Physiological Studies in Plant Anatomy 77 
carbohydrate equilibrium with the starch storing tissues in this 
region. 
Discussion 
From a consideration of the foregoing observations and experi¬ 
ments there emerges a definite view of the part played by exudation 
in regulating the flow of sap in the plant. The basal conception is that 
of an aqueous solution moving within a readily permeable xylem 
tube and able to diffuse outwards laterally along the permeable walls 
of the neighbouring protoplasts. 
At any moment certain protoplasts near the xylem tube have 
specially permeable plasma membranes and thus permit the ex¬ 
change of organic substances between their sap and the solution in 
the xylem tube. The direction in which the solute would pass depends 
upon its relative concentration in xylem vessel and protoplast so 
that in one region the liquid in the xylem vessel may be receiving the 
substance from the surrounding cells, in another region the same 
solute may be diffusing away from the xylem into the neighbouring 
tissue. Thus we arrive at the conception of an equilibrium concentra¬ 
tion for any solute, which is illustrated experimentally by cases 
where sugar solution passes through the vascular tracts of a living 
stem without appreciable gain or loss of concentration in passage. 
It is the presence of these organic solutes in the xylem sap which 
maintains the osmotic pressure of the sap at a level permitting the 
manifestation of exudation pressures when the surrounding parenchy¬ 
matous tissues are in contact with a more dilute aqueous solution. 
In the root these exudation pressures may be vigorous and well 
maintained because the lateral leakage of these solutes from the 
xylem tube is so efficiently prevented by the endodermis. In stem 
and leaf exudation pressures may be temporarily demonstrated 
under suitable conditions, but the absence of a functional endodermis 
permits too rapid a leakage of the necessary solutes from the xylem 
for the pressures to be long maintained. 
The water column in the xylem is usually ascending in a leafy 
plant under the pull exerted as the result of transpiration from the 
leaves, and exudation pressure is a relatively unimportant pheno¬ 
menon. Neglect of this consideration has possibly led to the attitude 
of opposition in which theories of the mechanism of exudation 
pressure appear to find themselves, in relation to the admirably clear 
case for the mechanism of transpiration suction presented by 
Dixon (10) and his co-workers. Thus in the brief preliminary account 
of her work given by the late Sarah Baker (2), after drawing attention 
