hem; _semi-permeable_, which permit the passage of the
solvent, but not that of dissolved crystalloidal substances; or
_permeable_, which permit the free passage through them of both solvents
and solutes. The first and last of these types of membranes have no effect
upon osmotic pressure; but osmotic pressure is at once set up whenever a
semi-permeable membrane is interposed between solutions of different
concentrations. It is due to the molecular motion of both the liquid and
the dissolved solids, as a result of which a greater number of molecules
are "bombarding," or pressing upon the membrane from the side of the more
concentrated solution. This sets up an unequal pressure upon the two sides
of the membrane, and if the latter be semi-permeable there will result a
passage of the liquid through the membrane toward the denser solution so as
to equalize the pressure. The resultant tendency is for the solutions on
the two sides of the membranes to become equal in concentration by movement
of the liquid from the less dense to the more dense portion, instead of by
movement of the dissolved materials toward the less dense part of the
solution as in the case of diffusion when solutions of different
concentrations are brought in contact with no membrane to interfere with
free diffusion.

Osmotic pressure tends, therefore, to force the movement of solvents
through semi-permeable membranes from more dilute toward more concentrated
solutions. Protoplasm acts in general as an approximately semi-permeable
membrane or material. For example, if the concentration of sugar in any
given mass of protoplasm becomes greater, by reason of the photosynthetic
activity, osmotic pressure is set up and water enters the mass, thus
preventing loss of turgidity due to increased concentration. Similarly, any
other increase in concentration of synthetic products is compensated for by
entrance of water because of increased osmotic pressure, unless the
products are insoluble and, therefore, incapable of effecting the osmotic
pressure.

Hence, osmotic pressure provides for the movement of water into and out of
protoplasm and so tends to keep the proportion of water uniform throughout
the entire tissue. It will at once occur to the reader, however, that if
the statements in the preceding paragraph were unqualifiedly true, and if
the protoplasmic mass were absolutely semi-permeable in character, there
would be no possibility of the passage of dissolved so