The question considered is whether pinocytosis is the principal
means of transporting major nutrients across the plasmalemma.
It is argued that ultrastructural evidence previously advanced
in favour of pinocytosis in plant cells can be interpreted in
other ways and does not uniquely support the hypothesis of
pinocytosis. Physiological evidence - particularly, selectivity
in uptake, competition between substances, differential effects
on influx of changes in internal states, and the associated
electrical currents - is incompatible with the characteristics
of pinocytosis in animal cells. The suggestion that pinocytosis
in plants involves selective binding prior to uptake cannot be
accepted because the required density of binding sites is at
least three orders of magnitude greater than is possible.  On
theoretical grounds it is shown that the water flow that would
be associated with pinocytosis would generate high values of
turgor which are not observed and probably could not be
sustained. Further, it is calcul