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Fig. S1 A typical root showing the cross-section, cell wall network and axes.

Fig. S2 An idealised root cross-section.

Fig. S3 Geometric labels for a ‘slice’ of root.

Fig. S4 A space–time diagram for cells moving through the EZ.

Table S1 Turgor pressure data as plotted in Fig. 3(a)

Table S2 Statistical significance of comparison of cell thicknesses Z1A vs Z1B

Notes S1 Model description for upscaling cell wall properties to the tissue level.

Notes S2 Model description for using an idealised root geometry.

Notes S3 Model description for bending induced by asymmetric extensibility.

Notes S4 Turgor pressure data.

Notes S5 Further cell wall thickness analysis.