Physiological Studies in Plant Anatomy 79 
Summary 
In this paper attention has been confined to the nature and 
conditions of the exchange of solutes between the xylem and the 
parenchyma around the vascular strand. Experimental grounds are 
given for the following conclusions: 
(1) At any moment certain cells around the vascular strand and 
within the vascular cylinder have especially permeable protoplasts ; 
in some cases these protoplasts would seem to be more readily per¬ 
meable by acid or basic ions. This phenomenon may account for the 
continued presence of solutes within the xylem vessel during the 
manifestation of exudation pressure. 
(2) Given suitable experimental conditions, exudation pressures can 
be exhibited by stems and leaves as well as by roots. In this connection 
some of the earlier work by Pitra, Kraus and Boehm is reviewed. 
(3) In the case of Fuchsia, failure to obtain the evidence of 
exudation pressure in the case of leafy twigs, can be traced to leakage 
from the hydathodes at the vein ends. If these exits from the veins 
are blocked, exudation pressures can easily be demonstrated under 
suitable conditions. 
(4) The solutes present in the xylem sap are both organic and 
inorganic. As the organic solutes fell off in quantity in the sap rising 
from a cut vine, the exudation pressure began to fail whilst the 
supply of inorganic solutes still remained approximately constant. 
Organic solutes would thus seem to be more directly responsible for 
the osmotic pressure effective in causing the flow of sap. On theo¬ 
retical grounds this result would be expected as the inorganic solutes 
present have probably diffused across the endodermal barrier. 
(5) The organic solutes in the case of the vine were mainly sugars, 
disaccharides and hexoses, but other organic substances were present, 
and one of these could be extracted by hot ether and subsequently 
obtained crystalline. Amongst the inorganic solutes, the presence of 
nitrites previously reported was confirmed, nitrites being found both 
in Vine and Fuchsia. 
(6) The fact that the water dropping from the hydathodes of 
Colocasia antiquorum (Schott) is practically free from solute, can 
readily be explained as due to the absorption of the solutes during 
the upward passage of the sap. This absorption appears to involve 
both physical adsorption and physiological absorption, and both 
processes are reversible. 
(7) The conception is developed of an equilibrium concentration 
for a sugar solution which would neither lose nor gain sugar during 
