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.
Structural color has captured the fascination of optical researchers through numerous observations throughout history, both in naturally occurring structures and in the animal world (1–3). Plants have also evolved structural colors to fulfill a variety of functions (4–7): Structurally colored leaves (8–10), flowers (11, 12), and fruits (4, 5, 13, 14) are used by plants to regulate light harvesting (8, 15–17) and attract pollinators (6, 7), while they are also believed to promote seed dispersal (4, 5). The few, so far, described plants whose fruits are structurally colored are understory species living in tropical regions, whose fruits reflect light spanning from deep metallic blue to green when ripe.
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