OSMOTIC PRESSURE IN PLANTS 77 



or upon a drop according as its vapour pressure is greater 

 or less than that of its neighbours. This causes the drops 

 to decrease or increase in size, an effect which can be ob- 

 served by a measuring microscope. The method has 

 recently been employed by Halket (1913) for plant saps, 

 and its value is obvious, as very minute quantities can 

 be employed. 



QUANTITATIVE LAWS OF OSMOTIC PRESSURE. 



Before proceeding to trace the influence of various 

 factors upon the osmotic pressure of cell sap, it seems 

 advisable to give the term a precise meaning, and briefly 

 to describe its nature and the conditions under which it 

 acts. For fuller information Findlay's " Osmotic Pres- 

 sure " should be consulted, and to it the author is much 

 indebted in the following account. According to Findlay, 

 the osmotic pressure of a solution may be defined as " the 

 equivalent of the hydrostatic pressure produced when the 

 solution and solvent are separated by a perfectly semi- 

 permeable membrane; or as the equivalent of the excess 

 pressure which must be imposed on a solution in order to 

 prevent the passage into it of solvent through a perfectly 

 semi-permeable membrane." To be exact, the solution 

 itself does not have any osmotic pressure, but if solvent 

 and solution were separated by a membrane permeable 

 to the former only, then such a pressure would be pro- 

 duced. Thus it is the excess of the tendency of the solvent 

 to diffuse inwards over that to diffuse outwards that pro- 

 duces the osmotic pressure. 



Van't Hoff, stimulated by Pfeffer's direct measure- 

 ments, undertook the thermodynamical deduction of the 



laws of solutions, and arrived at the equation P = ^r- ' 

 where P is the osmotic pressure, T the absolute tempera- 



