ABSTRACT
An outstanding problem of plant physiology is the nature of the mechanism which controls the movement of water-soluble substances into, and out of, the living cell. Recent investigations confirm the long established impression that in the vacuolar sap of living cells substances are present, which, although soluble in water, are almost unable to diffuse out when the cell is surrounded by water. It is clear that the living protoplast is responsible for this, since complete permeability is associated with death. Moreover, it is well established that the degree of permeability of living cells to solutes is dependent upon the composition of the medium with which they are in contact, and its consequent effect upon the plasma surface. Leaving on one side the controversy concerning the existence and constitution of a specific plasma membrane, which may be. responsible for these attributes of living cells, it is clear that the phenomena associated with boundary phases give adequate grounds for anticipating that the external surface of the protoplast will have a different composition from the main protoplasmic mass. It is natural that the composition of this surface layer should be stressed in theories of cell permeability, but it is clearly undesirable that our knowledge of its nature should rest largely on speculation.
