(a) Wild type barley (Himalaya) and della mutants (Sln1d, Sln1d5, Sln1d6) at comparable stages after flowering. (b) Mature spikes of genotypes shown in (a), with alternating spikelets attached to nodes of the main axis (rachis) indicated for the wild type. (c) 3D reconstruction of confocal image of mPS-PI-stained wild-type inflorescence meristem, fixed when the first spikelet primordia were initiated; a plane (red) fitted to landmarks placed on the boundaries of spikelet primordia was used to select SAM cells (details in Supplementary Information). (d, e) 3D reconstructions of segmented images of wild-type (d) and Sln1d (e) apices, with selected SAM cells coloured. (f) Correlation between meristem size (cell numbers) and inflorescence size (rachis nodes) in the wild type and sln1 mutants; points and green lines show average and standard deviation per genotype, the red line shows the linear regression of averages, with correlation coefficient r; for Hymalaya, Sln1d, Sln1d5 and Sln1d6, respectively, n was 18, 15, 18, 18 spikes (for node numbers) and 8, 7, 3, 4 apices (to image cell numbers). Bars: 10 cm (a), 1 cm (b), 100 µm (c-e).