its purpose is unclear, it has been suggested that the depression may mark the position of a sporangium. As the branch of a Lepidodendron tree grew the leaf-cushion only grew to a certain extent, past which the leaf-cushion stretched. This stretching widened the groove that separated the leaf-cushions, creating a broad, flat channel. Hyphae are occasionally present in the tissues of Lepidodendron trees, indicating the presence of mycorrhizal associations. Different fossil genera have been described to name the various levels of decay in Lepidodendron bark fossils. The name Bergeria describes stems that have lost their epidermises, Aspidiariu is used when cushions have been removed by deep decay, and Knorria is used when the leaf cushions and the majority of cortical tissues has decayed, with a shallow "fluted" surface remaining. However, it has been suggested that these are more likely growth forms than preserved bark types, as entire fossilized trunks have been discovered with dissimilar forms; if decay is assumed to be constant throughout the trunk, then different forms indicate growth rather than levels of decay. It is likely that the trunk of Lepidodendron trees were subject to the growth forms Knorria, Aspidiaria, and Bergeria progressing up the trunk, respectively. Reproduction Lepidodendron species had a life cycle of 10 to 15 years composed of a growth cycle, in which the trees grew to a predetermined height, and a subsequent reproductive cycle, in which the trees produced reproductive organs, after which the trees died, similar to the life cycle of a Mauna Kea silversword. Rather than reproduce with seeds, Lepidodendron trees reproduced with spores. The spores were stored in sporangia situated on fertile stems that grew on or near the main trunk. The fertile stems grew together in cone-like structures that clustered at the tips of branches. Distribution The lack of growth rings and of dormant buds indicate no seasonal growth patterns and modern plants with similar characteristics tend to grow in tropical conditions, but Lepidodendron species were distributed throughout subtropical conditions. The trees inhabited an extensive area compared to tropical flora of the same time period, with trees growing as far north as Spitsbergen and as far south as South America, in a latitudinal range of 120°. Decline and extinction By the Mesozoic era, the giant lycopsids had died out and were replaced by conifers as well as smaller quillworts. This may have been the result of competition from the emerging