Sieve elements of various ages were examined in petioles and midribs ofPlatycerium bifurcatum(Cav.) C. Chr. andPhlebodium aureum(L.) J. Sm., only older ones in similar parts of leaves ofPolypodium schraderiMett. andMicrogramma lycopodioides(L.) Copel. Nacreous walls apparently are formed by most, if not all, protophloem and metaphloem sieve elements in all four species. InPlatyceriumandPhlebodiumnacreous wall formation is closely correlated with the appearance of numerous membranes or vesicles in the region of the wall. These extracytoplasmic membranes apparently are derived from protrusions of the plasmalemma. After the nacreous layer is fully thickened, many endoplasmic reticulum (ER) membranes apparently end up outside the plasmalemma ofPlatycerium, where they degenerate and gradually intergrade in appearance with the fibrillar material comprising the nacreous thickening. InPhlebodium, Polypodium, andMicrogrammathe ER forms multivesicular bodies. As the cells approach maturity, the membranes delimiting the multivesicular bodies fuse with the plasmalemma and their vesicular contents, which are not discharged into the region of the wall, disappear. Gradually, the nacreous layer decreases in thickness and disappears. At maturity the enucleate sieve‐element protoplasts of all four species are essentially similar. They are lined by a plasmalemma and a parietal, anastomosing network of ER and contain both plastids and mitochondria. The plastids inPolypodiumandMicrogrammaare chloroplasts, but those inPlatyceriumandPhlebodiumlack grana and intergrana lamellae.