1900 | BRIEFER ARTICLES 351 
pressure against the cell wall depends on how rapidly water can get 
into the sap, and this in turn depends, inter alia, upon the resistance 
the protoplasm offers to its passage; this is very slight, but still must 
be something. Now in practice the osmotically active matter in the 
sap varies constantly,’ and therefore the osmotic pressure available for 
work does depend constantly upon the ease with which water traverses 
the semi-permeable membrane. 
To return to the osmometer: the column of liquid supported rises 
more slowly in some cases than in others because the membrane offers 
greater resistance to the movement of water through it. It seems 
evident to me that, if the osmotic pressure is the only force at work, 
there will be a certain minimum of pressure below which it would not 
suffice to put the water in the membrane into motion at all. And in 
this case the height at which the column finally comes to rest will 
measure the total impact of the molecules against the membrane mznus 
the resistance the membrane offers to the passage of the solvent. 
Whether the movement of the water in the membrane by its own “ liv- 
ing force” will be more inward than outward until the entire theoreti- 
cal osmotic pressure gets expression in the column supported, I do 
not know, but doubt it: and anyway that would be by the introduc- 
tion of another source of energy. Talk is not likely to get far on a 
subject where there have been no experiments for twenty-three years, 
but it is about all that we have, pro or contra. So long though as he 
based his opinions on experiments, Pfeffer thought that the pressure 
was influenced by the resistance of the membrane. And Krabbe® says 
that at temperatures near the freezing point a state of equilibrium 
is reached while there is a decided tension between the periphery and 
axis of a cylinder of live pith submerged in water, because of the 
resistance to the passage of water through the outer cells. 
All these considerations apply equally to the tubes closed by a mem- 
brane permeable only to the gases on one side. And the fact that the 
“Croton” leaf did not keep its lead does not destroy my position, 
because the inward diffusion of the gases of the atmosphere is a 
5 As by movement of salts or food, formation or hydrolysis of starch, etc. “ 
found some years ago, the turgor in the leaf of Funaria is regularly higher at night, 
by the pressure of 0.5 per cent. KNOg, than it is in the morning. 
: Krassg, G.: Jahrb. f. wiss. Bot., 29: 447. 1896. As the water drawn in by 
a is held by imbibition in the wall, the experiments on imbibition are valid for 
Ssmosis. Review in Bot. Gaz. 23: 303. 1897. 
