• Helicoidally arranged layers of cellulose microfibrils in plant cell walls can produce strong and vivid coloration in a wide range of species. Despite its significance, the morphogenesis of cell walls, whether reflective or not, is not fully understood. Here we show that by optically monitoring the reflectance of Pollia japonica fruits during development we can directly map structural changes of the cell wall on a scale of tens of nanometres. • Visible-light reflectance spectra from individual living cells were measured throughout the fruit maturation process and compared with numerical models. • Our analysis reveals that periodic spacing of the helicoidal architecture remains unchanged throughout fruit development, suggesting that interactions in the cell-wall polysaccharides lead to a fixed twisting angle of cellulose helicoids in the cell wall. • In contrast to conventional electron microscopy, which requires analysis of different fixed specimens at different stage of development, the non-invasive optical technique we present allowed us to directly monitor live structural changes in biological photonic systems as they develop. This method is therefore applicable to investigations of photonic tissues in other organisms.