Cinnamon, sensitive, royal, ostrich, and marsh ferns all thrive in Isle Royale's swamp forests, fens, and other wetland areas. Drier places on the island such as rocky ridges and cliffs are also prime real estate for ferns including rusty cliff fern, common polypody, and American rock brake, a Michigan threatened species. Bracken fern is a very common island species occurring in upland forests and woodlands where it often forms dense colonies spreading by rhizomes. Moonworts, unlike brackens and other larger ferns, are easily missed due to their rarity and small size.
Ferns and fern friends such as horsetails and clubmosses do not produce seeds like flowering plants, but instead bear one-celled spores. Structures develop from these microscopic spores that result in a new fern, horsetail, or clubmoss plant. Fern spores are usually in small clusters called sori, located on the underside of a fern leaf blade. The characteristic sori shapes (round, elongate, etc.) along with fern size and how finely a leaf blade is divided into smaller and smaller segments, are useful in identifying the island's nearly thirty different fern species.
There are eight horsetail and eight clubmoss species on Isle Royale. Horsetails, also known as scouring rushes, are single-stalked with regular joints (nodes) that give the plant the appearance of a miniature bamboo. Some species have a whorled form with branches that radiate out from the nodes. Clubmosses, which incidentally are not mosses at all, may form dense carpets on the forest floor spreading by long horizontal stems.