The fundamental principles of osmosis are reviewed. A scheme is presented for mathematically dealing with the osmotic quantities, expressed as osmotic specific free energies, and graphically representing the same in a diagram with appropriate coordinates. Theoretical cases are introduced, utilizing various osmotic action capacities which possibly control water movement between the plant and its environment. Four typical sets of data are analysed by the present scheme and limitations discussed. A survey is made of methods currently employed in obtaining various osmotic quantities in plants. The controversy as to the reality or non-existence of metabolic specific free energies (variously termed active, vital or non-osmotic pressures) must await more critical experimentation.