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Bright, iridescent colors observed in nature are often caused by light interference within nanoscale periodic lattices, inspiring numerous strategies for coloration devoid of inorganic pigments. Here, we describe and characterize the septum of the Lunaria annua plant that generates large (multicentimeter), freestanding iridescent sheets, with distinctive silvery-white reflective appearance. This originates from the thin-film assembly of cellulose fibers in the cells of the septum that induce thin-film interference–like colors at the microscale, thus accounting for the structure’s overall silvery-white reflectance at the macroscale. These cells further assemble into two thin layers, resulting in a mechanically robust, iridescent septum, which is also significantly light due to its high air porosity (>70%) arising from the cells’ hollow-core structure. This combination of hierarchical structure comprising mechanical and optical function can inspire technological classes of devices and interfaces based on robust, light, and spectrally responsive natural substrates.