permeability 187 
determinations tabulated by Dixon and Atkins (1913 b) the osmotic 
pressure was less than 11 atmospheres in only 13 cases, the lowest 
value found being 5-83 atmospheres in the leaf of Saccharum offici- 
narum and the highest 38-32 atmospheres in the fruit of Vitis vini- 
fera. According to Pfeffer (1900) the osmotic pressure, even in 
starved cells, rarely falls below 3-5 atmospheres (Stange, 1892; 
Copeland, 1896), while at the other end of the range saprophytic 
fungi, such as Aspergillus niger and Penicillium glaucum, growing on 
concentrated nutrient solutions, may develop an osmotic pressure of 
the sap of as much as 157 atmospheres (Eschenhagen, 1889). It is 
clear that in such cases the suction pressure is very great when the 
cells are transferred from the concentrated medium in which they 
were growing to distilled water, and it is not surprising that under 
such circumstances the cell walls may be ruptured (Pfeffer, 1900). 
The highest value recorded for the osmotic pressure of the sap of a 
flowering plant appears to be 153-1 atmospheres found by Harris, 
Gortner, Hofman and Valentine (1921) for a plant of Atriplex con- 
fertifolia growing in the neighbourhood of the Great Salt Lake. 
The Relation between the Position of a Cell in the 
Plant and the Osmotic Pressure of the Cell 
This is a question to which some considerable attention has been 
given, chiefly in relation to the ascent of water in trees. Ewart (1905) 
made some determinations of the osmotic pressure of leaf cells by 
the plasmolytic method and thought he had shown that the con¬ 
centration of the sap of leaf cells increases from lower to higher levels 
of the plant, but later (1906) came to the conclusion that the errors 
inherent in the method were too great to allow the drawing of definite 
conclusions. From the determinations of Dixon and Atkins (1910) it 
is concluded by Dixon (1914) that on the whole leaves at a lower level 
contain sap with a lower osmotic pressure than that of leaves inserted 
at a higher level, but that their results are not altogether satisfactory 
from this point of view. It is to be noted that these determinations 
were made before these workers realised the importance of a pre¬ 
liminary freezing of the tissues in order to obtain a real sample of 
the sap when pressed from the tissues. Hannig (1912), using the 
plasmolytic method, found the osmotic concentration higher in leaves 
than in roots. His statement that Dixon (1910) found the osmotic 
concentration of leaves independent of the level of their insertion 
is scarcely an exact statement of this author’s results. Harris, 
Gortner and Lawrence (1917) made 26 sets of determinations, in- 
13—2 
