Permeability 195 
(1884 a) analysed the expressed sap from a number of plant organs 
and calculated the osmotic pressure due to each of the substances 
present. He found the sum of the osmotic pressures so obtained 
approximately equal to the osmotic pressure of the expressed sap 
as determined by the plasmolytic method. A number of his results 
are summarised in Table XXIII. 
Table XXIII 
Osmotically active Constituents of Cell Sap 
(Data from de Vries) 
Osmotic pressures (as saltpetre values) of the various constituents in different species 
Gunnera scabra 
Heracleum 
Leafstalk 
Rheum 
Rheum 
Rochea 
Rosa 
sphondylium 
X- A ~ 
-N 
officinale 
hybridum 
falcata 
hybrida 
Constituent 
Leaf Stalk 
Young 
Old 
Stem 
Leaf Stalk 
Leaf 
Petal 
Organic acids 
0*020* 
0*023* 
0*028* 
o*o63 2 
0*I24 2 
0*055* 
0-023 3 
Potassium salts of 
0*013 
0*004 
0*004 
0*012 
0*013 
0*004 
0*012 
organic acids 
Glucose 
OI52 
0026 
0*021 
0*085 
0*052 
0*030 
0*218 
Potassium chloride 
— 
0*062 
O-OgO 
— 
— 
— 
— 
Sodium chloride 
0*014 
— 
— 
— 
— 
0*015 
— 
Potassium phosphate 
— 
— 
0*003 
0*012 
0*007 
— 
— 
Total 
0*199 
0*115 
0*146 
0*172 
0*196 
0*104 
0*253 
Found saltpetre value 
0*22 
0*12 
0*16 
0*20 
0*22 
0*13 
0*27 
of sap 
1 Principally malic acid and calculated as such. 
2 Principally oxalic acid and calculated as such. 
3 Not identified with certainty but probably malic acid and calculated as such. 
These results and others show that the osmotically active sub¬ 
stances of the cell sap are very varied. De Vries (1879, 1883) had 
previously emphasised the importance of organic acids in the main¬ 
tenance of turgor, but his later work showed that this is by no means 
a general rule. Nevertheless, in Rheum hyhridum oxalic acid, and in 
Rochea falcata malic acid, play a leading part in maintaining the 
osmotic pressure of the cell sap, while the observations of Kraus 
(1886) on a number of Crassulaceas show that in these plants half the 
dry weight may consist of soluble salts of malic acid. High per¬ 
centages of oxalic acid in the sap were found by de Vries in the cases 
of leaf stalks of Begonia Rex and B. manicata, in which free oxalic 
acid and potassium oxalate accounted together for about the osmotic 
pressure of the sap. 
The percentage of the osmotic pressure of cell sap due to sugars 
was found by de Vries to be very varied. The sugar is always calcu¬ 
lated as glucose by de Vries, but it will probably be more accurate to 
regard his numbers for glucose as approximately those for total sugar, 
