The Mechanism of Root Pressure 43 
sides of the cell seemed a necessary assumption to explain the facts 
obtained (Priestley, loc. cit. p. 195). This would certainly seem to be 
true for superficial hydathodes. In the case of the mass of parenchyma 
enclosed within the root endodermis however, the needs of the 
mechanism invoked will be adequately met if some of the proto¬ 
plasts enclosed within the endodermal cylinder are relatively per¬ 
meable to solutes, presumably organic solutes, to which other proto¬ 
plasts within the cylinder are impermeable. Experimental evidence 
for the existence of these more readily permeable protoplasts has 
been briefly put forward, and certain other cogent reasons for as¬ 
suming their existence will now be given. 
The absorbing region of a root is always close behind the actively 
growing apex, and in this region, so long as conditions are suitable 
for the manifestation of root pressure they are also suitable for 
growth. At this distance behind the growing apex growth will be 
manifest as differentiation, and within the endodermal cylinder such 
differentiation involves the continual metamorphosis of normal pro¬ 
toplasts into empty xylem vessels and sieve tubes. These changes 
consequent upon differentiation must release a relatively large bulk 
of organic solutes which are unable to leak outwards to the cortex 
because of the functional endodermis and which must therefore find 
their way into the xylem. It does not seem at all improbable that 
this source of solute alone, always available whilst the root is grow¬ 
ing, would be adequate to account for the normal exudation pressure 
developed within the root. When it is considered that such changes 
are proceeding in every active terminal branch of a large root system 
it is clear that they will supply a very large bulk of organic solutes 
to the ascending sap and so long as the root is growing the supply 
will be constantly maintained. 
Reference may now be made to an earlier investigation upon 
root pressure which may throw an important light upon the condi¬ 
tion under which protoplasts, bordering upon the already differ¬ 
entiated xylem, may gradually become increasingly permeable until 
their contents are allowed to diffuse into the xylem stream or are 
deposited upon their walls. Chamberlain( 4 ), in an incomplete in¬ 
vestigation, examined the effect upon the rate of flow of altering 
the pressure in the ascending sap column. Somewhat to his surprise, 
apparently, he found that if the root system was driving sap into 
a manometric system fixed on to the cut stem, when the pressure 
upon the sap was increased, the rate of flow after a temporary check, 
actually increased. Conversely if the pressure upon the head of sap 
