44 J. H. Priestley 
was decreased the rate of upward flow of the sap increased for the 
moment, but soon fell below its original value. Chamberlain regarded 
his results as inexplicable in terms of physics and chemistry and 
therefore as an argument in favour of “vital” activity in exudation 
pressures. Unfortunately, when critically examined, the data given 
by Chamberlain do not permit of a definite decision between two 
obviously possible explanations that involve no introduction of a 
mysterious “vital” force. 
If the exudation mechanism, briefly outlined at the outset of the 
paper, be reviewed, it will be seen that the driving force for the 
upward flow of sap is the difference of osmotic pressure existing 
between the sap in the xylem vessel and the soil solution. The 
concentration of the soil solution may be regarded for the present 
discussion as constant; increased or diminished flow of the ascending 
sap will therefore be directly dependent upon increased or diminished 
concentration of solutes in the xylem vessel which are unable to pass 
the semi-permeable membrane of some of the surrounding protoplasts. 
If therefore the sap in the xylem vessels be suddenly placed by 
experimental means under increased pressure, the temporary halt 
in the flow of sap as the pressures adjust themselves will be accom¬ 
panied by an increased concentration of the solutes in the xylem, if 
their discharge continues as before whilst they are no longer carried 
away so rapidly. Obviously, therefore, after the root system has 
adjusted itself to the new pressure, the flow of sap will recommence 
and for a time at a greater rate. If, however, the more rapid flow is 
only due to the temporary accumulation of solutes, the final equi¬ 
librium rate, depending as it does upon the rate of leakage of solutes 
into the xylem vessel as balanced against their loss in the upward 
flow, should be practically the same as the rate of flow existing before 
the pressure was altered. 
If, however, the rate of flow remains steadily higher when a 
steady rate is reached, then it would be clear that the concentration 
of solutes in the xylem sap was actually now maintained at a higher 
level although the rate of flow was greater. The natural cause to 
assign for this on a first analysis would be an increased permeability 
in some of the cells adjacent to the xylem vessel as a result of the 
increased pressure. This is a conclusion of very great significance and 
might well have great bearing upon the problem of the nature of the 
causal factors controlling xylem differentiation. 
The converse argument would lead us to expect that a smaller 
flow under diminished pressure might be explained by a lessened 
