A tough 3D puzzle in the walnut shell

Plant organs initiate as a group of tiny meristematic cells. After expansion, three basic shapes of organs may be defined: cylindrical, laminar, and spherical (Trinh et al., 2021). The development of a lamina can be followed by various surface and subsurface microscopy methods. However, organs shaped as opaque spheres are most conveniently studied in sections. Antreich et al. (2021) applied 3D reconstructions based on optical and scanning electron microscopy to study the development of cells building the walnut shell. Restricted by cellulose, the growing cells bulge and interdigitate with neighboring cells, leaving gaps at the regions of high curvature. Examining the cell interfaces with Raman microspectroscopy, they show that these gaps are lined with pectin.