Figure 16. Photograph of a cross section of a calcareous coal ball from the Middle Pennsylvanian Murphysboro coal bed equivalent, Indiana. The coal ball contains permineralized plant material that enables scientists to identify the individual plants and the plant communities (flora) that contributed to the peat, which later became a coal bed. The coal ball contains some identifiable remains of branches and twigs of lycopsid trees (Diaphorodendron vasculare), labeled D on the photograph. The structures, labeled S, are sections of lycopsid rootlets, called stigmaria. Compare these structures with the shapes of the macerals in the photomicrographs of coal (fig. 13). Modified from a photograph by W.A. DiMichele (National Museum of Natural History, Smithsonian Institution, Washington, D.C.).