Iridescent blue leaf coloration in four Malaysian rain forest understory plants,Diplazium tomentosumBl. (Athyriaceae),Lindsaea lucidaBl. (Lindsaeaceae),Begonia pavoninaRidl. (Begoniaceae), andPhyllagathis rotundifoliaBl. (Melastomataceae) is caused by a physical effect, constructive interference of reflected blue light. The ultrastructural basis for this inD. tomentosumandL. lucidais multiple layers of cellulose microfibrils in the uppermost cell walls of the adaxial epidermis. The helicoidal arrangement of these fibrils is analogous to that which produces a similar color in arthropods. InB. pavoninaandP. rotundifoliathe blue‐green coloration is caused by parallel lamellae in specialized plastids adjacent to the abaxial wall of the adaxial epidermis. The selective advantage of this color production, if any, is unknown.