Walter Stiles 
196 
although of course where sucrose is present the osmotic pressures 
calculated on the assumption that the whole of the sugar is glucose 
are too high as the osmotic pressures of equal weights of sucrose and 
glucose are in the ratio of approximately 1:2. As free acids were 
present in the sap and as the method of analysis of de Vries involved 
heating the sap it is probable that any sucrose would be inverted 
and so appear as hexose reducing sugar. The highest values for glu¬ 
cose were found in the petals of Rosa and the leaf stalks of Heracleum 
sphondylium, in which the percentage of the osmotic pressure at¬ 
tributed to glucose was found to be 807 and 69-1 per cent, respec¬ 
tively. At the other extreme come leaves of potato in which the 
part of the osmotic pressure of the cell sap attributable to sugar was 
found to be less than 4 per cent. Between these two extremes all 
possible intermediate conditions can be found. 
Potassium chloride is responsible for more than half the osmotic 
pressure in the cells of leaf stalks of Gunnera scabra, while in other 
plants other inorganic salts may form an important fraction of the 
total osmotically active substances, as for instance in growing shoot 
apices of Helianthus tuberosus, where de Vries found potassium 
nitrate responsible for 41*4 per cent, of the total osmotic pressure. 
Copeland (1896) concluded that the same salt was mainly responsible 
for the osmotic activity in Fagopyrum, Pisum, Phaseolus and Zea. 
The work of Nathansohn (1901) on Codium tomentosum suggests that 
in this plant sodium chloride may be an active osmotic constituent 
of the cell sap, while the more recent work of Wodehouse (1917) 
indicates the presence of a number of inorganic salts in the sap of 
the marine alga Valonia. It is probable that in marine plants 
generally, and also in halophytes, sodium chloride particularly may 
play a prominent part in the maintenance of osmotic pressure of the 
sap. That such is indeed the case cannot, however, be accepted with¬ 
out question (cf. T. G. Hill, 1919). Von Faber (1913) states that in 
mangroves the high osmotic pressures are due to sodium chloride in 
some species but not in others. 
It will be noted that a dissolved substance in the cell sap can 
only be a factor in the maintenance of a permanent osmotic pressure 
if the protoplasm is for all practical purposes impermeable to the 
substance. Now it is to be expected from the general considerations 
in earlier chapters that substances of small molecular weight such 
as potassium chloride and potassium nitrate and other inorganic 
salts would penetrate the protoplasm with comparative ease and 
rapidity as they do through membranes artificially prepared. There 
