Physiological Inferences from the Evidence of Translocated Tracer: a Caution

It is shown that tracer movement in a simple steady-state diffusion system does not give any evidence of the net movement of non-tracer. Published evidence points to the fact that the translocation system in plants is a diffusion-analogue system, described by Fick's Law but with a very large value of the diffusion coefficient. From these bases the prediction is made that although mature leaves of a plant do not appear to import label from a stem carrying labelled assimilate, yet aphids feeding on these mature leaves ought to accumulate label. An experimental verification of this prediction is presented.