Spores were borne in long cylindrical cones at the tips of the branches. Those cones referred, or assigned, to the genus _Lepidostrobus_ bore both small spores (microspores) and large spores (megaspores) in the same cones. Those in which only a large single spore, a somewhat seedlike structure, was developed in a spore sac (sporangium) are referred to the genus _Lepidocarpon_. The rather commonly found genus _Stigmaria_ comprises so-called “appendages” which, although stemlike in structure, apparently served as roots for the scale and seal trees. These appendages are identified by irregular spirals of circular scars (pits) that mark the attachment points of former rootlets. [Illustration: Reconstruction of _Sigillaria_ (after Hirmer)] _Sigillaria_, although less common than _Lepidodendron_, was widely distributed during the Pennsylvanian Period. It differed in growth habit from _Lepidodendron_ in that it generally had fewer branches and not uncommonly was unbranched. Some species also possessed a thicker trunk, with hexagonal to elongate leaf cushions separated by vertical ribs. The trunk was crowned, in the manner of the modern palm tree, by a cluster of large, grasslike leaves. The detached leaves of _Sigillaria_, extremely difficult to distinguish from _Lepidophyllum_ (leaves of _Lepidodendron_), are referred to the genus _Sigillariophyllum_ if preserved as compressions and to _Sigillariopsis_ if preserved as petrifactions. Unbranched _Sigillaria_ trunks have been found that are more than 100 feet long and six feet in diameter near the base, but the average height probably was closer to 50 feet. [Illustration: Reconstruction of _Calamites_ (After Hirmer)] Not all Pennsylvanian trees were large, however. Small forms are known, including the important undergrowth genera _Lycopodites_ and _Selaginellites_. In woody types the trunk consisted of an inner region of conducting and supporting tissues, surrounding concentric cortical layers, and an outer layer of corklike bark. Although the fossil impressions of the various bark layers have been given separate generic names, these are not commonly used. SCOURING RUSHES (Plate 2) Although related to the small, inconspicuous horsetails of today, the ancient scouring rushes of the Pennsylvanian Period grew to the size of trees and were among the most widely distributed plant groups. Some of these plants attained heights of 40 feet or more, but the average was closer to 20 feet. The trunks were jointed and bore a whorl of branches at the joints (nodes). Their small leaves also grew in whorls at nodes along the smaller branches. Internodal regions were ribbed in the same manner as present day horsetails. Fossils of the trunks are assigned to the genus _Calamites_ and quite commonly are preserved in sandstone and shale.