Short term influx as a measure of influx across the plasmalemma.

It is shown that the influx of tracer to a plant cell must be measured for less than one third of the half-time for exchange of cytoplasmic tracer, and any subsequent wash must be negligibly brief, if the initial influx measured over a finite period is to be a good estimate of the plasmalemma influx. Complications due to lack of knowledge of the cytoplasmic exchange rate constant and to extracellular contents make it difficult to make such an estimate from influx measurements alone. The use of influx measurements is further discussed.