Horizontal and vertical 2D slices in the single-layer (No. 1 & 2) and bilayered samples (No. 3 to 11) 4–7 days after sowing, when the root growth rate vanished in the stiffer bottom layer. The greyscales background image shows the sand in a specific section, while the root system projection is represented in green. For the cases marked with a blue asterisk the main root continues growing until it hits the interface between the two layers without touching the sidewall, whereas for those marked with a red asterisk main root touches the sidewall. The scale-bar is reported at the bottom right