Physiological Inferences from the Evidence of Translocated Tracer: a Caution

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