198 
] H. Priestley. 
This conclusion of Bayliss leads to another consideration it 
may be well to keep in mind. If the permeability of a protoplasmic 
membrane to glucose is a function of the difference of concentration 
on the two sides of the membrane, then the accumulation of sugar 
within L, or the dilution of this sugar by entrance of water, might 
be factors leading to a change in the permeability of the proto¬ 
plasmic membrane on the side bordering on the xylem vessel, and 
such a change might have the result of rendering the membrane on 
this side temporarily permeable to sugar. In other words, instead of 
the constant excretion of water, noted by Lepeschkin in the 
hydathodes under observation, we may here be dealing with an 
intermittent phenomenon in which changes in permeability follow 
upon the changes in concentration within the protoplasts 
bordering upon the xylem, the original permeability, or lack of 
permeability, being restored as these concentrations readjust them¬ 
selves by temporary discharge of water and solute into the xylem 
stream. This process, intermittent in the individual cell, would 
appear as a continuous process in the phenomenon of root pressure, 
where the final result is the sum of the activities of many thousands 
of cells in different phases of activity. 
Returning from this digression to the problem as to the 
constant provision of the solute which passes out with the water 
into the xylem vessel, the hypothesis that this solute is an organic 
acid derived from sugar has much attractiveness. As pointed out 
by Bayliss (2, loc. cit. p. 165), such chemical changes permit of 
rapid increase in the osmotic concentration, one molecule of 
glucose giving rise to several molecules of acid. Puthermore, the 
experimental data already accumulated (see for instance Hind 7, 
1914, p. 229, et. seq.) show that protoplasm is relatively rapidly 
permeable by organic acids under some conditions, though the facts 
of translocation drive us to assume a similar permeability for 
sugars under certain unknown conditions. 
Another difficulty is the apparent contradiction between the 
assumption that solutes enter the xylem vessel with the water of 
the root pressure stream, and the facts recorded by Flood (5, 1919) 
in the case of the excretion from the leaf tips of Colocasia anti¬ 
quorum. This very vigorous excretion has been shown to be due 
to the activity of the roots in pumping water up, and, during its 
upward passage until its exit at the tip, the water passes through 
no filtration mechanism whatever. But the water excreted is 
practically free from impurities. In that case what becomes of the 
