xo6 
V. H. Blackman 
acquaintance is assumed here with the properties of colloids. Refer¬ 
ence may be made to the works of Hatschek (1913), Taylor (1915), 
Bayliss (1915, 1918) and Wolfgang Ostwald (1909) in which colloids 
are treated from various standpoints. 
OSMOTIC PRESSURE, ROOT PRESSURE, 
AND EXUDATION 
By V. H. BLACKMAN 
(With 3 figures in the text.) 
he exudation of sap from the cut stem of a rooted plant is such 
X a striking phenomenon that it is not surprising that many 
attempts have been made to elucidate the physics of the process. A 
number of the older workers attempted to explain the phenomenon 
on simple physico-chemical principles and recently Rowell (1918), 
and Priestley (1920) in this Journal, have again discussed its 
mechanism. 
It is usually assumed that the osmotic pressure of the living cells 
of the root and stem play a main part in the production of root 
pressure, and it is usually recognised that one of the chief difficulties 
is the passage of water or a weak solution from the living parenchyma 
cells into the cavities of the dead wood-elements. Priestley accord¬ 
ingly suggests that the explanation put forward by Lepeschkin 
(1906) of the exudation of water by the sporangiophore of Pilobolus 
and by the multicellular epidermal hydathodes of the leaves of 
Phaseolus and other plants may be used to account for the passage 
of water from the living cells into the dead wood-elements. Since 
Lepeschkin’s views seem to have been accepted by some other 
botanists, and have received the support of Bayliss by inclusion in 
his Principles of General Physiology (1918), it would seem worth 
while to consider them in some detail. 
Pfeffer (1877), at the time that he was making his classical 
researches on osmotic pressure, was the first to deal critically with 
the mechanism of the exudation of fluid from living cells. He 
formulated three hypotheses: (1) that the plasma membrane develops 
unequal osmotic pressures in different parts of the cell; (2) that there 
