Class of the Pteridophytina, represented today by relatively small plants, the club mosses, although in the Carboniferous they included large trees which dominated the coal-swamp vegetation of the time. The oldest lycopods date from the Devonian and derived from the most primitive of vascular plants, the psilophytes. Lycopods are characterized by dichotomizing branches; spores borne in spore cases (sporangia) on the upper side of the fertile leaves (sporophylls), which are sometimes organized into cones; and ‘microphyllous’ leaves, i.e. with a single strand of vascular tissue proceeding unbrokenly (without a ‘leaf gap’) from the vascular tissue of the stem. The leaves of some lycopods were long and grass like. When they were shed, the leaves left distinctive, spirally arranged leaf scars on the trunk of the lycopod tree. Fragments of the bark are common as fossils in coal balls and the roof shales of Carboniferous coal seams. Prominent genera included Lepidodendron, Lepidophloios, and Sigillaria (in which the spiral leaf-scar pattern fell into discrete vertical rows).