Despite their common name, clubmosses are not technically mosses, but rather are evergreen herbs usually included in the group of plants known as fern allies. Like ferns, clubmosses are flowerless and seedless, and have true leaves, stems, and roots. The primitive plants were once among the most dominant flora on Earth, growing nearly 100 feet high and being particularly abundant during the Carboniferous Era, 360 million to 286 million years ago. Regardless of their size, the stems of clubmosses never contained woody tissues like those of trees. The massive amounts of clubmoss vegetation present during that time greatly contributed to the coal deposits that are utilized today as a human source of fuel as they decayed.