Permeability 193 
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concentration of the cell sap of Cheetomorpha increased as a result of 
transferring it to a solution of higher salinity, while Mayenburg (1901) 
found a similar increase in osmotic pressure in Aspergillus niger when 
this fungus was grown on strong nutrient solutions. The osmotic 
pressure of the cell sap of root hairs of halophytes appears to be con¬ 
siderably higher than that of mesophytic plants (cf. Drabble and 
Lake, 1905,1907), while T. G. Hill (1908) has shown that the osmotic 
concentration varies with that of the external medium and in the 
same direction. It is, however, not at all clear whether the increase 
in osmotic concentration is due to simple absorption of the salt or 
other osmotically active substance from the external medium, or to 
a complicated series of internal changes not understood (cf. T. G. Hill, 
1919, for halophytes; Mayenburg, 1901, for Aspergillus). According 
to von Faber (1913) the osmotic concentration of the cell sap of 
mangroves is considerably higher than the normal, but only in some 
cases is this due to a high concentration of sodium chloride. Stange 
(1892) could find no accumulation of potassium nitrate in the cells of 
the root tips of Pisum and Lupinus growing in strong solutions of 
this salt although the parenchyma cells behind the root tip contained 
the salt in considerable quantity. 
Wind, no doubt by its influence on transpiration and consequent 
removal of water from the leaf cells, induces an increase of osmotic 
pressure in them (Ursprung and Blum, 1916 c). 
The data bearing on the influence of temperature on the osmotic 
pressure of plant cells have been brought together by Ursprung and 
Blum (1916 c; see also Blum, 1917). From their own observations 
and the earlier ones of Copeland (1896), Pantanelli (1904), Lidforss 
(1907), Kny (1909), Winkler (1913), Buchheim (1915) and Meier 
(1915) it would appear that the osmotic pressure of the cells examined 
is a minimum in the region of io° to 12 0 C. and increases with both 
increase and decrease in temperature. Yet the influence of tempera¬ 
ture has not been wholly disentangled from the influence of other 
factors, and it would be premature to accept such an opinion as con¬ 
clusively proved. 
With regard to the influence of light on the osmotic pressure of 
plant cells, de Vries (1884 a) and Copeland (1896) have shown that 
etiolated plants give lower values of the osmotic pressure in their 
cells than normal plants. Buchheim (1915) concluded from observa¬ 
tions on the alga Cylindrocystis that strong illumination brought 
about an increase in the osmotic pressure of the cell sap of this plant. 
Dixon (1914) found that illumination increased the osmotic pressure 
