Many species reproduce vegetatively by stolons. The plant reproduces via seed or vegetatively by underground rhizomes. The water horsetail reproduces both by spores and vegetatively by rhizomes. The plant can be propagated sexually by seed or vegetatively by dividing the rhizome. "Anthurium" can be propagated by seed or vegetatively by cuttings. The plant reproduces sexually by seed and vegetatively by sprouting. There is greater variability than with the vegetatively propagated rootstocks. "Echinacea purpurea" can be propagated either vegetatively or from seeds. Since vegetatively propagated plants are clones, they are important tools in plant research. This means that useful forms and hybrids must be propagated vegetatively. Lichens reproduce by means of spores or vegetatively. It is sterile and can only reproduce vegetatively through suckers or cuttings. Fire moss is dioecious, reproducing generatively with spores and vegetatively through protonemata. The triploid forms are sterile, and reproduce only vegetatively, via bulbs. Self-heal propagates both by seed and vegetatively by creeping stems that root at the nodes.