From the standpoint of their relation to osmosis, membranes may be either _impermeable_, in which case neither solvent nor dissolved materials can pass through them; _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.