In the field of ion transport, animal physiologists have made substantial progress during the last decade and a great deal of information
is now available concerning nerve, muscle, and various epithelial tissues.
Before the techniques evolved by the animal physiologists were applied to
plants a considerable delay occurred which may have been in part due to
the morphological complexities of plant cells. Recently, however, these
techniques have been applied with considerable success to plant cells,
especially to the large internodal cells of the Characeae.
It is now clear that the plant cell may be regarded as a three compartment
system consisting of the cell wall, the protoplasm, and the vacuole.
The cell wall acts as a physical extension of the environment but the
protoplasm is bounded by the plasmalemma and tonoplast membranes which act
as the diffusion barriers containing the sites at which ?active transports
takes place. Electrolytes move from one compartment to another by
diffusion, mass flow, solvent drag, or active transport but, in systems
in which osmotic pressure is constant, diffusion and active transport are
the main processes involved. It is now almost universally recognised
that the electrochemical potential gradient, not simply the concentration
gradient, is the driving force on an ion moving passively across a
membrane, and several measurements of the driving forces on particular ions
have been made. However, the forces measured are usually between the
vacuole and the external medium and not the forces across the individual
membranes. When the ionic fluxes have also been measured it has been
possible to suggest whether active transport takes place. In a few cases,
connections with metabolism have been established.Several works on the ionic relations of plants have been published
recently including books by Briggs, Hope, and Robertson (1961), Sutcliffe
(1962), and Jennings (1963). The general approach adopted in this thesis
is that outlined in a review by Dainty (1962). It is not yet possible to
treat the problem of ion transport in plants at the molecular level but,
by using a suitable member of the Characeae, an attempt has been made to
determine more accurately the forces acting on the ions, the membranes at
which active transport takes place, and the parameters which control the
electrical properties of the membranes. Electrical measurements have
also been made on a higher plant organ, the exuding root system, in order
to obtain evidence on the controversial problem of transport into the
xylem
