We found that opals covering a rice plant are composed of loosely packed silica nanoparticles ∼50 nm in diameter. The plant opal-mimetic structures of bunching nonspherical silica nanoparticles were produced through hydrolysis of tetraethoxysilane in an alcoholic solution containing a long-chain branched polyethyleneimine (BPEI). Bunching silica nanoparticles 10–50 nm in diameter were spontaneously grown in polymer-mediated polycondensation in the BPEI matrix that accumulated hydrolyzed silicate species.