Physiological Studies in Plant Anatomy 73 
In some cases no indication of sugar was obtained, even by 
Molisch’s reaction, but usually this test gave clear indication of the 
presence of sugar. Sometimes a marked reduction of Fehling’s 
solution took place immediately, more frequently no reduction with 
Fehling’s was obtained until after inversion with acid and subse¬ 
quent neutralisation, indicating that in these cases cane sugar was 
the only sugar present. In other cases the formation of glucosazone 
gave indubitable evidence of a hexose sugar; the solutions were too 
dilute or too opalescent to enable these indications to be extended by 
the use of a polarimeter. In spite of the opalescence frequently noted 
in the liquid, the bi-uret test never gave indication of the presence 
of a protein or peptone in the solution. 
The osmotic concentration of the sap was obtained in some cases 
by Barger’s method. (See Barger ( 3 ) and Yamakami ( 24 ). This method 
has already been used in plant physiological work by Halket(i 4 ).) The 
most concentrated saps examined were found to be isotonic with 
0-25 M sodium chloride solution. 
IV. Exchange of Sugar between the Xylem and 
the Surrounding Protoplasts 
During the course of this work, Flood’s paper ( 13 ) appeared upon 
the nature of the liquid issuing from the hydathodes of Colocasia 
antiquorum (Schott). The liquid issues through the hydathode when 
the sap in the xylem is under positive pressure. Flood’s investiga¬ 
tions show clearly that it passes through no filtering mechanism in 
its transit through the hydathode and yet the water that issues is 
as pure as conductivity water. How can this statement.be reconciled 
with the statements and experimental evidence referred to above, 
showing that the xylem sap contains sugar or other organic solute? 
The reconciliation is not difficult and has already been indicated; 
see, for instance, Ewart (12) {loc. cit. pp. 349-351, Phil. Trans. 199 B), 
where cogent • experimental evidence is given for the absorption of 
solutes in their passage along the xylem vessel by the process of 
adsorption. 
In the course of our own work we endeavoured to demonstrate 
quantitatively the removal of sugar from the stream of water passing 
along the xylem. These experiments are perhaps more open to 
criticism than those of Ewart, as it is very difficult to obtain quan¬ 
titative results on this point which will have the precision of the 
qualitative demonstrations given by Ewart. 
