The Diversity of Ferns and Their Relatives
Investigating Horsetails, Club Mosses, and Ferns
Aug 15, 2009
Plants may be informally grouped into three categories based on structure and mode of reproduction: nonvascular spore plants which are simple in structure (no vascular system) and reproduce by spores, vascular spore plants which are advanced in structure (possess a vascular system) but reproduce by spores, and vascular seed plants which are advanced in structure and reproduce by seeds.
The vascular spore plants incorporate four divisions from the formal plant classification system: several species of whisk ferns, 15 species of horsetails, 1,000 species of club mosses (which are not really mosses at all), and roughly 20,000 species of true ferns.
Class Psilotopsida (Whisk Ferns)
This is a small group consisting of only two living genera with several species. These plants have a green branching stem with tiny scale-like appendages and they produce their spores in yellow globular structures. They are unique among the vascular plants in that they lack both complete roots and leaves, although they do possess rhizomes (horizontal underground stems) like other vascular spore plants.
Whisk ferns are tropical and subtropical in their distribution. In the United States, they are found in Florida, Louisiana, Arizona, Texas, Hawaii, as well as Puerto Rico. Some types are found growing as epiphytes on tree ferns in Australia and islands of the southwest Pacific. (An epiphyte is a plant that is not rooted but grows nonparasitically attached to and often hanging down from other larger plants).
Class Equisetopsida (Horsetails)
Consisting of only one genus with 15 species, these plants have ribbed, jointed green stems with whorls of thin wire-like leaves that soon become brown and no longer undergo photosynthesis. To some, this gives them the appearance of a horse’s tail while others compare them to the wire-handled brushes used to clean glassware in the laboratory. The various species of horsetails are widespread in moist or damp places by streams, and along the edges of woods.
Humans have taken advantage of the fact that horsetails have a great deal of silica in their cell walls. This makes their stems abrasive allowing the pioneers of North America to use them to scour their pots and pans clean. That gave rise to their second common name – scouring rushes. Musicians also use them to sand the reeds on wind instruments.
Division Lycopodiophyta (Club Mosses)
Ranging from small, flat forms commonly found on the forest floor to large epiphytes in the tropics, this phylum contains 10-15 genera that consist of around 1,000 species.
These plants are not true mosses and only superficially resemble them with small overlapping scale-like leaves. The spore cases are arranged in club-like clumps at the ends of erect stems. Creeping Jenny and ground pine are two well-known club mosses often used in Christmas wreaths. In the early days of flash photography, the spores of the club moss known as Lycopodium were the original flash powder. These powdery spores were also used in the past as a type of talcum powder, as a coating for pills, to stop bleeding from wounds, and for other medicinal purposes.
One of the plant realm's strangest inhabitants must surely be a club moss known commonly as the “Resurrection Plant” (Selaginella lepidophylla). Most club mosses are tropical but this type lives in the desert where it often dries up and curls into a brittle brown ball. However, when water again becomes available, it seems to miraculously resurrect from the dead turning green and uncurling to lie flat on the ground.
Division Pteridophyta (Ferns)
With around 20,000 species, ferns are by far the largest and most familiar group of vascular spore plants. They also show great variation in their size and structure ranging from tiny floating aquatic forms less than 1.25 cm (0.5 in) across to tropical tree ferns that grow to 24 meters (80 feet) in height.
Ferns are the most diverse in the tropics, where about three-fourths of the species are found. Only about 380 species of ferns occur in the United States and Canada. Approximately a third of all species of tropical ferns grow upon the branches or trunks of trees as epiphytes.
Vascular Spore Plants Have an Ancient Lineage
The fossil record reveals that about 300 million years ago, great parts of North America were covered by shallow swamps and seas. In the warm and wet climatic conditions of that time thick forests of giant relatives of today’s club mosses, horsetails, and ferns covered the land. Some of these plants were more than 20 m (66 feet) tall.
Then as the climate changed, these early forests died out and disappeared. Tremendous amounts of partially decayed remains of these plants were buried and slowly compressed over long periods of time under high temperature and great pressure. Eventually, they became what are known as fossil fuels – coal, oil, and natural gas.
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