The key difference between operculum and peristome is that operculum is a cap-like covering found in some plants, mosses and fungi while peristome is a ring of tooth-like appendages around the opening of the capsule of mosses.
Mosses are nonvascular plants belonging to phylum Bryophyta. They are flowerless plants that don’t have true roots. They grow in damp or shady areas and reproduce by spores. Mosses produce spores inside the spore-bearing capsules or sporangia. The spore-bearing capsule has an apical lid which is called an operculum. It also has a ring of tooth-like appendages surrounding the mouth of the capsule. This is called the peristome. Both operculum and peristome are important structures of mosses. When the operculum falls off, peristome is exposed to the outside.
