club moss, name generally used for the living species of the class Lycopodiopsida, a primitive subdivision of vascular plants. The Lycopodiopsida were a dominant plant group in the Carboniferous period, when they attained the size of trees, and contributed to the coal deposits then being formed. They are now considered relictual. Although they resemble the mosses, they are considered to be evolutionarily more advanced because they are vascular, that is they have specialized fluid-conducting tissues. Club mosses are usually creeping or epiphytic and often inhabit moist places, especially in tropical and subtropical forests. They reproduce by means of spores, either clustered into small cones or borne in the axils of the small scalelike leaves. The principal genera are Lycopodium and Selaginella. Some species of Lycopodium are called ground pine or creeping cedar, especially those that resemble miniature hemlocks with flattened fan-shaped branches, and are often used for Christmas decorations. The spores of L. clavatum are gathered and sold as lycopodium powder, or vegetable sulfur, a highly inflammable yellow powder sometimes used for pharmaceutical purposes (e.g., as an absorptive powder) and in fireworks. Selaginella species, often incorrectly called Lycopodium, are occasionally grown as ornamentals. One of the best known is a resurrection plant. Club mosses constitute the division Lycopodiophyta, class Lycopodiopsida.