Club Mosses

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Club Mosses

Club mosses, also called lycophytes, are flowerless, seedless plants in the family Lycopodiaceae, that belong to an ancient group of plants of the division Lycophyta. The lycophytes were one of the dominant plants during the Coal age (360–286 million years ago) and many were shrubs or large trees. By 250 million years ago, most of the woody species had died out. Between 10 and 15 living genera have been recognized, consisting of about 400 species. Lycopodiaceae are cosmopolitan, occurring in arctic to tropical regions. Nowhere do they dominate plant communities today as they did in the past. In arctic and temperate regions, club mosses are terrestrial; in the tropics they are mostly epiphytes near the tops of trees and seldom seen. The classification of club mosses has changed radically in recent years. Most temperate species were grouped within the genus Lycopodium, from the Greek lycos, meaning wolf, and pous meaning foot, in an imaginative reference to the resemblance in some species of the densely leaved branch tips to a wolf’s foot. However, it is now clear that fundamental differences exist among the club mosses with respect to a variety of important characters. Seven genera and 27 species have been recognized in the flora of North America. Four of the common genera, formerly all within the genus Lycopodium, are Lycopodium, the tree club mosses (six species), Diphasiastrum, the club mosses (five species), Huperzia, the fir mosses (seven species), and Lycopodiella, the bog club mosses (six species); all are terrestrial. The sole epiphytic member of the club moss family in North America is the hanging fir moss (Phlegmariurus dichotomus ), which is common in subtropical and tropical Central and South America. In North America it is known found only in the Big Cypress Swamp, Florida.

KEY TERMS

Gametophyte— Individual plant containing only one set of chromosomes per cell that produces gametes, i.e., reproductive cells that must fuse with other reproductive cells to produce a new individual.

Sporophyte— The diploid, spore-producing generation in a plant’s life cycle.

Unlike other ancient plants, such as liverworts, the sporophytes of club mosses are clearly differentiated into root, stem, and leaves. All living club mosses are perennial herbs that typically possess underground stems that branch and give rise to shoots that rarely exceed 7.9 in (20 cm) in height. Although the photosynthetic organs of club mosses are commonly called leaves, technically speaking they are microphylls, which differ from true leaves in that they contain only one unbranched strand of conducting tissue. The “micro” prefix does not necessarily mean that these photosynthetic organs are small; in fact some extinct tree lycophytes were 3.3 ft (1 m) long. It refers instead to an initially very small flap of tissue that grew along the stem of primitive leafless plants; eventually, through evolution, this grew larger and had a strand of conducting tissue enter it to produce the modern microphyll. Microphylls are generally needle-like, spear-shaped, or ovate, and arranged spirally along the stem, but are occasionally opposite or whorled. The habit of evergreen leaves on stems that in some species run along the ground has given rise to the common name of ground or running pines. Stems have a primitive vascular tissue composed of a solid, central column.

Spores, all of one type, are produced in sporangia that occur either singly on fertile leaves (sporophylls) that look much like nonfertile leaves or on modified leaves that are tightly appressed on the tip of a branch producing a cone or club-like structure, hence the name club moss. The cones may or may not be stalked. The spores germinate to produce bisexual gametophytes that are either green and photosynthetic on the soil surface or are underground and nonphotosynthetic, deriving some of their nourishment from mycorrhyzae. The maturation of a gametophyte may require six to 15 years. Biflagellated sperm are produced in an antheridium (male reproductive organ) and an egg is produced in a flask-shaped archegonium (female reproductive organ). Water is required for the sperm to swim to another gametophyte and down the neck of an archegonium to reach the egg at the bottom. The young sporophyte produced after fertilization may remain attached for many years, and in some species the gametophyte may continue to grow and produce a succession of young sporophytes.

Club mosses are ecologically minor components of all the ecosystems in which they occur. Their economic importance is also slight. Many club mosses produce masses of highly inflammable sulfur-colored spores that were once used as a constituent of flash powder in early photography and in fireworks. The spores were also formerly used by pharmacists to coat pills. In parts of eastern North America, local cottage industries have sprung up to collect club mosses in order to make the most elegant of Christmas wreaths. Spores of common club moss (Lycopodium clavatum ) are used by paleoecologists to calibrate the number of fossil pollen grains in samples of lake mud. Some Druid sects considered club mosses to be sacred plants and had elaborate rituals to collect club mosses and display them on their altars for good luck.

See also Liverwort; Spore.

Resources

BOOKS

Flora of North America Editorial Committee, eds. Pteridophytes and Gymnosperms. Vol.2of Flora of North America. New York: Oxford University Press, 1993.

Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of Plants. 6th ed. New York: Worth Publishers Inc., 1998.