The Mechanism of Root Pressure 47 
are withdrawn from solution. If, for instance, sugar diffuses from a 
weak solution in the xylem vessel into the sap of a neighbouring 
protoplast and is then removed from solution because used in meta¬ 
bolism or stored as starch, then the diffusion gradient will continue 
to drive sugar into this relatively permeable protoplast and “physio¬ 
logical absorption” will be in progress. 
In this connection it is difficult to see why Blackman lays so 
much stress upon the necessity of oxygen and suitable temperature 
relations for the manifestation of root pressure, as indicating that 
“vital” factors other than simple osmotic relations are involved. 
Surely this dependence of root pressure upon external conditions 
necessary for healthy existence may be completely explained by the 
fact that the semi-permeable membrane of the normal protoplast is 
only semi-permeable when healthy. And however adequate physi¬ 
cally the definition of osmotic pressure given by Blackman (loc. cit. 
p. 108), it is necessary to remember that, for practical purposes, the 
manifestation of osmotic activity requires the presence of the so- 
called “semi-permeable” membrane. 
Botanical Department, 
University of Leeds, 
December 1921. 
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(10) Pfeffer, W. Osmotische Untersuchungen. (1877.) 
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