As plant physiology increased steadily in the latter half of the 19th
century, problems of absorption and transport of water and of mineral
nutrients and problems of the passage of metabolites from one cell to
another were investigated, especially in Germany. JUSTUS VON LIEBIG, who
was born in Darmstadt in 1803, founded agricultural chemistry and
developed the techniques of mineral nutrition in agricul- ture during
the 70 years of his life. The discovery of plasmolysis by NAGEL! (1851),
the investigation of permeability problems of artificial membranes by
TRAUBE (1867) and the classical work on osmosis by PFEFFER (1877) laid
the foundations for our understanding of soluble substances and osmosis
in cell growth and cell mechanisms. Since living membranes were
responsible for controlling both water movement and the substances in
solution, "permeability" became a major topic for investigation and
speculation. The problems then discussed under that heading included
passive permeation by diffusion, Donnan equilibrium adjustments, active
transport processes and antagonism between ions. In that era, when
organelle isolation by differential centrifugation was unknown and the
electron microscope had not been invented, the number of cell membranes,
their thickness and their composition, were matters for conjecture. The
nature of cell surface membranes was deduced with remarkable accuracy
from the reactions of cells to substances in solution. In 1895, OVERTON,
in U. S. A., published the hypothesis that membranes were probably lipid
in nature because of the greater penetration by substances with higher
fat solubility.