Stipules can be discerned at the leaf bases of these ferns. The hardened leaf bases are persistent and overlap to form a hardened layer surrounding the stem. The mantle of sclerenchymatous leaf bases and intermixed roots can form a woody trunk when the stem emerges above ground, up to in "Todea barbara". Extinct members of the family, which flourished during the Mesozoic, could reach the stature of trees and be termed tree ferns. The leaves are either holodimorphic, with separate fertile and sterile fronds assuming an entirely different structure, or have fertile and sterile portions of the frond very distinct in structure.