Plastid-dividing rings (PDs) are described for the first time in the ferns Ophioglossum, Gleichenia, Hymenophyllum, Trichomanes, Athyrium, Dryopteris, Ceratopteris and Pteridium. They are found in the constricted isthmuses of undifferentiated plastids, amyloplasts, amylochloroplasts and chloroplasts in gametophytes and in a range of sporophytic tissues including apices, differentiating leaf mesophyll and vascular parenchyma from roots, stems and leaves. Fern PDs comprise two concentric structures; a dense granular ring 60-100 nm wide adhering to the stromal face of the constriction and a smaller cytoplasmic annulus only 20-40 nm wide. The diameters of the PD-containing constrictions, particularly those in vascular parenchyma, where the outer component is usually lacking, are extremely uniform (means of 177-188 nm in different species and tissues) and closely in line with those in angiosperms. PDs in ferns differ from those previously described in other groups of lands plants in that they are far more persistent and in multilobed plastids one is present at each constriction. The occurrence of multiple PDs in the giant plastids found in up to 30% of vascular parenchyma cells may be associated with organelle fusion rather than division.Copyright 1993, 1999 Academic Press