Figure 3.

Peel test for model calibration. The red epidermal surface grows slower than the green parenchymal core. When the outer surface layer is peeled off the inner core, the epidermal surface contracts by ~ 1%, i.e. its stretch is λ = 0.99, while the parenchymal core expands by ~ 4%, i.e. its stretch is λ = 1.04. The peel test can serve to calibrate the relative surface growth to $\widehat{\vartheta }=0.99/1.04=0.952$.