Permeability 173 
to which reference is made; but as the osmotic pressure can only 
exist when the solution is separated from another or from pure water 
by a membrane, a term is useful to indicate the concentration of 
substances in the cell sap in terms of the osmotic pressure that would 
be produced when such a solution is separated from pure water by 
a semi-permeable membrane. For this quantity the term osmotic 
concentration may be employed. Ursprung and Blum (1916 a) prefer 
to denote this quantity by the term “osmotic value,” which also 
appears free from all academic objections. 
Thirdly, there is the inwardly directed pressure of the cell wall 
(wall pressure) which is equal and opposite to the hydrostatic 
pressure exerted against it by the liquid in the cell. This 
hydrostatic pressure has also been spoken of as turgor or turgor 
pressure, but it will be observed that it is not the same quantity as 
the osmotic pressure for which the term is sometimes used. Here 
the term turgor pressure will be used to designate the total hydro¬ 
static pressure exerted by the cell liquid against the protoplast and 
cell wall. 
Fourthly, there is the net pressure sending water into the cell. 
This, as we have seen, is equal to the difference between the osmotic 
pressure of the external solution and the osmotic pressure of the cell, 
less the turgor pressure. This quantity is called by Ursprung and 
Blum (1916 a, 1916 d , 1916 e) and other writers in German (e.g. 
Holler, 1920), often with the assumption that the external liquid 
is water, the suction force (“Saugkraft ”), a term that has been 
applied to the net pressure sending water into whole organs and 
tissues. As this quantity is a pressure rather than a force it will here 
be termed the suction pressure (Stiles, 1922), and it will be convenient 
to distinguish between the net suction pressure, whatever the external 
liquid, and the full suction pressure 1 when the external liquid is water. 
1 Ursprung and Blum (1916 d) clearly recognise the dimensions of the 
suction pressure as being those of a pressure, but for reasons which do not 
appear very forceful prefer to retain the term “Saugkraft.” They say, “Was 
die Terminologie betrifft, so soli die Bezeichnung ‘ Kraft ’ beibehalten werden, 
obschon es sich ja um J^. ra ~handelt, also um eine Grosse die nicht in Kg 
Flache 
sondern in Atm. gemessen wird. Man pflegt ja auch in der Physik von Zug- 
und Druckkraften zu reden.” Nor has the term “water absorbing power” 
proposed by Thoday (1918 6) for this quantity much to recommend it, for the 
quantity is certainly not a power but a pressure, and the term suction force 
has the claim of priority and the advantage of brevity, and in the form of 
suction pressure appears to be free from all disadvantages. When the term 
“suction pressure” alone is used in the following, it indicates the full suction 
pressure. 
