68 J. H. Priestley and Dorothy Armstead 
plasts by the osmotic action of these solutes. In Kraus’ experiments, 
these solutes were free to gather at the cut surface of the parenchy¬ 
matous tissues, whilst such sap as collected in the xylem was sinking 
through the vessels into the moist sand below. 
In concluding this section reference may be made to an incidental 
observation. 
One of the earliest leafy twigs to be used for experiments was a 
branch of Fuchsia and negative results were obtained both on this 
occasion and whenever this plant was used. It seemed possible that 
the regularity with which the liquid sank in the tube above the cut 
end of the leafy Fuchsia branch might be due to the constant leakage 
proceeding through the multicellular hydathodes to be found at the 
termination of the xylem in the marginal teeth of the leaves. These 
hydathodes are known to exude liquid when the sap in the xylem is 
under positive pressure and it may well be that, simply under the 
head of water above the hydathode, the sap is flowing through 
the xylem out from the hydathodes into the water in which the leaves 
are immersed. Fuchsia twigs were therefore taken and the margin 
of the leaves dipped in melted paraffin wax. When now immersed 
in water, the rise of sap in the capillary tube above the cut end was 
noted after a few hours. 
The experiments described in this section, then, together with 
the literature cited, seem to supply a considerable body of evidence 
for the existence under suitable conditions of an exudation into the 
xylem cavity from the surrounding parenchyma in stem and leaf as 
well as in root. This exudation would probably depend upon the 
two factors defined in a previous paper (Priestley (20)), (i) Osmotic 
withdrawal of water from parenchyma cells incapable of further 
extension and ultimately supplied from a watery medium less con¬ 
centrated in solutes than the sap within the xylem vessel; and 
(2) A supply of solutes to the xylem sap adequate to maintain this 
relatively high osmotic concentration. This leads to a consideration 
of the solutes present in the sap of the xylem under different con¬ 
ditions, a question which is discussed in the succeeding section. 
III. Solutes in the Xylem Sap. 
Gelston Atkins (i) ( loc . cit. p. 187) has already pointed out how 
widespread is the erroneous impression that the water in the xylem 
vessel is an exceedingly dilute solution of inorganic salts, and the 
work emanating from the Botanical Department of Trinity College, 
