Osmotic Pressure, Root Pressure, and Exudation 115 
by means of which this exudation could be brought about, though 
how far such mechanisms are actually at work in the living cell it 
is at present impossible to say. Much more knowledge of cell 
dynamics is required before we can deal satisfactorily with such 
difficult problems as exudation and root-pressure. Quantitative 
data are particularly necessary for example as to the force with 
which fluid can be exuded from the cell as the result of the action 
of electro-endosmosis or of the difference of concentration of 
solutions in different parts of the cell. 
Imperial College of Science and Technology, 
London, S.W. 7. 
May 1921. 
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