An important aspect of root research is the relationship between structure and function. Few attempts at in-situ assessment of function exist, but many studies have attempted to relate observable and measurable root morphologies and anatomies to function. One of the most successful has been Specific Root Length( SRL - meters of root per gram of root biomass). SRL requires a few basic assumptions to be efficacious: a) all roots measured are of the same mass density (dry weight per unit volume), b) actual root diameter does not change with treatments or environmental perturbations. Data will be presented that demonstrates that these two assumptions are not valid characterizations of active root systems. This data will be used to demonstrate that neither of these assumptions are valid for a range of species.