Calamites is an extinct arborescent (tree-like) horsetails to which the modern horsetails are closely related. The plant was medium sized, growing to heights of more than 100 feet (30 m) and were components of the coal mires of the Caerboniferous Period, 360 to 300 million years ago. The trunks of Calamites had a distinctive, bamboo-like appearance and vertical ribbing. The branches, leaves and cones were all borne in whorls. The leaves were needle-shaped with up to 25 leaves per whorl. The outer portion of the trunk was made of wood (xylem). The stems of modern horsetails are typically hollow or contain numerous air-filled sacs or pith. Calamites had the same stem structure. Calamites reproduced by spores which were produced in small sacs organized into cones. The plant had massive underground rhizomes which allowed for the production of clones of one tree. This is the only group of trees of the period to have clonal habit. The genus Calamites became extinct in the Lower Permian, a time that also saw the origin and diversification of the herbaceous genus Equisetum, the only living sphenophyte genus.