What can tracer profiles tell us about the mechanisms giving rise to them?

For a long time, tracer profiles observed at various positions along the phloem transport pathway have been used to infer details about both the mechanisms and the amount of flow through this pathway within whole plants, But the mechanisms still elude us, This paper investigates why this is so and shows that although the amount of mechanistic information available from tracer profiles is rather limited, they do provide reliable quantitative measurements of the amount of flow. Detailed quantitative analysis of tracer profiles in barley leaves is used to illustrate techniques of mechanistic interpretation from input-output analysis and indicates the need of two compartments to describe carbon flow through a leaf from fixation to phloem loading.