350 BOTANICAL GAZETTE [MAY 
Most physical chemists agree that so long as the semi-permeable 
membrane really allows none of the solute to permeate it any resist- 
ance it may offer to the passage of the solvent is without influence 
upon the osmotic pressure. Ostwald,’ Speyers,? and Pfeffert make 
the same statement. The argument—and it a perfect one —is, in 
brief, that the osmotic pressure is the aggregate impact of the mole- 
cules of the solute against a medium which they cannot permeate : 
this medium must contain, and hence be permeable to, the solvent, 
else the solute could not touch it: but as the solvent has no share in 
the actual pressure, it is indifferent whether its movement be free, or 
barely possible. Osmotic pressure, in this sense, is computed from 
boiling and freezing point determinations : it cannot be directly meas- 
ured. Zhe osmotic pressure useful in plant cells, and the osmotic pres- 
sure measured by so-called osmometers, ¢s mot the pressure which the 
flying molecules of the solute exert against the restraining membrane (or 
other medium), du¢ it ts the pressure which this membrane passes along 
against some other, more resistant medium, or the solvent in question 
which is outside it. And this practical, available osmotic pressure 
depends very largely upon the resistance offered by the membrane to 
the passage of the solvent. If several osmometers be set up with the 
same solution, osmosis to take place through equal areas of different 
membranes, it will be found after a time that the columns of liquid 
they support are far from equivalent. In each osmometer the height of 
the column measures the pressure then available for work ; but it may 
be objected that the experiment is not yet complete. 
In a plant all the osmotic pressure which does anything — whether 
it is to keep the plant fresh and stiff (turgescent), or to absorb water 
from the soil, or to transport water or food, or to fulfill an usual con- 
dition of growth —is the pressure of the protoplasm against the cell 
wall. If there is an increase in the osmotically active material in the 
cell sap, z. ¢., in the pressure of the sap against the vacuole wall, the 
promptness with which there can be a corresponding increase in the 
2? OSTWALD : Solutions 103. 
3 SPEYERS: Text-book of physical chemistry 63. 
4 PFEFFER: Pflanzenphysiologie 1:120 [2 ed.]. Zur Kenntniss der Plasmahaut 
und der Vacuolen, etc., 302. 1890. ‘“ Die Qualitat der Haut kann, so lange geloste 
Stoffe nicht exosmiren, den osmotischen Druck nicht beeinflussen und meine friihere 
gegentheilige Annahme, die sich der allgemein verbreiteten Vorstellung anschloss ist 
demgemiss irrig.’ 
