The origin of the concept of the plasma membrane is rather ambiguously identified. It antedated Pfeffer and deVries, and, after searching the literature, the writer attributes it to Carl Nägeli (1855), who had been working with algae, fungi, mosses, and unicellular plants. He noted particularly that the surface of discontinuity of the cell is impermeable to pigments contained within the cytoplasm or added to the solution around the cell. He found the protoplasmic surface to be more dense, viscous, and otherwise distinguishable from the cytoplasm, and hence he called the surface layer the plasma membrane . From experiments on osmosis he concluded that the plasma membrane endows the cell with its osmotic properties.